Mind the Gap
Peter M. Kappeler
l
Joan B. Silk
Editors
Mind the Gap
Tracing the Origins of Human Universals
Editors
Prof. Dr. Peter M. Kappeler
Deutsches Primatenzentrum
Abt. Verhaltensökologie & Soziobiologie
Kellnerweg 4
37077 Göttingen
Germany
pkappel@gwdg.de
Dr. Joan B. Silk
University of California,
Los Angeles
Dept. Anthropology
Los Angeles CA 90095
USA
jsilk@anthro.ucla.edu
ISBN: 978 3 642 02724 6 e ISBN: 978 3 642 02725 3
DOI 10.1007/978 3 642 02725 3
Springer Heidelberg Dordrecht London New York
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Cover: Photo by Etsuko Nogami, Kyoto University
A mother chimpanzee uses a pair of stones to crack open oil palm nuts; watched by the son, 7 years old,
and the daughter, 1.5 years old.
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Preface
This volume features a collection of essays by primatologists, anthropologists,
biologists, and psychologists who offer some answers to the question of what
makes us human, i.e., what is the nature and width of the gap that separates us
from other primates? The chapters of this volume summarize the latest research on
core aspects of behavioral and cognitive traits that make humans such unusual
animals. All contributors adopt an explicitly comparative approach, which is based
on the premise that comparative studies of our closest biological relatives, the
nonhuman primates, provide the logical foundation for identifying human universals as well as evidence for evolutionary continuity in our social behavior. Each of
the chapters in this volume provides comparative analyses of relevant data from
primates and humans, or pairs of chapters examine the same topic from a human or
primatological perspective, respectively. Together, they cover six broad topics that
are relevant to identifying potential human behavioral universals.
Family and social organization. Predation pressure is thought to be the main
force favoring group-living in primates, but there is great diversity in the size and
structure of social groups across the primate order. Research on the behavioral
ecology of primates and other animals has revealed that the distribution of males
and females in space and time can be explained by sex-specific adaptations that are
sensitive to factors that limit their fitness: access to resources for females and access
to potential mates for males. The interaction of these selective pressures has favored
the formation of stable social groupings, which range from pair-bonded family
groups to large multi-male, multi-female groups. In Chap. 2, Bernard Chapais
explores and reconstructs the deep social structure of human societies based on his
extensive knowledge of primate social systems. He provides a convincing scenario
for the transition from a chimpanzee-like system to one that characterizes all human
societies. According to his analyses, the development of weapons has broken the
polygyny potential of males, leading to the formation of stable breeding bonds.
Reciprocal exchange of (female) mates among neighboring groups subsequently
created a new dimension of kinship-mediated bonds across groups, which uniquely
characterize human societies.
v
vi
Preface
In Chap. 3, Ryne Palombit takes a broad look at bonding and conflict between
the sexes, emphasizing commonalities between humans and other primates. He
identifies sexual conflict as one constant in both human and primate societies and
explores its consequences in depth. The susceptibility to infanticide is one important underlying feature of intersexual relations in primates with important consequences for their physiology and behavior. Sexual coercion and other forms of
aggression between the sexes also have their origins in sexual conflict. This
evolutionary approach can, therefore, explain some aspects of human behavior at
least as well as culture-based alternatives.
Socio-ecological theory is based on the assumption that fundamental sex differences related to parental investment play an important role in shaping mammalian
reproductive strategies. Differences in sexual and reproductive strategies between
human males and females are, therefore, expected and have been documented in
many cases. In Chap. 4, Monique Borgerhoff Mulder uses data from a rural foragerhorticultural population in Tanzania to emphasize the fact that humans are not
invariably restrained by deep physiological constraints into stereotypical gender
roles. Her insightful analyses of the human pair bond reveals much more flexibility
than previously assumed, raising interesting new questions about other prominent
related aspects of human social behavior, including paternal care, female bonding,
and multiple mating by females.
Politics and power. Apart from reproduction, much social behavior of primates
revolves around dominance and power. Competition for access to resources or
mates is ubiquitous in the animal world, and in long-lived species such as primates,
where individuals interact on a daily basis, dominance offers a mechanism to
minimize the immediate costs of competition. In Chap. 5, David Watts reviews
our current understanding of dominance and power in primate societies. Following
a most welcome critical discussion of the various terminologies used to describe
agonistic asymmetries in primatology, he reviews the assumptions and predictions
of the socioecological model that aims at explaining species differences in dominance styles. Watts argues that power and dominance are widespread among
primates, but that politics, i.e., polyadic coalitions and social manipulations that
require third-party awareness, are limited to great apes and differ importantly in
kind from human politics.
In Chap. 6, Aime´e Plourde turns to human power asymmetries. She argues that
dominance is also pervasive in human societies, albeit based on a much richer
repertoire of coercive behaviors. In addition, she identifies prestige as a unique
source of social power in humans. She discusses the evolutionary origins of
prestige, focusing on the hypothesis that prestige arose in parallel with the increasing importance of transmitting complex information culturally because individuals
who successfully acquired and applied these vast sources of knowledge were
treated with respect and admiration. Subsequently, signaling wealth and success
took on an important role in reinforcing prestige asymmetries. Plourde goes on to
depict how prestige has pervaded human social life in a unique manner, ranging
from group competition to politics.
Preface
vii
Laura Betzig focuses on a particular example to illustrate the consequences of
power asymmetries in human societies in Chap. 7. Her detailed analysis of one
period in Roman history documents pronounced skew in reproductive opportunities
as a result of socially imposed and politically controlled power differences between
emperors and eunuchs at the extreme ends of the social hierarchy. As such, this
example attests to the social flexibility of our species already emphasized in the first
section.
Intergroup relationships. Interactions with neighboring social units play an
important part in the daily lives of most animals. In territorial species, groups
defend resources located within their territories against covetous neighbors. Intergroup relations are, therefore, primarily characterized by mutual aggression. In
Chap. 8, Margaret Crofoot and Richard Wrangham take a closer look at the nature
and function of primate intergroup aggression. They show that the essential functional reasons for competition between groups are very similar across species,
despite variable feeding ecologies and modes of interactions, and that numerical
superiority is the best predictor of long-term success. In contrast to chimpanzees
and humans, however, deliberate planning and regular lethal violence do not
characterize intergroup violence in other primate species.
In Chap. 9, Azar Gat zooms in on human warfare as a form of collective,
organized intergroup aggression that is not found in this form in other primates.
He uses examples from various cultures and periods to illustrate the factors that
favor this form of collective aggression. Again, competition for resources and
reproductive opportunities loom large as important incentives, but religion and
other supernatural beliefs also generate conflict among groups of humans. Gat
also examines the role of proximate mechanisms, such as prestige, retaliation,
and ecstasy. His informed evolutionary interpretation of the different aspects of
warfare provides a compelling example of how a (near) human universal is shaped
by general evolutionary processes.
Foundations of cooperation. One striking feature of human intragroup social
behavior is the frequency and scope of cooperation. This problem is of special
interest to students of behavior because cooperation is, by definition, associated
with a cost for the actor and a benefit for the recipient, and therefore counterintuitive for both evolutionary and economic analyses of behavior. In Chap. 10,
Joan Silk and Robert Boyd use a comparative approach to identify ways and
mechanisms in which human cooperation differs from that of other primates.
They review how kinship, reciprocity, and mutualism structure cooperative behavior of nonhuman primates in different functional domains. The same processes can
be identified in humans, but Silk and Boyd argue that cultural evolution has created
new opportunities for group-level cooperation.
In Chap. 11, Venkat Lakshminarayanan and Laurie Santos focus on the
seemingly irrational aspects of cooperative behavior: foregoing individual pay-off
maximization or, in other words, violating the norms of economic decision-making.
They identify several key features of apparently irrational economic behavior in
humans and explore whether similar features exist in other primates. They find
similarities in aspects of this “economic cognition,” such as loss aversion and
viii
Preface
inequity aversion. Their conclusions put claims about human economic irrationality
into a broader perspective and identify new and exciting avenues for experimental
work on primates.
Some of the seemingly irrational decisions of individuals, and prosocial acts in
particular, may be proximately governed by emotions. In Chap. 12, Daniel Fessler
and Matthew Gervais take a closer look at these emotions and their evolutionary
origin. They broaden this approach by expanding their inquiry to all major emotions. Their comprehensive review reveals that many emotions have an evolutionary origin well outside the primate order. On the other hand, this broad comparative
perspective helps identify likely universally human emotions, such as shame and
norm-based guilt.
Language, thought, and communication. The unusually large human brain
harbors the hardware for our cognitive abilities. After all, these abilities underlie
our intelligence and behavioral flexibility. Language is the most salient aspect of
human social cognition and communication. In Chap. 13, Dorothy Cheney and
Robert Seyfarth examine the continuities and discontinuities between human language and vocal communication in nonhuman primates by focusing on vocal
production and perception. Their review shows that humans are unique in the
flexibility with which they produce learned, modifiable sounds, whereas fundamental differences with respect to call usage and perception are less clearly pronounced.
Their experiments with wild baboons also reveal that a relatively small repertoire of
fixed calls with specific meaning can, nonetheless, generate a formidable communication system.
In Chap. 14, Chris Knight takes a semantic look at the evolution of language. He
considers fundamental aspects of digital and analog communication systems to
illuminate the possible transition from primate vocal communication to language.
Animal play provides an interesting situation in which animals modify their signals
in a way that shares fundamental features with language. With this theoretical
background, Knight develops a hypothesis about the origin of language in which
menstruation and sexual conflict play a pivotal role in the socio-cultural evolution
of human language.
In Chap. 15, Robin Dunbar examines the origin and functions of human brains
within the broader context of primate brain evolution. After all, primates, as a
group, are distinct from other mammals because of their large brains for their body
size. He discusses three hypotheses about primate brain evolution that focus on
ecological, life history, and social explanations. He stresses the often overlooked
fact that these hypotheses imply different benefits and constraints so that a comprehensive approach that specifies causes, constraints, and emergent properties is
required. The summary of his earlier empirical analyses strongly implicates group
size as the main driving force in primate brain evolution. The special position of
humans (and to a lesser extent of chimpanzees and bonobos) can be explained by
the special cognitive demands of the dispersed, nested structure of their social
groups.
In Chap. 16, Michael Tomasello and Henrike Moll outline their view of the
uniqueness of human cognition. Accordingly, individual cognitive abilities, while
Preface
ix
impressive compared with other primates, are not what distinguish us ultimately
from our close relatives. Instead, the synergies resulting from the combination of
many individual brains in constructing and culturally transmitting social institutions and rules constitute a truly unique human achievement. The psychological
mechanism facilitating these effects is shared intentionality. Tomasello and Moll
compile an impressive array of experiments with great apes and human children to
muster support for their claim. Their work also provides a compelling case for the
coevolution of cognition and culture that can help to understand problems discussed
in the last section (Innovation and Culture).
In Chap. 17, David Bjorklund, Kayla Causey, and Virginia Periss take up the
theme of shared intentionality and focus on its developmental aspects. They
emphasize the importance of mothers, in particular, in the development of the
necessary cognitive abilities and mechanisms, such as gaze following and empathy.
These authors chose chimpanzees raised by human caretakers as another interesting
model for exploring the gap between humans and chimpanzees. They broaden their
comparative analyses to social learning in general as well as the necessary theory of
mind and conclude that, despite several striking similarities between human children and enculturated chimpanzees, the active role of human mothers in these
interactions is unique.
In Chap. 18, Robert Trivers devotes his attention to two phenomena of the mind
that are closely related but differ enormously in what we know about their effects on
our daily behavior and decision-making. While deception is known to exist at all
levels of life and expressed in the behavior and other traits of organisms, selfdeception is only known from humans. Trivers presents evidence from several
experiments that can only be explained by assuming that our unconscious mind
hides true information from the conscious mind, thereby affecting the latter’s
performance. This constellation provides a fascinating playground for studying
the interactions among deception, its detection, and self-detection, as well as a
stimulating source of questions about the nature and organization of our minds.
This section concludes with yet another comparative perspective on primate
cognition. In Chap. 19, Claudia Fichtel and Peter Kappeler look at the other side of
the coin of human universals by asking which cognitive abilities humans share with
other primates because they represent shared ancestral traits. They review the
literature on cognitive abilities and the social behavior of strepsirrhine primates,
which represent the best living models of ancestral primates. The available
evidence suggests that strepsirrhines are by no means inferior to New World
primates on a number of tests of technical intelligence, so that there appears to be
a solid cognitive baseline common to all primates. In the realm of social behavior,
however, group-living lemurs differ in a number of details, including coalition
formation. Whether these discrepancies reflect a lack of social intelligence in
lemurs or adaptations to particularly competitive regimes remains an open question
for the time being.
Innovation and culture. The existence of multiple traditions that are transmitted via social learning is a hallmark of human societies. Once thought to be one of
the main human universals, culture is now also known to have deep roots among the
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Preface
common ancestors of humans and other great apes. In Chap. 20, Andrew Whiten
explores the depth and nature of these roots. He provides a useful summary of the
culture debate and discusses the results of natural observations and clever experiments with chimpanzees to sharpen the distinction between different components
and mechanisms of culture in these two species. He shows that there are many
similarities in the patterns and mechanisms of cultural behaviors, but finds major
differences in the complexity of human culture.
In Chap. 21, Richard McElreath describes human culture as an effective inheritance system for ecological and social information. In addition to genetic information and individual learning during development, socially mediated transfer of
information provides a very flexible mechanism to accumulate locally relevant
knowledge. He is interested in understanding how and why the human genome
has learned to extract and transmit environmental information in such a unique and
complex manner. His focus is on the origins of culture. How could it get started
initially in the absence of large amounts of information, and, hence, immediate
benefits? In this chapter, he introduces a model that suggests a possible scenario and
highlights the important role of innovations in the initial process.
In Chap. 22, Carel van Schaik and Judith Burkhart develop the hypothesis that
the need for assistance in rearing offspring and the development of cooperative or
communal breeding systems has favored the evolution of the suite of derived traits
that distinguish humans from other primates. Their chapter integrates the findings
of many of the previous chapters in the volume, and provides a powerful example of
the value of the comparative approach for understanding what makes us human.
Taken together, the chapters in this book provide the context for understanding
the similarities and differences between humans and other primates. The data
reviewed here provide fertile ground for developing and testing additional hypotheses about the origins and adaptive value of universal human traits, and for
evaluating competing claims about the significance of the traits that distinguish
us from other primates. The chapters in this book illuminate the magnitude and
historical depth of the gap between humans and other primates, and help us to
understand why and how our ancestors traversed the particular historical path that
brings us to the present.
Acknowlegements
This volume is largely based on contributions to a conference held in Göttingen
(Germany), in December 2007, the VI. Göttinger Freilandtage. We thank the
Deutsche Forschungsgemeinschaft (DFG), the Deutsches Primatenzentrum
(DPZ), the City of Göttingen, and the University of Göttingen for their support of
this conference.
We subsequently solicited several additional contributions for this volume to
cover topics not addressed during the conference. We are particularly grateful to
those colleagues for contributing chapters to this volume at much shorter notice,
and we appreciate their professional collegiality.
Every chapter of this volume was peer-reviewed, and we thank the following
colleagues for providing helpful and constructive comments on at least one chapter:
Filippo Aureli, Josep Call, Bernard Chapais, Dorothy Cheney, Lee Cronk, JeanLouis Desalles, Frans de Waal, Charles Efferson, Nathan Emery, Azar Gat, Sue
Healy, Karin Isler, Richard McElreath, Joseph Manson, John Mikhail, Susan Perry,
Markus Port, Hannes Rakoczy, Laurie Santos, Brooke Scelza, Robert Seyfarth,
Bernard Thierry, Michael Tomasello, Peter Turchin, Carel van Schaik, Andrew
Whiten, Michael Wilson, and Richard Wrangham.
Special thanks to Ulrike Walbaum for formatting chapters, figures, and tables,
and for carefully double-checking all references. P.M. Kappeler thanks Claudia,
Theresa, and Jakob for moral support. J.B. Silk thanks P.M. Kappeler for the
opportunity to help shape this volume.
Göttingen and Los Angeles
May 2009
Peter Kappeler & Joan Silk
xi
Contents
Part I Introduction
1
Primate Behavior and Human Universals: Exploring the Gap . . . . . . . . . . . 3
Peter M. Kappeler, Joan B. Silk, Judith M. Burkart, and
Carel P. van Schaik
Part II
Family & Social Organization
2
The Deep Structure of Human Society: Primate Origins
and Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Bernard Chapais
3
Conflict and Bonding Between the Sexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Ryne A. Palombit
4
The Unusual Women of Mpimbwe: Why Sex Differences
in Humans are not Universal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Monique Borgerhoff Mulder
Part III
Politics & Power
5
Dominance, Power, and Politics in Non human
and Human Primates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
David P. Watts
6
Human Power and Prestige Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Aimée M. Plourde
7
The End of the Republic (Human Reproductive Strategies) . . . . . . . . . . . 153
Laura Betzig
xiii
xiv
Contents
Part IV
Intergroup Relationships
8
Intergroup Aggression in Primates and Humans:
The Case for a Unified Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
Margaret C. Crofoot and Richard W. Wrangham
9
Why War? Motivations for Fighting in the Human
State of Nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Azar Gat
Part V
Foundations of Cooperation
10
From Grooming to Giving Blood: The Origins
of Human Altruism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
Joan B. Silk and Robert Boyd
11
Evolved Irrationality? Equity and the Origins of Human
Economic Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
Venkat Lakshminarayanan and Laurie R. Santos
12
From Whence the Captains of Our Lives: Ultimate
and Phylogenetic Perspectives on Emotions in Humans
and Other Primates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
Daniel M.T. Fessler and Matthew Gervais
Part VI
Language, Thought & Communication
13
Primate Communication and Human Language: Continuities
and Discontinuities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
Dorothy L. Cheney and Robert M. Seyfarth
14
Language, Lies and Lipstick: A Speculative Reconstruction
of the African Middle Stone Age ‘Human Revolution’ . . . . . . . . . . . . . . . 299
Chris Knight
15
Brain and Behaviour in Primate Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
Robin I.M. Dunbar
16
The Gap is Social: Human Shared Intentionality and Culture . . . . . . . 331
Michael Tomasello and Henrike Moll
17
The Evolution and Development of Human Social Cognition . . . . . . . . 351
David F. Bjorklund, Kayla Causey, and Virginia Periss
Contents
xv
18
Deceit and Self-Deception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
Robert Trivers
19
Human Universals and Primate Symplesiomorphies: Establishing
the Lemur Baseline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395
Claudia Fichtel and Peter M. Kappeler
Part VII
Innovation & Culture
20
Ape Behavior and the Origins of Human Culture . . . . . . . . . . . . . . . . . . . . 429
Andrew Whiten
21
The Coevolution of Genes, Innovation, and Culture in Human
Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451
Richard McElreath
Part VIII
22
Conclusions
Mind the Gap: Cooperative Breeding and the Evolution of Our
Unique Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477
Carel P. van Schaik and Judith M. Burkart
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497
Contributors
Laura Betzig The Adaptationist Program, Ann Arbor, MI, USA, lbetzig@
gmail.com
David F. Bjorklund Department of Psychology, Florida Atlantic University, Boca
Raton, FL, USA, dbjorklu@fau.edu
Monique Borgerhoff Mulder Department of Anthropology, University of
California at Davis, Davis, CA, USA, mborgerhoffmulder@ucdavis.edu
Robert Boyd Department of Anthropology, University of California, Los Angeles,
Los Angeles, CA, USA, rboyd@anthro.ucla.edu
Judith M. Burkart Anthropological Institute and Museum, University of Zürich,
Zürich, Switzerland, judith.burkart@access.uzh.ch
Kayla Causey Department of Psychology, Florida Atlantic University, Boca
Raton, FL, USA, klabeth@mac.com
Bernard Chapais Department of Anthropology, University of Montreal, Montreal,
Canada, bernard.chapais@Umontreal.Ca
Dorothy L. Cheney Department of Biology, University of Pennsylvania,
Philadelphia, PA, USA, cheney@sas.upenn.edu
Margaret C. Crofoot Department of Anthropology, Harvard University,
Cambridge, MA, USA, crofoot@fas.harvard.edu
Robin I. M. Dunbar Institute of Cognitive & Evolutionary Anthropology,
University of Oxford, Oxford, England, robin.dunbar@anthro.ox.ac.uk
xvii
xviii
Contributors
Daniel M. T. Fessler Center for Behavior, Evolution, & Culture, Department
of Anthropology, University of Anthropology, Los Angeles, CA, USA
dfessler@anthro.ucla.edu
Claudia Fichtel Department of Behavioral Ecology & Sociobiology, German
Primate Center, Göttingen, Germany, claudia.fichtel@gwdg.de
Azar Gat Department of Political Sciences, Tel-Aviv University, Tel-Aviv, Israel,
Azargat@post.tau.ac.il
Matthew Gervais Department of Anthropology, Center for Behavior, Evolution, &
Culture, University of California Los Angeles, Los Angeles, CA, USA, mgervais@
ucla.edu
Peter M. Kappeler Department of Sociobiology/Anthropology, University of
Göttingen, Göttingen, Germany, pkappel@gwdg.de
Chris Knight Department of Anthropology, School of Social Sciences, Media,
and Cultural Studies, University of East London, London, England
Chris.Knight@uel.ac.uk
Venkat Lakshminarayanan Department of Psychology, Yale University, New
Haven, CT, USA, venkat.lakshminarayanan@yale.edu
Richard McElreath Department of Anthropology, University of Utah, Salt Lake
City, UT, USA, rmcelreath@gmail.com
Henrike Moll Max Planck Institute for Evolutionary Anthropology, Leipzig,
Germany, moll@eva.mpg.de
Ryne A. Palombit Department of Anthropology, Center for Human Evolutionary
Studies, Rutgers University, New Brunswick, NJ, USA, rpalombit@anthropology.
rutgers.edu
Virginia Periss Department of Psychology, Florida Atlantic University, Boca
Raton, FL, USA, vperiss@gmail.com
Aimée M. Plourde AHRC Centre for the Evolution of Cultural Diversity, Institute
of Archaeology, University College London, London, England, aimee.plourde@
ucl.ac.uk
Laurie R. Santos Department of Psychology, Yale University, New Haven, CT,
USA, laurie.santos@yale.edu
Part I
Introduction
Part II
Family & Social Organization
Chapter 1
Primate Behavior and Human Universals:
Exploring the Gap
Peter M. Kappeler, Joan S. Silk, Judith M. Burkart,
and Carel P. van Schaik
1.1
Introduction
What makes us human? This question has occupied people for millennia. A
conclusive answer continues to elude us as we learn more about ourselves and
other animals. A series of important discoveries over the last 50 years have led us to
largely abandon the search for single traits that are unique to humans. We now
know that tool use, language-like communication, lethal intergroup aggression, and
an ability to anticipate future events can also be found in other species. However,
humans are still quite different from other animals. So, the principal question has
become: “What is the nature and the width of the gap that separates humans from
primates and other animals?” This edited volume features a collection of essays by
primatologists, anthropologists, biologists, and psychologists, who offer some
partial answers to this question. In this introductory chapter, we briefly outline
the background of this fundamental question about human universals and explain
our emphasis on behavioral traits.
P.M. Kappeler (*)
Department of Sociobiology/Anthropology, University of Göttingen, Göttingen, Germany
e mail: pkappel@gwdg.de
J.S. Silk
Department of Anthropology, University of California, Los Angeles, CA, USA
e mail: jsilk@anthro.ucla.edu
J.M. Burkart and C.P. van Schaik
Anthropological Institute and Museum, University of Zürich, Zürich, Switzerland
e mail: judith.burkart@access.uzh.ch, vschaik@aim.unizh.ch
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 1, # Springer Verlag Berlin Heidelberg 2010
3
4
1.2
P.M. Kappeler et al.
The Gap is Behavioral
An obvious place to start an inquiry into the distinctiveness of Homo sapiens, at
least for anthropologists, is the formal description of our species. When Carolus
Linnaeus provided the first scientific description of humans in 1758, he deviated
from the rules he had developed for the definition of other species of plants and
animals in two ways. First, he did not designate a holotype. Instead, his own
remains were declared as a lectotype 200 years later (Stearn 1959). Second, and
more importantly, instead of presenting the usual differential diagnosis of anatomical traits, he provided only a prompt: “Nosce te ipsum!” (“know thyself”). It was,
therefore, left to later anatomists to identify and describe the few anatomical
synapomorphies that distinguish our species from the other hominids, such as
bipedalism, lack of an opposable big toe, an enlarged neocortex, and permanent
breasts (Lovejoy 1981). Most other anatomical and physiological traits that make
up the human body can be explained as homologies, reflecting our biological past as
chordates, vertebrates, tetrapods, amniota, mammals, and primates, respectively.
Similarly, the analysis of the hominid fossil record provides a rough outline
of the timing and sequence of anatomical changes leading up to the emergence
of H. sapiens about 160,000 years ago, but most of the details have to do with
bipedalism, changes in dentition and cranial volume, or they reflect changes in
degree (e.g., in skull shape or brain size) rather than fundamental innovations
(Henke and Tattersall 2007). Thus, comparative anatomists and paleoanthropologists can clearly identify a human being and distinguish it unequivocally from our
closest biological relatives in the present or past, but their list of criteria does not
answer the big question in a manner that would satisfy scholars of other disciplines.
The first complete sequencing of the chimpanzee genome (Mikkelsen et al.
2005) revealed that the genetic blueprints of humans and our closest living relatives
are 98.77% identical. This result raises two important questions. First, given larger
genetic differences between some other primate species, one can ask whether the
separation of humans and chimpanzees at the species and, especially, at the genus
level, is justified (cf. Diamond 1992). However, species concepts and definitions
continue to be in flux (de Queiroz 2005), so the question about the formal taxonomic status of H. sapiens is, perhaps, only an academic one.
Second, the comparison of the human and chimpanzee genome suggests that
everything that distinguishes us from chimpanzees must be encoded in the very
small amount of uniquely human DNA. This hypothesis is based on the assumption
that all morphological, physiological, and behavioral traits are controlled by the
genes that we can sequence. If we assume that the majority of the interesting and
fundamental human universals are directly or indirectly linked to our behavior (see
below), this explanation could only be correct if relatively small genetic differences
correspond to big behavioral differences. In voles (Microtus spp.), for example,
relatively minor genetic differences in a vasopressin receptor gene correspond to
major species differences in social organization and the mating system (Hammock
and Young 2005). However, a recent study of the same vasopressin receptor gene in
12 Old World primates with variable mating systems revealed no covariation
(Rosso et al. 2008).
1
Primate Behavior and Human Universals
5
Another potential example for minor genetic differences with major behavioral
consequences is provided by the FOXP2 gene, which is critically involved in the
control of the neural circuitry controlling speech and language (Vargha-Khadem
et al. 2005). Its present form in humans, which differs from that of other great apes
by only a few mutations, has been present for about 200,000 years, roughly
coinciding with the emergence of modern humans (Enard et al. 2002). Similarly,
Microcephalin, a gene involved in the regulation of brain growth, is more variable
in humans than in other primates, and it has been under positive selection since the
origin of the last common ancestor of humans and great apes (Wang and Su 2004).
Moreover, one genetic variant of this gene in humans has been under positive
selection in the past 40,000 years (Evans et al. 2005). Thus, some important
behavioral innovations of humans appear to have a genetic underpinning, but,
crucially, it remains largely unclear to what extent which aspects of human behavior are under direct genetic control.
Since humans do not differ qualitatively in their anatomy from great apes, except
for the adaptations related to bipedalism, and because the genetic differences
between these taxa to the extent that we understand them beyond the primary
sequences do not appear to be tremendous, the main difference must and does
exist in the realm of behavior and cognition. There is indeed little doubt that
H. sapiens is the most intelligent and socially complex animal. Human cultural
and technological achievements, powered by our large brains and capacities for
language, are astounding. Within a few thousand years, we have come to build
spacecraft that explore the solar system, work with nuclear power, manipulate the
genome of plants and animals, eradicate and heal diseases, and transmit information
instantaneously around the globe via computer technologies. It is widely accepted
that intelligence and rationality are the salient driving forces of human behavior,
which facilitate all those achievements. However, the very same rational and
intelligent individuals engage in futile contests over social status, discriminate
against members of other social groups, and rape, torture, or loot whenever social
control mechanisms fail. But humans also donate money to support common
welfare, help strangers, respond in predictable ways to particular stimuli of beauty
or emotion, and consistently exhibit sex differences in many aspects of social
behavior across cultures. Evolutionary processes, therefore, have also profoundly
shaped the patterns of human social organization and behavior. How exactly
evolutionary and cultural mechanisms interact in shaping human social behavior
is still to be discovered. What is clear, however, is that the gap is behavioral and
cognitive; what is less clear is how wide it is.
1.3
A Brief History of the Gap
The questions as to how and why humans differ from (other) animals have occupied
philosophers, theologians, psychologists, and anthropologists long before the genetic basis of adaptations was discovered. Their reflections have been summarized and
6
P.M. Kappeler et al.
discussed at length, so that only a few examples may suffice to illustrate preDarwinian attempts to address this question. Aristotle, for example, pioneered a
broad comparative approach by carefully comparing details of anatomy, reproduction, and behavior of the animals familiar to him. Humans were explicitly included,
as they represented after all the best-known species, and he observed that, in contrast
to animals, “humans are not exclusively occupied with the two basic purposes of life:
to maintain themselves and to perpetuate their kind.” This first indirect allusion to
human culture was to dominate this debate 2,400 years later.
Other eminent philosophers focused more specifically on the human psyche.
Immanuel Kant (1797/1798), for example, concluded that “the ability to set themselves any kind of purpose is what sets humans apart from animals.” This ability
exemplifies one aspect of human rationality, long thought to be our main distinguishing feature as a species. Ever since Plato, it had been held that the human mind
and matter are two ontologically separate categories, giving rise to a philosophy of
dualism (Descartes 1641) that is also in accordance with many religious beliefs,
whose influences have dominated this discussion for centuries. The scientific study
of the mind of animals began in earnest only in the late twentieth century (Griffin
1976), however, so that these assertions about (unique) qualities of the human mind
remained unchallenged for a long time.
Charles Darwin revolutionized human self-conception. He not only developed a
theory that firmly established man’s place in nature (1859), but he also made
influential speculations about unique human traits and their origin (1871). He
realized the importance of our intellectual abilities in explaining our success as a
species “. . .the intellect must have been all-important to [man], even at a very
remote period, enabling him to use language, to invent and make weapons, tools,
traps etc; by which means, in combination with his social habits, he long ago
became the most dominant of all living creatures.” He also noted that we have
“special social instincts,” which underlie our unusual cooperative tendencies:
“These instincts (of moral qualities), are of a highly complex nature, and in the
case of the lower animals give special tendencies towards certain definite actions;
but the more important elements for us are love, and the distinct emotion of
sympathy.” While arguing that these instincts have “in all probability been acquired
through natural selection,” he also maintained that some aspects of our social
behavior must have a different origin: “man resembles those forms, called by
naturalists protean or polymorphic, which have remained extremely variable,
owing, as it sees, to their variations being of an indifferent nature, and consequently
to their having escaped the action of natural selection.”
The notion that the capacity for culture has profoundly transformed evolutionary
dynamics within the human species gained momentum among social and natural
scientists alike. Sigmund Freud ((1927) 2005), for example, was even more specific
and considered human culture as the hallmark of humans: “Human culture and I
mean all that in which human life extolled over its animalistic conditions and
distinguishes itself from the life of animals.” The influential social anthropologist
Leslie White (1949) made Darwin’s second point explicit: “Culture may thus be
considered as a self-contained, self-determined process; one that can be explained
1 Primate Behavior and Human Universals
7
only in terms of itself.” But also eminent evolutionary biologists, such as Theodosius Dobzhansky, saw human culture as a unique phenomenon or process that is
beyond the influence of natural selection: “Man is a unique product of evolution in
that he, far more than any other creature, has escaped from the bondage of the
physical and biological into the multiform social environment” (Dobzhansky and
Montagu 1947) and “culture is an adaptive mechanism supplemental to, but not
incompatible with biological adaptation” (Dobzhansky 1961).
This question about the control of human behavior has played both a central and
crucial role in the debate about human uniqueness. The humanities and social
sciences in western society developed a Weltanschauung, in which humans were
completely isolated from the rest of nature and all psychological traits beyond our
sensory abilities and a small number of basic, general-purpose rules guiding
homeostatic behaviors were considered the product of socially constructed
learning, socialization processes, and conscious reasoning. In following the behaviorists’ paradigm, many social scientists maintained that humans are emancipated
from the genetic control of behavior, and the resulting ability to create and
perpetuate complex culture sets us apart from primates and all other animals
(Lewontin et al. 1984). This view assumed that “Man is not committed in detail
by his biological constitution to any particular variety of behavior,” and as a result,
“culturally conditioned responses make up the greater part of our huge equipment
of automatic behavior” (Benedict (1934) 2005). Thus, “evolved psychological
components place only the broadest constraints on what a human mind can
become” (Mameli 2007).
The aversion of the social sciences and humanities to evolutionary analyses of
human behavior had much to do with early attempts to biologize human behavior,
which emphasized the genetic determinism of eugenics and reified racial classifications and their presumed mental correlates. This work was eventually used to justify
restrictive immigration policies, unpalatable social policies, and even genocide.
This sordid history induced a collective denial of any biological influence on the
mind and behavior of humans, and was facilitated by the notion that to explain
human behavior we only needed to turn to culture, which hermetically sealed
human behavior against any biological influences. In Ridley’s (1996) words:
“today [cultural] anthropologists demand that the existence of culture, reason or
language exempt us from biology.” Thus, the isolation of the social sciences and
humanities from biology was understandable, if unfortunate.
1.4
Explaining the Gap
One might argue that the social sciences have been considerably less successful in
building a strongly predictive, integrative theoretical structure than their counterparts in the life sciences. The proper infusion of biological thinking might improve
their explanatory power. To a modern biologist, the so-called “standard social
sciences model” (Tooby and Cosmides 1992) almost reads like a creationist
8
P.M. Kappeler et al.
manifesto. It produced positions such as the Seville Statement on Violence (1986,
see de Waal 1993), which in retrospect are remarkable in their denial of any
biological influences on human behavior.
All this began to change after 1975, when E.O. Wilson published Sociobiology:
The New Synthesis. Wilson generated vehement controversy over his last chapter,
in which he boldly extended the evolutionary approach to the study of humans:
“In this macroscopic view, the humanities and social sciences shrink to specialized
branches of biology; history, biography and fiction are the research protocols of
human ethology; and anthropology and psychology together constitute the sociobiology of a single primate species.” In the beginning, sociobiology largely focused
on behavior, rather than its cognitive and emotional underpinnings (but see Trivers
1971), and also assumed that human behavior largely reflected genetically canalized adaptations rather than culture. Still, it rekindled the debate about biological
influences on human nature.
From among the various movements that applied evolutionary thinking to
humans (Laland and Brown 2002), one emerged as dominant during the 1990s.
Evolutionary Psychology (EP) focused on psychological mechanisms (modules)
that underlie behavior and decision-making (Tooby and Cosmides 1992), but
largely ignored the fitness consequences of actual behavior in contemporary settings. EP assumed that our cognitive abilities are massively modular. These
modules evolved during the Pleistocene, when hominins were hunter-gatherers
living in small groups in savanna-like habitats, called the “environment of evolutionary adaptedness.” Hence, a certain number of these psychological mechanisms
exist as adaptations to problems that we no longer face. They can be identified by
the so-called evolutionary functional analysis, which amounts to imagining the
problems faced by Pleistocene foragers and suggesting plausible solutions as
hypotheses for psychological mechanisms. This line of reasoning elegantly
explained the existence of patently maladaptive behaviors in modern humans.
EP generated important insights into sex differences in the criteria for mate
choice and clarified the notions of standards of beauty, attractiveness, sexual
jealousy (Buss 1994), the incidence of rape (Thornhill and Palmer 2000), patterns
of infanticide and aggression, including murder (Daly and Wilson 1988), aggression by male coalitions (Tooby and Cosmides 1988, in Buss 2008), and military
history (Diamond 1997; Turchin 2003). It also inspired the functional analysis of
disease, both physical and mental (Nesse and Williams 1995), art, music, and dance
(Miller 2000), and even literature (Carroll 2007). EP nicely explained why some
phobias are much more prevalent than others, in blatant disproportion to current
risks (e.g., we are more afraid of snakes than car crashes), and explained our
addiction to sweet, fatty, and salty foods. These fears and appetites made perfect
sense under hunter-gatherer conditions but are hopelessly maladaptive in our
current environment.
This approach has not been without its critics, however (Scher and Rauscher
2003; Buller 2005; Mameli 2007). Among the various points of criticism, two are
especially relevant here. First, echoing Wilson’s (1978) famous dictum that “genes
hold culture on a leash,” EP downplays the explanatory power of culture and the
1
Primate Behavior and Human Universals
9
force of the autonomous forces that culture creates. Variation in human behavior is
due to interactions between a universal set of adaptations shared by all people and
the external conditions a child encounters during development. This can produce
alternative psychological phenotypes within populations, but can also create variation across populations that may look deceptively like culture (i.e., behavioral
innovations that spread and are maintained by often conformity-based social
learning), but really is not EP calls this variation “evoked culture.” This shortcoming has been rectified by the successful development of cultural evolution
theory (Richerson and Boyd 2005; Henrich and McElreath 2007), which recognizes
that much variation in human behavior and the human mind can be due to historically determined processes of innovation and biased social transmission: “Culture
is on a leash, all right, but the dog on the end is big, smart, and independent. On any
given walk, it is hard to tell who is leading who” (Richerson and Boyd 2005).
Second, and most relevant to our argument, EP operates in a historical and
phylogenetic vacuum because it ignores the distinction between ancestral and
derived human features. EP implicitly assumes that all interesting human traits
are derived responses to Pleistocene conditions. Yet, human evolution did not start
in the Pleistocene and many of our traits may have been around much longer
(Fichtel and Kappeler, this volume; Whiten, this volume). Due to the explosive
growth of detailed, long-term primatological studies over the past 50 years, we have
now, for the first time in the history of our species, a detailed picture of our closest
living relatives.
1.5
Primatology and the Gap
The findings of behavioral primatologists have been spectacular. Monkeys and apes
form long-term social relationships that they use to exchange services and favors,
including grooming, sex, and coalitionary support in conflicts (Silk and Boyd,
this volume); reconcile when they have conflicts; commit infanticide and deploy
various social and sexual strategies to reduce that risk (Palombit, this volume);
demonstrate sophisticated and surprisingly detailed knowledge about the social
goings-on in their groups and, to some extent, in the population at large (Watts,
this volume; Dunbar, this volume); grow slowly and acquire numerous social and
subsistence skills, in part by social learning, and thus show signs of culture (Whiten,
this volume); and so on. Chimpanzees also engage in lethal intergroup aggression,
up to the point of eliminating males from neighboring communities (Crofoot and
Wrangham, this volume), they make and use tools, and field studies continue to
refine our knowledge of how cultural traditions are maintained in wild populations
(Matsuzawa 1994; Matsuzawa et al. 2001).
As a result, we now know more about primate behavior than about the behavior
of almost any other taxon except temperate diurnal birds. Along with developments
in molecular biology, which showed that humans are African great apes, who split
off from the rest of the hominoid lineages a mere 6 8 mya (Glazko and Nei 2003),
10
P.M. Kappeler et al.
everybody should now be fully aware of our behavioral and genetic similarity to
the great apes, and to primates more generally. Thus, we can no longer afford to
ignore our primate roots, and must explore the consequences of these hard facts.
Indeed, primatologists have gradually begun to use the broad insights into primate
behavior to weigh in on topics such as the evolution of human sexuality (Hrdy
1997), language (Savage-Rumbaugh 1999, Cheney and Seyfarth, this volume),
parenting (Hrdy 2009), between-group violence (Wrangham and Peterson 1996;
(Crofoot and Wrangham, this volume), technology and culture (McGrew 2004; van
Schaik 2004; Laland and Galef 2009), and morality (Ridley 1996; de Waal 2006).
The development of these ideas has been largely independent of work within the EP
movement.
Comparative primatology looks for evidence of both convergence (homoplasy)
and common descent (homology) in specific traits. At first, it may seem futile to
look for convergences in traits that seem to be unique, but each complex trait can be
found to have elements in common with traits in other, sometimes distantly related
lineages, which can shed light on their function in each lineage. Culture is a prime
example. Although human culture is clearly different than culture in other taxa, it
shares some elements with the cultural traditions of other primates, particularly
great apes (Whiten, this volume). This approach, thus, enriches our understanding
of human evolution. Second, by explicitly reconstructing the ancestral states of
human traits, primatology can distinguish between shared and derived human
features, something the history-free EP approach is unable to do. Ironically, the
examples we quoted above from EP tend to refer to behavioral tendencies we share
with many other organisms, whereas primatologists have generally focused on
explaining the most clearly derived ones, building on observations on nonhuman
primates.
Like any other species, H. sapiens is connected to its relatives by descent from
common ancestors. No species is fundamentally distinct from its close relatives,
certainly not if they shared a common ancestors as recently as humans and the two
chimpanzees. The similarities between humans and apes generated by the research
of primatologists are numerous, and they do not require any other explanation than
that they have been present for a long time, and apparently are not patently
maladaptive, allowing them to persist in both taxa. However, in the euphoria of
finding numerous fundamental similarities and in defiance of the remaining defenders of human uniqueness, many have proclaimed that humans are not fundamentally different from the other great apes. Fundamentalists can argue that the
differences are nonessential features that have been simply layered onto the primate
core. We are just a “third chimpanzee” (Diamond 1992), with an “inner ape” inside
of us (de Waal 2005). Hence, there are quantitative, not qualitative differences
between us and other primates.
Nothing could be further from the truth. Every species is not just connected to
others, it is also unique, or else it would not be a separate species. Perhaps the most
remarkable thing about humans from a comparative perspective is how different we
have become from our fellow great apes in the rather short time that separates us
from them. We will briefly summarize these differences below. The real challenge
1 Primate Behavior and Human Universals
11
is to explain these derived traits. Which of the myriad aspects of the mind and
behavior of humans are unique, and why did they evolve only in our own species?
1.6
Uniquely Human
We noted above that defining humans is a parlor game with roots going back to the
Greek philosophers, but during the past decades, this exercise has acquired a more
solid foundation based on comparative analyses. But the difficulties of adequately
characterizing humans as a species and distinguishing humans from other primate
taxa are often overlooked. The overwhelming majority of humans now live in
settled societies, surrounded by written texts or even more modern media technology,
in very large societies that have multilayered organizations and with numerous
institutions. However, all of this is very recent, with the oldest elements not even
12,000 years old. The few remaining hunter-gatherers have life styles that are most
similar to those in which humans lived for most of their history, including natural
levels of fertility (Hawkes 2006). In addition, we can assume that human universals
reflect our most ancient human roots. This relies on the argument that if humans
display a common trait across our vast range of social and environmental conditions, this common trait must also have been in the early modern humans that
evolved in Africa, and then populated the world. Especially, where the two sets of
traits overlap, we can have some confidence in their deep roots. Finally, important
insights into human nature have emanated recently from cross-cultural experimental studies of human economic choices (e.g., Fehr and Rockenbach 2004;
Henrich et al. 2005).
These developments have made it possible to update and organize the existing
lists of the derived traits of humans relative to the reconstructed traits of the last
common ancestor. Whole books have been devoted to the subject (e.g., Antweiler
2007), and this is not the place to develop a detailed list. But we want to do two
things here: first, to clarify the procedure and, second, to present a selected
summary of the major differences between ourselves and other primates.
A summary of derived features requires that we not only know the differences
but also their polarity. A difference between two sister taxa can be due to a change
in one, a change in the other, or a divergence in both. Polarity in mind and behavior
is usually fairly straightforward (the issue is much less obvious at the genomic
level), but to make sure, it is useful to compare humans with the genus Pan and with
the other great apes, as well. Fortunately, in spite of their remarkably variable social
organization and subsistence, the great apes, as a group, are rather homogeneous
with respect to cognition (Deaner et al. 2006; Burkart et al. in press), brain size
(Schoenemann 2006), and life history (Robson et al. 2006). This homogeneity
implies that they are rather conservative, making it easier to infer polarity.
A summary of the nonmorphological and nonphysiological features that are
derived in humans relative to the great apes and not discussed in subsequent
12
P.M. Kappeler et al.
chapters (see also Flinn et al. 2005; Richerson and Boyd 2005; Burkart et al. in
press) would include the following unusual features:
l
l
l
l
Cumulative material culture and social institutions and rituals, all critically
dependent on language. Culture is perhaps our preeminent adaptation.
Unusual subsistence ecology, involving skill-intensive hunting and gathering
and extremely intense cooperation, also in between-group conflict (Ridley 1996;
Kaplan et al. 2000; Gurven 2004).
Slower development, longer lifespan, accompanied by higher female reproductive rates and midlife menopause (Robson et al. 2006) and extensive allomaternal help (Hrdy 2009).
Unusual cognitive abilities, including language, long-term planning, causal
understanding, and episodic memory. These abilities build on shared intentionality, i.e., the ability to participate with others in collaborative activities with
shared goals and intentions (Tomasello and Moll, this volume), which also
involves language-based teaching.
So, there is a gap, and it would be foolish to deny it. A mere extrapolation of any of
the great ape phenomena is very unlikely to explain the dramatic differences, and it
would seem that what we are looking for is a set of selective pressures not
encountered by the other great apes or a confluence of capacities, ecological
circumstances, and a certain amount of serendipity that set our ancestors on a
different path than other great apes. This could be a completely novel set of
pressures encountered by no species before, such as pressures emanating from
cultural evolution (see Silk and Boyd, this volume; McElreath, this volume).
However, that still begs the question why cultural evolution became so much
stronger in humans than in the other apes. Thus, we should also look for selective
pressures that are novel for the apes but convergently present among other primates,
other mammals, or among birds, and which may have precipitated the operation of
the truly unique cultural selection. It seems unlikely that we will ever settle on a
single account of how we became such an unusual species. But we now have a
much richer body of theory and comparative data that allow us to develop more
complete and compelling hypotheses that we can critically evaluate.
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Chapter 2
The Deep Structure of Human Society: Primate
Origins and Evolution
Bernard Chapais
“. . .our primate cousins have ‘kinship systems’ which contain the elements of human
kinship systems, but . . . no other primate combines elements in the way that we do. . .The
elements are common: the combination is unique. My contention is, therefore, that it is to
the combination of elements that we must look for clues to the uniqueness of human systems,
not to the elements themselves.”
Robin Fox 1975: 10 11
Abstract On theoretical grounds, one expects all human societies to share a
common structural denominator, or deep social structure, which would describe
both the unity of human society across cultures and its uniqueness in the animal
world. Here, I argue that a powerful model of humankind’s deep social structure is
the concept of reciprocal exogamy described by Claude Lévi-Strauss a social
arrangement in which groups are bound together through the particular linkage of
pair-bonds and kinship bonds. The present analysis provides a phylogenetic test of
the exogamy model of human social origins. It shows that reciprocal exogamy
breaks down into a number of phylogenetically meaningful components and that
the evolutionary history of the whole system may be reconstructed parsimoniously
in terms of the combination of a Pan-like social structure with a new mating system
featuring stable breeding bonds. The concept of deep structure points to the
following human universals: stable breeding bonds and their correlate, fatherhood;
the multifamily community; strong siblingships; bilateral (uterine and agnatic) kin
recognition; incest avoidance; out-marriage (exogamy); matrimonial exchange;
dual-phase residence (pre/postmarital); lifetime bonds between dispersed kin;
bilateral relations between in-laws; kin-biased and affinity-biased marriage rules;
and between-group alliances (supragroup levels of social organization).
B. Chapais
Department of Anthropology, University of Montreal, Montreal, Canada
e mail: bernard.chapais@umontreal.ca
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 2, # Springer Verlag Berlin Heidelberg 2010
19
20
2.1
B. Chapais
Introduction
The idea that all chimpanzee societies share a number of structural features that set
them apart, collectively, from all other animal societies
that there exists a
chimpanzee deep social structure sounds not only reasonable but is also rather
obvious. But the same idea applied to human societies is much less evident. Human
societies are so variable cross-culturally that the notion that a common structural
denominator underlies all of them, past and present, is not easily conceptualized.
Yet, all human societies are the product of a unique set of mental constraints and, if
only for that reason they must share, at some level of abstraction, a universal deep
structure. What that structure is and how it evolved are questions that I addressed at
length in a previous book (Chapais 2008), of which the present chapter is a précis.
Owing to space constraints, other parts of the book concerned with methodological,
theoretical, epistemological, and historical considerations, and with the evolution of
descent groups, are largely ignored.
The deep structure of human society can only be abstracted from the comparative analysis of human societies. It is a matter of cross-cultural sociology. Significantly, however, the topic has never been recognized as such by the discipline best
positioned to tackle it, namely, sociocultural anthropology discussions of the
nature of human society’s deep structure are not found in anthropology textbooks,
for example. A number of reasons account for this. Several influential schools of
thought in sociocultural anthropology historical particularism, the Culture and
Personality school, cognitive anthropology, symbolic/interpretive anthropology,
postmodernist approaches, among others consistently emphasized the uniqueness
of every culture and focused, accordingly, on differences rather than similarities
between societies (for general discussions of relativism in anthropological theory,
see Harris 1968; Gellner 1985; Barnard 2000; Deliège 2006). These theoretical
perspectives stressed the importance of understanding cultures from the inside, kept
away from wide-scale cross-cultural comparisons, and consequently from the
search of the general principles governing cultural variation, and in some cases,
even denied that such principles existed. Other perspectives, such as structuralfunctionalism (Radcliffe-Brown 1957), Murdock’s “statistical comparatism”
(Murdock 1949; Goodenough 1970), cultural ecology (Steward 1955), or cultural
materialism (Harris 1979) did conceive of anthropology as a comparative science
whose main objective was to organize cultural variation and/or cultural change into
a set of general principles. But comparativists were harshly criticized for not
delivering the generalizations sought for and for producing, instead, principles
that were deemed simplistic and reductionist, or self-evident and meaningless. In
any case, the comparativists themselves did not address the issue of the deep
structure of human society.
For these reasons and many others, sociocultural anthropology came close to
skipping the topic altogether, and it did leave it out of its preoccupations as far as an
explicit treatment is concerned. Fortunately, however, anthropological research has
produced one particularly powerful model of humankind’s deep social structure.
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The deep structure of human society: Primate origins and evolution
21
In his book “Les Structures E´le´mentaires de la Parente´” (The Elementary Structures of Kinship), published in 1949, Claude Lévi-Strauss, the father of the French
structuralist school, implicitly proposed that reciprocal exogamy intermarriage
between members of distinct groups was the defining characteristic of human
societies. Lévi-Strauss never discussed reciprocal exogamy in terms of the deep
structure of human society and, moreover, he consistently opposed the idea that it
should be understood in terms of a chronologically primitive structure. Nonetheless, his description of it fits particularly well with the criteria and attributes one
would think of to characterize an entity such as the common denominator of all
human societies: namely, a structure (1) that defines the uniqueness of human
societies in relation to all other animal societies, (2) whose evolution coincided
with the birth of human society, (3) which embodies the unity of human societies,
cross-culturally, (4) which is described at such a high level of abstraction that it may
be construed as a correlate, in the social sphere, of the human mind, and (5) which
reflects the operation of some of the most elementary principles governing human
social relationships.
Lévi-Strauss’s description of reciprocal exogamy appears to meet all five criteria
formally, if not empirically. First, reciprocal exogamy was said to mark “the
transition from nature to culture,” or the passage from nonhuman to human society.
If one assumes that such a transition took place at some point in time, reciprocal
exogamy would coincide with the birth of human society and define its uniqueness.
Second, the single most important cognitive process involved in reciprocal exogamy
the ability to engage in relationships based on exchange
was described by
Lévi-Strauss as a “universal mental structure”: in other words, as an integral
property of the human mind. At the same time, the identification of women as the
“most precious possession” men could exchange was deemed so fundamental a
principle that Lévi-Strauss did not even justify this assumption. Taken together, the
last two points suggest that the core principle of reciprocal exogamy, matrimonial
exchange, would be the outcome of some underlying biological factors. Third,
reciprocal exogamy featured what Lévi-Strauss called the “atom of kinship” and
which he defined as the most elementary kinship unit and the basic building block of
human societies. As we shall see, the atom of kinship is a structure that amalgamates
some of the most basic types of human bonds. For all these reasons and from a
strictly sociological viewpoint that is, independently of any evolutionary considerations Lévi-Strauss’s concept of reciprocal exogamy is a strong candidate for the
deep structure of human society.
2.2
What is Reciprocal Exogamy?
What follows is a synthetic summary of reciprocal exogamy as far as it relates to my
objective of characterizing the deep structure of human society. This summary is
written from a primatological and evolutionarily informed perspective. Accordingly,
I stress aspects that Lévi-Strauss did not necessarily emphasize, and I use terms that
22
B. Chapais
he did not necessarily employ. In particular, I spell out the structural connections
between what I consider to be the core features of reciprocal exogamy from a
comparative interspecific perspective: intermarriage, supragroup kinship networks, alliances between in-laws, the atom of kinship, sister exchange, and marriage
between cross-cousins.
Simply stated, reciprocal exogamy is a social arrangement in which groups are
bound together through marital unions and kinship. Reciprocal exogamy is best
illustrated with the simplest system described by Lévi-Strauss, restricted exchange
between two exogamous kin groups, A and B (Fig. 2.1). Intermarriage between
groups A and B is exemplified here with a single family per group. The two groups
are patrilocal and the two families trade their kinswomen to obtain wives in return,
building alliances in the process. Upon marriage, wife Ego moves to her husband’s
group. Because Ego breeds and raises her children there, the A family will have
grandchildren, nieces, nephews, and cousins living in group B, with whom they will
come into contact when the two families, or the two groups, visit each other. Given
that this process is generalized to all marriages and works in both directions, the
resulting kinship network encompasses the two intermarrying groups which become quite intricately connected.
Simultaneously, Ego’s marriage reinforces bonds between the A and B families
because Ego and her husband act as natural intermediaries between their respective
families; that is to say, marriage connects and unites in-laws (or affines). Significantly, preferential bonds between in-laws often translate into marriages among
them. Two widespread practices are the levirate and the sororate (Murdock 1949:
29). The levirate is the rule according to which a widow must marry the brother of
Fig. 2.1 Reciprocal exogamy between two patrilocal kin groups, illustrated by marriage between
female Ego and male B1, and marriage between Ego’s brother and male B1’s sister, the two unions
exemplifying sister exchange (or daughter exchange depending on one’s viewpoint). Triangles:
males. Circles: females. Thin U shaped lines indicate marriage. Thick inverted U lines indicate
siblingships. Arrows give the direction of between group transfer (postmarital residence)
2
The deep structure of human society: Primate origins and evolution
23
her deceased husband (her brother-in-law). The sororate is the reciprocal rule, a
widower marrying the sister of his deceased wife (his sister-in-law). To
Lévi-Strauss, the sororate and the levirate were facets of reciprocal obligations
between exchanging units.
In sum, reciprocal exogamy binds social groups through two different processes.
First, out-marriage distributes close kin across distinct groups, these relatives
pursuing their relationship on the long run despite their physical separation, the
outcome being further kinship-based bonds between groups. Second, marriage
creates or reinforces bonds between more distantly related individuals, the spouses’
respective families, generating affinity-based alliances between the groups.
To proceed further with the description of reciprocal exogamy, it is useful to
consider how Lévi-Strauss himself accounted for it. His explanation holds in the
following assumptions and principles: (1) for some reason, men living in distinct
groups needed to ally with each other, (2) reciprocity is a universal mental structure,
(3) acts of reciprocity create social partnerships (Mauss 1923), (4) women are the
most precious commodities of exchange, (5) men exert some level of control over
their kinswomen, and (6) marriage is a means of exchange. From this, it follows that
men seeking to form solid alliances with other men would best do so by exchanging
their daughters and sisters as spouses for each other. But a major problem crops up
at this point. Lévi-Strauss also assumed (7) that men were sexually attracted to their
kinswomen that incest was natural. Therefore, to be in a position to exchange their
daughters and sisters, men first had to renounce marrying them, and to achieve this
they had to invent the incest prohibition. In that perspective, exogamy and alliance
formation are men’s ultimate goal, while the incest taboo is the fundamental
prerequisite; exogamy and the incest taboo are, thus, two sides of the same coin,
and they mark the birth of human society.
With this general picture in mind, we may now introduce what Lévi-Strauss
called the atom of kinship and which he described as “the most elementary form of
kinship that can exist” (1963, p 46) and “the sole building block of more complex
systems” (1963, p 48). One might argue that the smallest unit of human kinship is
the mother child bond, but Lévi-Strauss was concerned with social structure, not
with dyadic relationships. The atom of kinship rests upon four terms: a brother, his
sister, the sister’s husband, and their son (Fig. 2.2). A theoretical argument invoked
by Lévi-Strauss is that the atom of kinship includes the three types of relations
always present in any human kinship structure: a relation of consanguinity (between siblings), a relation of affinity (between spouses), and a relation of descent
(between parent and child). A more direct argument, still according to Lévi-Strauss,
is that the atom of kinship is the immediate outcome of the incest impediment
between brothers and sisters. Owing to the incest taboo, a brother cannot have
children with his sister. He, thus, elects to lend her to another male for breeding
purposes; so, the sister’s children are “the product, indirectly, of the brother’s
renunciation” as Fox put it (1993: 192). More bluntly, because the brothers cannot
reproduce with their sisters, they do so via their sisters’ husbands, hence the
intimate interconnection between the three basic categories of bonds. It should
also be noted that the atom of kinship embodies another chestnut of human kinship
24
B. Chapais
Fig. 2.2 Lévi Strauss’s
“atom of kinship” (circled
individuals). The individuals
pictured here are the same as
in Fig. 2.1. Definitions of
symbols as in Fig. 2.1
Fig. 2.3 Structural relations
between sister exchange (or
bilateral marriage between
affines) and marriage between
cross cousins. Cross cousin
marriage is the extension of
sister exchange to their
offspring. Definitions of
symbols as in Fig. 2.1. See
text for explanations
studies, the widespread phenomenon of avuncular relationships, those special
bonds between maternal uncles and their sororal nephews. It is precisely in relation
to that problem that Lévi-Strauss (1963) discussed the atom of kinship.
From here, we are in a position to understand still another central aspect of
reciprocal exogamy, namely, cross-cousin marriage, which Lévi-Strauss described
as the “elementary formula for marriage by exchange” (1969, p 129) and “the
simplest conceivable form of reciprocity” (1969, p 48). Cousins are the offspring of
siblings and belong to one of two categories: cross-cousins, who are the offspring of
opposite-sex siblings, and parallel cousins, the offspring of same-sex siblings.
Marriage between cross-cousins is a widespread practice and the object of a prescription in a large number of societies. Let us return to males A1 and B1, in Fig. 2.1,
who are married to each other’s sisters and thereby, brothers-in-law. If the two males
extend the exchange principle to their own children, that is, if they exchange their
respective daughters as wives for their own sons, this produces marriage between
cross-cousins. This is illustrated in Fig. 2.3, extended from Fig. 2.2. If male A1’s
daughter (“a”) marries male B1’s son (“b”), this produces a marriage between crosscousins, female a’s father and male b’s mother being siblings. In a situation of sister
2
The deep structure of human society: Primate origins and evolution
25
exchange between two groups, it is also the case that female a’s mother and male b’s
father are siblings, so the two individuals are double, or bilateral, cross-cousins.
It may readily be seen that the atom of kinship, with its emphasis on the brother
sister bond, and cross-cousin marriage, which unites the offspring of a brother and a
sister, are intimately related structurally. Cross-cousin marriage is one particular
manifestation of the brother sister bond, in which the brother controls the marriage
of his sister’s children. It is brother A saying to his sister Ego: “Your son will marry
my daughter.” The brother sister bond, thus, lies in the very heart of reciprocal
exogamy and between-group alliances in Lévi-Strauss’s scheme. This is particularly
intriguing considering that in nonhuman primates, brothers and sisters are most
often separated by natal group dispersal so that brothers are not in a position to exert
any influence on their sisters.
In sum, the proposition that reciprocal exogamy embodies the deep structure of
human society implies that the distinctiveness of human social organization, compared with all other animal societies, holds in the conjunction and particular linkage
of kinship bonds and marriage bonds, a linkage that produces between-group
alliances and supragroup levels of social organization; put differently, the essence
of human society lies in its “federate” nature. Interestingly, another model concerned
with the simplest and earliest human kinship system the so-called tetradic theory
(Allen 2008) has much in common with reciprocal exogamy. Its central feature is a
rule of reciprocal recruitment of spouses between two kin groups (reciprocal exogamy), with pair-bonds uniting affines in the two kin groups. All human kinship
systems are supposedly derived from that stem structure. Although Lévi-Strauss’s
concept of reciprocal exogamy and Allen’s tetradic theory differ in a number of
respects, they belong to the same family of models and both are compatible with the
present evolutionary analysis.
2.3
Phylogenetic Evidence as a Test of the Exogamy Model
Lévi-Strauss described reciprocal exogamy as if it was a cultural creation. Moreover, in accordance with the synchronic perspective of structuralism, he consistently abstained from discussing elementary kinship structures within the evolutionary
paradigm. From such an unchronological perspective, reciprocal exogamy appears
to be an irreducible entity: a system whose elements owe their origin and existence
to the system itself and hence have no evolutionary history of their own. Ever since,
Lévi-Strauss has consistently dismissed the relevance of primate studies for understanding the origins of human society (Lévi-Strauss 1985, 2000). But if reciprocal
exogamy is, in effect, the deep structure of human society, it might well have an
evolutionary past, and its components their own evolutionary histories. This very
assertion is not self-evident. Any structure described as the earliest human social
system, be it Allen’s tetradic model or Lévi-Strauss’s reciprocal exogamy, is, by
definition, a normative (rule-governed), symbolically mediated structure. It is so
because all aspects of human behavior, from greetings to legal systems, are
26
B. Chapais
symbolically mediated and have culturally defined meanings. From this, it follows
that the very search for the evolutionary precursor of reciprocal exogamy rests on
the assumption that it existed under some presymbolic and more or less embryonic
form prior to the evolution of the symbolic capacity, and that this form had a
biological basis and phylogenetic roots. The following analysis may, thus, be
construed as a test of the hypothesis that a primitive version of reciprocal exogamy
thrived as a set of behavioral regularities well before the evolution of the symbolic
capacity generated several more sophisticated and institutionalized versions of it.
As pointed out earlier, Lévi-Strauss’s characterization of reciprocal exogamy
meets the formal criteria of a deep social structure from a sociological viewpoint.
But if that argument is a necessary condition for validating the present claim, it is
not a sufficient one. In theory, other candidate structures might satisfy the same
conditions, and here lies the relevance of the evolutionary perspective. If reciprocal
exogamy is, in effect, the deep structure of human society, it should also accord
with the criteria set by the phylogenetic analysis of the phenomenon. I identify three
such criteria. First, the candidate structure ought to have been described, or to be
actually describable, in terms of the same basic categories used to describe all other
primate societies i.e., group composition, mating system, dispersal patterns,
kinship structure, and so on. In other words, the candidate structure should fit
within the general framework of primate comparative sociology. Second, the
candidate structure ought to break down into phylogenetically sound components.
That is to say, its evolutionary roots should be manifest in a number of building
blocks observable in other primates; and the building blocks that are uniquely
human should also make sense from that perspective (discussion below). Third,
the evolutionary history of the candidate structure ought to be parsimoniously
reconstructible in view of our knowledge about primate behavior and human
evolution. In particular, it should accord with the characteristics of the last common
ancestor that we shared with other primates.
To appraise the significance of these criteria, consider the candidate structure of
early human society proposed by the cultural evolutionist Lewis Morgan (1877
1974), who hypothesized that early human society featured group marriage among
all members of ego’s generation, what he called the consanguine family; or the
scenario of his contemporary John McLennan (1865) 1970), who proposed a
structure featuring wife capture, female infanticide, and generalized polyandry.
Unsurprisingly, none of these structures meet even one of the above three criteria.
As I shall argue, Lévi-Strauss’s concept of reciprocal exogamy does satisfy the
three of them. Put differently, the phylogenetic test of the exogamy model of human
society’s deep structure lends support to it.
Very few authors have looked into the evolutionary origins of human society as a
whole. Sociocultural anthropologists White (1959) and Service (1962) briefly
explored the topic (Chapais 2008), but the comparative analysis of human kinship
and primate kinship was truly pioneered by Fox (1975, 1979, 1980, 1993), also a
sociocultural anthropologist. Fox identified exogamy as the cornerstone of human
kinship systems and argued that its two basic elements kin groups and stable
breeding bonds (or kinship and marriage) were present in nonhuman primates but
2
The deep structure of human society: Primate origins and evolution
27
never in the same species as is the case in humans, who form multifamily kin
groups. This led him to conclude that the originality of the human system lay not so
much in the two components themselves as in their merging in the same species,
and that if humans had not invented kinship and pair-bonds, they had invented other
elements such as affinal kinship (in-laws), out-marriage, and female exchange (Fox
1975, 1980). Some 10 years later, Rodseth and his colleagues picked up the thread,
noting that a few primate species such as the hamadryas baboon do combine kin
groups and stable breeding bonds, and that such species practice “exogamy” in that
they form stable breeding bonds after transferring into another group. Such bonds,
however, do not translate into alliances between groups because members of the
dispersing sex lose contact with their natal kin, so in-laws cannot recognize each
other. Rodseth et al. (1991) concluded that the distinctiveness of human society lies
in the bilateral recognition of in-laws and the exchange dimension of exogamy.
My own comparative analysis builds upon this earlier work, but takes into
account all major aspects of reciprocal exogamy described in the previous section.
This leads me to break the phenomenon down into the following phylogenetic
building blocks: a multimale multifemale group composition; stable breeding
bonds and its correlate, fatherhood; strong siblingships; kin recognition along
both the maternal line (uterine kinship) and the paternal line (agnatic kinship);
incest avoidance; out-marriage (exogamy) and its correlate, dual-phase residence
(pre/postmarital); lifetime bonds between dispersed kin; bilateral relations
between in-laws; kin-biased and affinity-biased marriage rules; female exchange;
and between-group alliances (supragroup levels of social organization). I call this
set of features the exogamy configuration, and in the remainder of this chapter, I go
on to demonstrate that a phylogenetic analysis of that configuration supports the
view that it describes the deep structure of human society.
2.4
Origins of the Multifamily Community
Where should one start when attempting to reconstruct the evolutionary history of
the exogamy configuration? Interestingly, the answer to this question is contained
in the timing of the evolution of its most basic feature, the modal composition of
human groups. That composition is the multifamily community (Rodseth et al. 1991)
and its evolutionary origin appears to date back to the Pan Homo split, some 6 7
million years ago (Fig. 2.4). The multifamily community is a rare form of group that
combines two independent elements: a multimale multifemale composition and a
mating system featuring stable breeding bonds (monogamous or polygynous). On
logical grounds, a multifamily system may evolve through two different paths.
According to the first possibility, the multimale multifemale composition came
first, followed by the evolution of stable breeding bonds, as illustrated in Fig. 2.5.
Humans and their two closest relatives, chimpanzees and bonobos, form multimale
multifemale groups, which suggests that this trait is homologous in the three species
and shared with their last common ancestor. Accordingly, early hominids formed
28
B. Chapais
Fig. 2.4 Phylogenetic
relationships of humans and
apes as assessed by several
sets of molecular data
(Goodman et al. 1998, 2005;
Enard and Pääbo 2004). Ou
orangutans, Go gorillas, Bo
bonobos, Ch chimpanzees,
Hu humans. The Pan genus
includes chimpanzees and
bonobos
Fig. 2.5 Two evolutionary paths leading to the modal composition of human groups (the multi
family community)
multimale multifemale groups and had a chimpanzee/bonobo-like promiscuous
mating system. It follows that stable breeding bonds and biparental families evolved
at some point after the Pan Homo split, producing the multifamily composition.
The second logical possibility is the reverse sequence: families (monogamous or
polygynous) appeared first and the multifamily group evolved later through the
merging of autonomous families. I call this sequence the “gorilla hypothesis”
because it fits with the observation that our third closest relative forms autonomous
polygynous social groups. According to one version of this hypothesis, the last
common ancestor of the Pan and Homo lineages had a gorilla-like social structure,
which evolved into the multimale-multifemale composition along the Pan line and
into the multifamily group along the hominid line (Imanishi 1965, Sillén-Tullberg
and Møller 1993). According to another, somewhat less parsimonious version,
gorilla-like groups had already evolved into the multifamily group prior to the
2
The deep structure of human society: Primate origins and evolution
29
Pan Homo split and stable breeding bonds were lost along the Pan line (Geary
2005).
As argued at length elsewhere (Chapais 2008), the chimpanzee/bonobo hypothesis is more likely based on a number of arguments. Briefly, it fits better with what
we know about the evolutionary history of multifamily groups in other primate
species (hamadryas baboons, gelada baboons, drills, and mandrills). A cladistic
analysis carried out by Barton (1999) suggests that the multifamily composition did
not evolve through the amalgamation of autonomous units, but from an ancestral
multimale multifemale group and the subsequent conversion of the mating system
from sexual promiscuity to stable breeding bonds. Second, compared with the
gorilla hypothesis, the chimpanzee/bonobo hypothesis requires the smallest number
of evolutionary changes to produce the multifamily structure. Third, the gorilla
hypothesis implies that independent polygynous units coalesce, presumably
through the evolution of reduced levels of competition or higher levels of cooperation between males. But such changes would rather favor the extension of male
philopatry in gorilla groups all males remaining in their natal group and their
transformation into multimale multifemale groups, not into multifamily groups.
Fourth, the chimpanzee hypothesis fits better with the observation that early
hominids were anatomically more similar to chimpanzees than to gorillas, and
therefore that their behavioral ecology and social structure more closely resembled
those of chimpanzees.
2.5
Kinship in Early Hominid Society
The chimpanzee/bonobo hypothesis is an integral part of the larger issue of the suite
of traits characterizing the last common ancestor of chimpanzees, bonobos, and
humans. The traits common to all three species have been described by Wrangham
(1987), Ghiglieri (1987), and Foley (1989), and provide a cladistic model (Moore
1996) of their common ancestor. Besides a multimale multifemale composition,
they include (1) territoriality, (2) male philopatry male localization coupled with
female transfer and (3) male kin groups. Territoriality characterizes both chimpanzees and bonobos (Newton-Fisher 1999; Wrangham 1999; Boesch and BoeschAchermann 2000; Watts and Mitani 2001; Fruth and Hohmann 2002; Wilson and
Wrangham 2003; Williams et al. 2004; Watts et al. 2006). Territoriality comes along
with hostility between males belonging to distinct groups and with the absence of
any supragroup social entity. There are no between-group alliances in chimpanzees
and bonobos; the local group is the highest level of social organization.
Male philopatry is the rule in chimpanzees and bonobos all males stay put
while a majority of female emigrate (Goodall 1986; Furuichi 1989; Nishida 1990;
Kano 1992; Boesch and Boesch-Achermann 2000; Doran et al. 2002; Nishida et al.
2003) with gorillas displaying a partial pattern of male philopatry (Bradley et al.
2004, discussion in Chapais 2008: 142). In humans, the structural equivalent of
male philopatry is the majority residence pattern in humans, with 70% of the 1,153
30
B. Chapais
Fig. 2.6 Composition of a
single patriline (a) and a
single matriline (b) in a male
philopatric primate group.
Circles: females. Triangles:
males. Black symbols:
members of the same
matriline or patriline. Empty
symbols: nonmembers.
Braces illustrate promis
cuous mating
a
b
societies in Murdock’s (1967) Ethnographical Atlas classified as patrilocal or
virilocal. But contrary to previous assertions (Steward 1955; Service 1962;
Ember 1978), recent studies indicate that bilocality rather than patrilocality is the
majority residence pattern among hunter-gatherers (Alvarez 2004; Marlowe 2004).
Notwithstanding this, the prevalence of residential diversity (bilocality, patrilocality, matrilocality) during more recent phases of human evolution is not incompatible with the view that male philopatry and human patrilocality are homologous. As
discussed at length elsewhere (Chapais 2008: 142 151, 238 243), ancestral male
philopatry in the hominid line remains the most parsimonious hypothesis.
An important correlate of male philopatry in chimpanzee and bonobo groups is
their genealogical structure. Male philopatry produces a strong asymmetry in the
composition of patrilines and matrilines. Male localization generates kin groups
comprised of extensive, multigenerational patrilines (Fig. 2.6a), in which a male
lives with his sons, brothers, father, uncles, grandfather, and other agnatic kin.
Concurrently, female transfer produces small, two-generation matrilines
(Fig. 2.6b). In a group in which all females emigrate, a male lives with his mother
and his maternal siblings, but not with his mother’s kin (e.g., his maternal grandfather and uncles) who live in the mother’s natal group. Nor does he know his
daughters’ and sisters’ offspring because females emigrate at puberty and lose
contact with their natal kin. The next question, then, is: of all the kin types a
male lives with, which ones does he recognize as such? To answer this question, it
is necessary to consider the domain and basic processes of kin recognition in
nonhuman primates in general.
2.5.1
Kin Recognition in NonHuman Primates
Kin recognition is inferred from the preferential treatment of known kin (nepotism).
Studies in which nepotism was analyzed according to kin types in groups in which
females are the resident sex female philopatric groups indicate that females
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The deep structure of human society: Primate origins and evolution
31
recognize their mother, daughters, sons, brothers, sisters, grandmother, grandoffspring, great-grandmothers, and great-grandoffspring, and, less consistently, their
aunts and nieces/nephews. Cousins and more distant relatives do not appear to be
part of the domain of kin recognition (Kapsalis and Berman 1996; Chapais et al.
1997, 2001; Bélisle and Chapais 2001; Chapais and Bélisle 2004; Silk et al. 2006).
The anthropologist, George Murdock, provided a useful classification of kin types.
Ego’s primary kin are its mother, father, brothers, sisters, sons, and daughters.
Ego’s secondary kin are the primary kin of each of its primary kin; they are Ego’s
grandparents, grandchildren, aunts, uncles, nieces, and nephews. Similarly, Ego’s
tertiary relatives are the primary kin of one’s secondary kin; that is, Ego’s first
cousins, great-aunts, great-grandparents, among many others (Murdock 1949: 94).
Viewed in terms of Murdock’s categories, nonhuman primates recognize their
primary uterine relatives (mother, sons, daughters, brothers, and sisters) and some
of their secondary uterine relatives, namely their grandmother and grandoffspring.
Other secondary kin such as aunts and nieces are part of the gray zone of kin
recognition. Nonhuman primates also recognize some of their tertiary uterine kin
(great-grandrelations), but apparently not others such as cousins.
The cornerstone of uterine kinship is the intimate and enduring bond between
mothers and offspring. It is that bond, in all likelihood, that mediates kin recognition between other categories of uterine kin; for example, between sisters. Two
different processes are probably at work here. In the first, the mother is merely a
passive mediator of familiarity between her daughters. A female would recognize
her sister as that particular individual she meets near her mother on a daily basis
because both sisters are independently attracted to the same mother and with
whom she becomes disproportionately familiar over the years. On this basis alone,
two sisters are in a privileged position to develop a preferential bond of their own.
Similarly, through proximity to her mother, a female is bound to become disproportionately familiar with her maternal grandmother and develop preferential bonds
with her (see also Berman and Kapsalis 1999; Berman 2004; Rendall 2004).
This first recognition process focuses on the fact that through proximity to her
mother, a female acquires information about her sisters. But in all likelihood, the
female is simultaneously acquiring information about her sisters’ relationships with
her mother. The ability to learn about the relationships of others is well documented
in nonhuman primates (Cheney and Seyfarth 1980, 1986, 1989, 1999; see also
Cheney and Seyfarth 1990, 2004) and provides a distinct cognitive basis for kin
recognition. From a female’s viewpoint, a sister is not only that particular individual she is disproportionately familiar with, but she may well be, in addition, that
close associate of her mother, the one that she protects against certain individuals,
grooms at certain rates, tolerates near food sources, and so on. Kin recognition,
here, involves Ego classifying others by using her mother as a reference point. In
sum, its likely that primates learn the identity of their uterine kin by acquiring
information both about their own relationships with their relatives and their
mothers’ relationships with these same kin. Crucially, the two processes depend
on the lasting character of the mother offspring bond. For a newborn sister to be
able to recognize her 5 year old sister, the older sister’s bond with her mother must
32
B. Chapais
last significantly longer than 5 years. Similarly, for a granddaughter to recognize
her maternal grandmother, mother daughter bonds must last significantly longer
than the generation length.
2.5.2
Kin Recognition in Early Hominids
With this background information, we may come back to the issue of kin recognition in a chimpanzee/bonobo society and, by way of inference, in early hominid
society. In chimpanzees, mother offspring recognition is well documented, and so
is the recognition of maternal siblings (Goodall 1986; Pusey 1990, 2001; Lehmann
et al. 2006; Langergraber et al. 2007). But given that males do not normally grow up
with their mother’s kin, the domain of uterine kin recognition is, in general, limited
to these two categories. Interestingly, however, mothers sometimes breed in the
group in which they were born, a situation which provides individuals with an
opportunity to recognize their mother’s kin. For example, in the Gombe colony of
wild chimpanzees, 50% of the females stayed and bred in their natal group
whereas in other communities, most females emigrate. In this context Goodall
(1990) provided anecdotal accounts of preferential bonds between grandsons and
their maternal grandmothers and between maternal uncles and their sororal
nephews. Clearly, then, the kin recognition potential of chimpanzees encompasses
not only primary maternal kin (mothers, daughters, sons, and maternal siblings),
but secondary maternal kin as well, which is not surprising given the cognitive
sophistication of chimpanzees.
If mother offspring recognition is well established in chimpanzees, father
offspring recognition is not. Paternity recognition based on disproportionate levels
of familiarity between fathers and offspring is unlikely because females mate with a
large number of males (Wrangham 1993, 2002) and do not maintain long-term
exclusive relationships with particular ones, including the males with whom they
had an offspring (Lehmann et al. 2006). Accordingly, Lehmann et al. (2006)
reported remarkably limited and weak effects of paternity on social interactions
between adult males and youngsters. Similarly, paternal siblings (half-siblings
related through the father) do not appear to recognize each other in chimpanzees
(Lehmann et al. 2006; Langergraber et al. 2007). Given that males do not maintain
long-term bonds with their fathers, they can hardly recognize their fathers’ kin on
this basis
e.g., their paternal grandfather and paternal uncles
as females
recognize their mother’s kin in female philopatric groups. Thus, even positing
some degree of paternity recognition, as described by Lehmann et al. (2006) in
chimpanzees, or by Buchan et al. (2003) in baboons, such levels of bonding
between fathers and sons are unlikely to reveal the agnatic kinship structure in
male kin groups as maternity recognition reveals the uterine kinship structure in
female kin groups.
Overall, then, the domain of kin recognition in our two closest relatives is
normally quite limited (Fig. 2.7). Assuming that early hominids mated promiscuously
2
The deep structure of human society: Primate origins and evolution
33
Fig. 2.7 Domain of kin
recognition from ego’s
viewpoint in a male philo
patric, chimpanzee like
group, assuming that all
females breed outside their
natal group. Five generations
are pictured but normally
only three coexist. Black
symbols: kin recognized by
Ego, assuming that Ego
recognizes his uncles, aunts,
nieces, and nephews. Empty
symbols: kin not recognized
by Ego
and were male philopatric, male relationships in these groups were minimally differentiated on the basis of kinship. This may appear somewhat intriguing considering
that kinship is a central organizing factor in small-scale human societies, which,
therefore, would have evolved from a comparatively “kinshipless” type of society. As
we shall see, the key to this apparent paradox is the evolution of stable breeding
bonds. Table 2.1 summarizes the state of the exogamy configuration prior to the
evolution of stable breeding bonds (Phase I).
2.6
The Evolution of Stable Breeding Bonds
The transition from sexual promiscuity to enduring breeding bonds in the course of
hominid evolution is the single-most important event that launched the exogamy
configuration on its evolutionary path. How did that happen? Answers to this
question have traditionally focused on the adaptive aspects of pair-bonding, but
they must also take into account the relevant phylogenetic constraints set by the
ancestral mating system of hominids. Up to 80% of human societies combine
monogamy with polygyny, with the majority of families being monogamous in
any given society. Logically, then, hominids went from chimpanzee/bonobo-like
sexual promiscuity to the predominantly monogamous multifamily structure. The
primate data suggest that this evolution involved two transitions: (1) from sexual
promiscuity to generalized polygyny (as in the multiharem structure of hamadryas
baboons), and (2) from generalized polygyny to generalized monogamy. A direct
passage from sexual promiscuity to generalized monogamy is unlikely for a number
of reasons. First, polygyny is the norm in mammals in general. Accordingly, some
primate species exhibit the multiharem structure, but none display the multimonogamous pair structure. (Monogamy exists in nonhuman primates but monogamous
pairs do not form cohesive groups). Second, the transition from sexual promiscuity
34
B. Chapais
Table 2.1 The cumulative construction of the exogamy configuration in the course of human
evolution. Phase I extended from the Pan Homo split to the evolution of stable breeding bonds.
Phase II began after the evolution of stable breeding bonds and ended with the evolution of the
tribe, which marked the onset of phase III
Phase I
Phase II
Phase III
Multimale multifemale groups
Yes
Yes
Yes
Uterine kinship
Yes
Yes
Yes
Incest avoidance
Yes
Yes
Yes
Outbreeding (dual phase residence)
Yes
Yes
Yes
Stable breeding bonds
Yes
Yes
Paternity recognition (fatherhood)
Yes
Yes
Strong siblingships
Yes
Yes
Multifamily communities
Yes
Yes
Agnatic kinship
Yes
Yes
Out marriage (exogamy)
Yes
Yes
Pre/postmarital residence
Yes
Yes
Lifetime bonds between dispersed kin
Yes
Bilateral affinity
Yes
Atom of between group alliances
Yes
Primitive tribe
Yes
Residential diversity
Yes
Marriage between siblings in law
Bias
Bilateral marriage between sibling pairs
Bias
Marriage between cross cousins
Bias
Exchange dimension of exogamy
to generalized polygyny finds cladistic support. The few primate species with a
multiharem structure belong to taxonomic groups in which the majority of species
have a multimale multifemale composition and a promiscuous mating system. This
suggests that the clade’s common ancestor had the latter type of mating system
(Barton 1999). Third, primate behavioral ecology is compatible with the transition
from sexual promiscuity to generalized polygyny, but hardly so with a direct
transition to generalized monogamy. The multiharem composition appears to be
an adaptation to a low food density that cannot support large aggregations, an
hypothesis that finds support in the observation that savanna baboons, which
typically form large multimale multifemale groups, may sometimes subdivide
into polygynous units in harsher ecological conditions (Barton 1999). Fourth, as
pointed out earlier, a majority of human societies combine monogamy with polygyny, a fact that strongly suggests that the ancestral hominid pattern was
generalized polygyny. This leads us to the second step, the transition from
generalized polygyny to generalized monogamy.
2.6.1
Monogamy as Maximally Constrained Polygyny
The classic view about the origin and function of human pair-bonding is the
parental collaboration hypothesis, which conceives of pair-bonds as parental
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The deep structure of human society: Primate origins and evolution
35
partnerships based on a sexual division of labor (Washburn and Lancaster 1968;
Isaac 1978; Alexander and Noonan 1979; Lovejoy 1981; Hill 1982; Fisher 1992,
2006; Kaplan et al. 2000). A competing view holds that pair-bonds in present-day
hunter-gatherers do not meet the criteria of cooperative partnerships, from where it
is inferred that pair-bonding evolved instead as part of a male’s mate guarding
strategy (Hawkes 1991, 1993, 2004; Hawkes et al. 2001). The two positions appear
mutually exclusive and irreconcilable, but a primatological perspective shows that
they are not. It suggests that human pair-bonds are parental partnerships, but
partnerships that initially evolved as mating strategies.
As discussed at length elsewhere (Chapais 2008, pp 162 168), two major
correlates of the parental collaboration hypothesis are empirically supported: the
costs of raising human children are disproportionately high owing to our larger
brain and its correlate, delayed maturation (Kaplan et al. 2000), and the father’s
economic contribution does alleviate the costs of maternity (Gurven 2004). But one
must not confuse the actual working of the human family with its origins. Studies of
the mating and parental care systems of mammals in general suggest that pairbonding did not initially evolve as parental partnerships. Stable breeding bonds in
mammals are primarily mating arrangements. In a phylogenetic analysis of mammalian mating and parental care systems, Brotherton and Komers (2003) found that
in most monogamously breeding species that exhibit parental collaboration, paternal care had evolved after monogamy was already established, and this for reasons
other than parental collaboration. This would explain why direct paternal care is
absent in several monogamously mating primate species (van Schaik and Kappeler
2003). According to this view, it is monogamy that sets the stage for the evolution
of paternal care, rather than parental collaboration driving the evolution of monogamy (Dunbar 1995; Ross and MacLarnon 2000; van Schaik and Kappeler 2003).
Applied to the human case, this reasoning suggests that pair-bonding originated as a
mating arrangement, which later operated as a preadaptation for parental collaboration when delayed maturation evolved and the costs of maternity began to
increase. In other words, the father was already present in the family when paternal
investment became advantageous and was, presumably, selected for. When viewed
sequentially, then, the mating arrangement hypothesis and the parental collaboration hypothesis are basically compatible.
If pair-bonding was not part of a paternal care strategy initially, why did it
evolve, and why did hominids forego polygyny for preponderant monogamy? A
relatively simple explanation is that monogamy “replaced” polygyny when the
costs of polygyny became too high. Consider the following thought experiment.
In chimpanzees, male dominance relations are well defined and higher-ranking
males have a higher reproductive success (Constable et al. 2001). Now suppose that
all males were given similar fighting abilities. In such a situation, conflicts would be
extremely costly and males would be better off switching to scramble competition
and attempting to copulate with as many females as possible. This would translate
into high levels of sexual promiscuity and sperm competition, and a low variance in
male reproductive success. At this point, let us carry out the same reasoning, but in a
different initial setting: the multiharem structure of hamadryas baboons. If all males
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B. Chapais
were given the same fighting abilities, the monopolization of females by a male
would be extremely costly. Any male attempting to defend more than one female
would be challenged by equally powerful males. Hamadryas males being harem
builders, they would keep doing this, but they would end up forming monogamous
bonds. The outcome would be an egalitarian distribution of females among males
generalized monogamy because it is the arrangement that minimizes conflicts,
hence the costs of aggression.
This thought experiment provides an hypothesis for the transition from
generalized polygyny to generalized monogamy in the hominid lineage. The pivotal
factor is the development of technology, in particular the discovery that tools could
be used as weapons. Any tool can be used as a weapon, provided it can inflict
injuries. Armed with a deadly weapon, any hominid male was in a position to
seriously hurt stronger individuals. In such a context, it should have become
extremely costly for a male to monopolize several females. Only males able to
monopolize tools, or males forming coalitions, could do so. But then all males could
make tools and form coalitions. Thus, a likely side effect of the evolution of tools
was a marked increase of the costs of aggression and a corresponding reduction of
the variance in male competitive ability. The reasoning rests on the assumption that
weapons increased the competitive power of all males in a manner largely independent of their physical prowess. If this assumption is correct, generalized polygyny,
with its permanent exclusion of a large fraction of males from the pool of reproductive individuals, would have become unfeasible. It was bound to give way, eventually, to generalized monogamy. Accordingly, the drive for polygyny was merely
checked, not eliminated. Polygyny could reemerge whenever some males were able
to attract females based on attributes other than physical prowess. Human societies
amply testify to this reemergence.
The present explanation is considerably more parsimonious than the parental
collaboration hypothesis in so far as the very origin of pair-bonding is concerned.
Monogamy is seen not as the outcome of specific selective pressures for paternal
investment, but as the mere by-product of other elements merging together over
evolutionary time, namely, prior polygyny and the rise of technology.
2.7
Fatherhood and the Expansion of Kinship
Whatever its timing and exact causes, the evolution of stable breeding bonds in the
hominid lineage transformed kinship from a low-profile factor in the social organization of early hominids to the prominent role it plays in simple human societies.
The key event was the evolution of systematic paternity recognition and fatherhood.
From the time the father associated with the mother, children were in a position to
recognize their fathers, and the fathers their children, even in the absence of any
form of direct paternal care. The processes involved here are similar to those
underlying the recognition of uterine sisters, in which the mother acts as the
mediator between the two sisters. A child and his father were bound to become
2
The deep structure of human society: Primate origins and evolution
37
disproportionately familiar with each other by virtue of their common preferential
bond with the same female, who was a mother to one and a “wife” to the other. The
child was also in a position to learn his father’s identity by recognizing the
characteristics of the relationship between his mother and father, the fact, for
example, that his father was his mother’s primary male associate, the one who
protected her against other males, had sexual interactions with her, and so on.
As soon as young hominids could recognize their father on such a reliable basis,
they were in a position to recognize their father’s relatives, including their paternal
grandfather and grandmothers, their paternal uncles and aunts, and their patrilateral
cousins. Again, the processes involved in the recognition of patrilateral kin may be
inferred from those underlying the recognition of uterine kin, where the mother is
the central reference point from Ego’s perspective. Here, it is Ego’s father who is
the central reference figure and operates as an intermediary between his son and his
close kin. Crucially, the recognition of one’s patrilateral kin was possible only if
fathers and sons engaged in enduring bonds with one another. This raises the
question of how such bonds might have evolved. One simple process is suggested
by relationships between adult males and adolescent males in chimpanzees. As they
start to travel independently of their mothers upon reaching adolescence, male
chimpanzees often attempt to form bonds with particular adult males. For example,
Pusey (1990) described the relationship between an adolescent male and the
group’s alpha male who had been a close associate of the adolescent’s mother at
a time when the latter was still traveling with his mother. The association persisted
into adulthood. Jane Goodall used the term “follower” to describe such relationships between a youngster and a particular adult male, stressing that the bond “is
almost entirely initiated and maintained by the follower” (Goodall 1986, p 202).
Such evidence suggests that nascent father son bonds in the hominid lineage could
have been initiated and maintained by the sons themselves, hence independently of
active forms of paternal care, and that they merely required fathers to be selectively
tolerant toward their sons, a condition predicted by kin selection theory.
Figure 2.8 illustrates the domain of kin recognition in hominid groups after the
evolution of stable breeding bonds. The contrast with Fig. 2.7 is striking. Prior to
the evolution of stable breeding bonds, the agnatic kinship structure was present but
socially indiscernible. The genealogical structure lay dormant. To reveal it, paternity recognition was needed, and this is precisely what stable breeding bonds
Fig. 2.8 Domain of kin
recognition from Ego’s
viewpoint in a male
philopatric, chimpanzee like
group after the evolution of
stable breeding bonds.
Definitions as in Fig. 2.7
38
B. Chapais
accomplished. With paternity recognition, the role of agnatic kinship in structuring
social relationships in male kin groups became comparable to the role of uterine
kinship in female kin groups such as macaques and baboons.
Another major consequence of fatherhood on kinship is that it created a whole
new type of family. Owing to space constraints, I must skip the reasoning (Chapais
2008, pp 202 215) underlying the following description. From a chimpanzee/
bonobo-like bi-generational and monoparental (mother offspring) unit, the
hominid family evolved into a biparental unit integrating three generations of
individuals owing to paternity recognition, grandmothers affiliate with their
son’s children and some affines as well, that is, into some sort of extended family.
On the basis of the assumption that fathers and sons developed lifetime cooperative
partnerships, such families included a well-defined core of primary agnates (father
and sons) whose cohesiveness stemmed, fundamentally, from the benefits associated with cooperating with a same-sex close kin. Importantly, daughters (or
sisters) were an integral part of such units. In chimpanzees, females have loose
bonds with their brothers, and with only a fraction of these. Pair-bonding changed
that situation drastically. Henceforward, among a young female’s most basic bonds
in the new hominid family were those that she developed with her primary kin: her
mother, father and brothers. This simple fact may be seen as the single most
important necessary condition for the evolution of practices such as sister exchange,
cross-cousin marriage, and avuncular relationships (discussion below).
2.8
The Origins of Exogamy and Postmarital Residence
“Exogamy lies far back in the history of man” wrote Edward Tylor long ago, “and
perhaps no observer has ever seen it come into existence, nor have the precise
conditions of its origin yet been inferred” (Tylor 1889, p 267). Tylor could hardly
have foreseen that the answer to this enigma lay in our close relatives. From an
evolutionary perspective, exogamy (primitive out-marriage) is simply the incidental by-product of the combination of two otherwise typical primate patterns:
between-group transfer and pair-bonding. Female dispersal was presumably the
rule in the ancestral male kin group. Upon the evolution of stable breeding bonds,
females kept emigrating into a new group as before, but instead of mating promiscuously in it, they formed an enduring breeding relationship; they were “marryingout” so to speak. Significantly, exogamy at that stage was deprived of any exchange
dimension: females were transferring between groups on their own initiative, they
were not part of transactions between males. That would come later.
We saw that for Lévi-Strauss, female exchange was the primary and most binding
form of reciprocity between men, an argument which he based on the empirical
observation that throughout the world women are men’s “most precious possession.”
Primatology vindicates this conclusion but through different arguments. Female
transfer between groups was the ancestral condition. This helps explains why a
female bias in dispersal not a male one is widespread cross-culturally. But
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The deep structure of human society: Primate origins and evolution
39
another, even more basic factor was involved. Throughout the animal kingdom,
including primates, females are certainly the most “precious possession” males
may compete for, as has been overwhelmingly documented ever since Darwin
(1871) 1981) first explained why this was so (for primates, see contributions in
Kappeler and van Schaik 2004). In this sense, Lévi-Strauss’s assertion fits nicely
with sexual selection theory.
Even as they were giving rise to a behavioral form of exogamy, stable breeding
bonds were generating a primitive form of what anthropologists call postmarital
residence. Chimpanzees and bonobos have a dual-phase residence pattern: females
spend a prebreeding phase in their natal group, followed by a breeding phase
elsewhere. Dual-phase residence is, thus, a phylogenetically primitive pattern. The
evolution of pair-bonding transformed that pattern into one comprised of a prepairbonding phase (or “premarital” phase) spent in the natal group, followed by a
postpair-bonding (or “postmarital”) phase spent in the new group. Like exogamy,
“postmarital” residence emerged from the integration of pair-bonding to male philopatry, a fusion that produced an embryonic form of patrilocality. To many sociocultural anthropologists, in contrast, prior to the invention of the incest taboo, residence
had been a single phase spent in one’s natal group (e.g., Murdock 1949, p 16).
Table 2.1 summarizes the state of the exogamy configuration immediately after
the evolution of stable breeding bonds (Phase II). Compared with the previous
stage, several new traits have emerged, but several others are still lacking. At this
point in human evolution, hominid groups were independent entities like all other
primate groups. Once individuals moved out of their natal group, they ceased to
interact with the relatives they left behind. Social life was limited to one’s local
group. The remaining components of the exogamy configuration had to await the
extension of social structure beyond the local group (phase III). They are attributes
of between-group alliances, or supragroup social structures. For the sake of simplicity, I use the term tribe in a generic sense to refer to such entities.
2.9
The “Atom of Between-Group Alliances”
As pointed out earlier, chimpanzees and bonobos are territorial: they avoid other
groups and may even attack strangers. In chimpanzees, intergroup fights are
initiated and conducted by adult males, and the targets include other adult males,
infants, and, sometimes, mothers. The local community is, thus, the most inclusive
level of social organization in our closest relatives. Assuming that early hominids
were territorial, it follows that a necessary condition for the evolution of betweengroup alliances the tribal level of organization was the pacification of adult
males living in distinct groups. The issue of the origin of the tribe brings LéviStrauss’s concept of the atom of kinship to the fore and, again, stable breeding
bonds appear to hold the key to this major transition in human evolution.
Figure 2.9 pictures two hominid groups after the evolution of stable breeding
bonds. The focus is placed on female Ego, born in group A and pair-bonded in
40
B. Chapais
Fig. 2.9 Between group pacification processes activated during meetings between two male
philopatric groups after the evolution of stable breeding bonds. The processes are illustrated by
focusing on a single female from group A (black circle) after she has transferred to and pair
bonded in group B. The female has children in group B and is recognized by her paternal kin living
in A. The female also acts as an intermediary between her kin living in A and her husband and his
relatives
group B. Suppose the two groups were to meet at their common border in some
nonaggressive way: intergroup meetings have been reported to occur from time to
time in bonobos (Idani 1990), but have not been observed in chimpanzees. In the
context of such meetings, female Ego would recognize, in addition to her mother
and maternal siblings, her father, grandfather, and uncles living in group A, and she
would be recognized by them. This could not be the case prior to the evolution of
stable breeding bonds, Ego not having experienced a preferential bond with her
father. Minimally, a male would be disinclined to attack his daughter, granddaughter, or sister, so Ego would have benefitted from some kind of immunity from her
male relatives. The same principle applies to all transferred females and to both
directions (group B females transferred in group A), hence to a significant fraction
of individuals in both groups. Moreover, a female’s immunity against aggression
should extend to her own offspring: A male who refrained from attacking his
daughter or sister should also refrain from attacking the individual that his daughter
or sister carried on her back or belly. Male chimpanzees are known to attack and kill
the infants of isolated mothers when they come across them at their common
border. From the time males could recognize such individuals as their close kin,
male infanticidal attacks should have dropped. In sum, owing to paternity recognition and its impact on agnatic kinship, group A males would be collectively
inhibited from attacking several females living in group B and, reciprocally,
group B males would be collectively inhibited from attacking several group A
females. A state of mutual, though fragmentary, tolerance stemming from the
existence of several “kinship bridges” between intermarrying groups would prevail.
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The deep structure of human society: Primate origins and evolution
41
Concurrently, another, distinct process would favor between-group pacification,
this one involving the mediation of affines. In-laws are the relatives of one’s spouse,
or the spouses of one’s relatives, depending on one’s viewpoint. Cognitively speaking, the recognition of in-laws is similar to kin recognition. It requires no more than
the ability to recognize preferential bonds between others e.g., between one’s
daughter and the latter’s husband. When groups A and B came into contact, ego’s
father could recognize his daughter’s husband (his son-in-law) and, reciprocally,
ego’s husband could recognize his father-in-law. Importantly, from an evolutionary
perspective, relationships between in-laws were bound to be, fundamentally, relationships between potential allies. Brothers-in-law, for instance, share a vested
interest in the same female, one as a husband, the other as a brother. Both males
derive benefits from the female’s well-being, the husband through his own reproductive interests with his wife, the brother by virtue of his genetic relatedness with
her inclusive fitness benefits. Crucially, this shared interest is not impeded by
sexual competition between the two males: owing to incest avoidance, a brother
does not compete with his sister’s “husband” for sexual access to his own sister.
Minimally, therefore, brothers-in-law should refrain from attacking each other, as
should, for that matter, fathers-in-law and sons-in-law and other affines. The
importance of the affinity route in the pacification of intergroup relations can hardly
be overstated because it is about peaceful relationships between adult males, the
individuals directly responsible for intergroup conflicts.
The foregoing reasoning presupposes that interbreeding groups met sporadically
at their common border in some nonaggressive way, in which case the structure of
kinship and affinity bridges just described would be activated. But if interbreeding
groups never came into contact in the first place, pacification could not start. In
other words, pair-bonding and the expansion of kinship were a necessary condition
for pacification, but not a sufficient one. For pacification to get going, some factors
had to favor nonaggressive meetings between groups, such as those described for
bonobos (Idani 1990). These factors might have operated through a reduction of the
levels of feeding competition between groups, an increase in the opportunities for
using the same resources simultaneously (food, water, or shelter), and/or an increase in the benefits of between-group cooperation against either other groups or
other species. This point needs further investigation.
In the pacification processes envisioned here, the alliance between groups A and
B hinges on female Ego who is simultaneously bonded to her male kin in group A
and to her husband in group B; or, in the words of Edward Tylor’s, on “the
peacemaking of the women who hold to one clan as sisters and to another as
wives” (Tylor 1889, p 267). The father daughter/wife husband triad is a dual-link
chain, with female Ego acting as a swivel joint between the two groups. The same
applies to the brother sister/wife husband triad. Each of these two Ego-centered
chains embody simultaneously, the kinship basis and the affinity basis of betweengroup alliances. Taken together, they may be described as the atom of betweengroup alliances, the smallest social element involved in between-group social
structures. This paraphrase of Lévi-Strauss’s “atom of kinship” is more than merely
analogical. Lévi-Strauss restricted the atom of kinship to the brother sister husband
42
B. Chapais
triad, neglecting the father daughter husband triad. He also erroneously ascribed
the atom of kinship to brothers repressing a built-in drive for incest and renouncing
marriage with their sisters, and he included the sister’s children in it. Notwithstanding these differences, the atom of kinship and the atom of between-group alliances
are basically the same thing, structurally speaking: a kinship bond connected to a
pair-bond through the intermediary of ego. Lévi-Strauss’s atom of kinship, the hub
of reciprocal exogamy, does have an evolutionary history.
2.10
The Nascent Tribe
The foregoing discussion points to the nature of the emerging tribe. At this stage in
its evolution, the tribe was merely a set of interbreeding local groups exhibiting
some levels of tolerance with one another, and the number of local groups forming a
tribe was determined by the exact pattern of female transfer between groups.
Simple models of female transfer make it possible to specify some of the conditions
favoring male pacification through the formation of kinship and affinity bridges
between groups (Chapais 2008). These models indicate (1) that male pacification
ensues whether female transfer is unidirectional or bidirectional between groups,
but that bilateral transfer promotes further congeniality, (2) that the pace of pacification between any two groups depends on the proportion of females in one group
moving to the other group: the larger that proportion, the larger the number of
kinship bridges between the two groups, and (3) that transfer between groups of
substantially different size works to the disadvantage of the smaller group. Combining the three principles, one may infer that intergroup pacification was especially
favored in situations where female circulation was bidirectional and concentrated
among a small number of groups that were not too dissimilar in size.
If the composition of the primitive tribe reflected the exact pattern of female
transfer between its constituent groups, that pattern was itself determined by the
geographical distribution of groups. In chimpanzees, for example, female transfer
was reportedly frequent between communities whose home ranges overlapped
extensively, but infrequent between communities whose home ranges did not
overlap (Kawanaka and Nishida 1974; Nishida 1979). Considering that the main
factors affecting the geographical distribution of local groups are the presence of
physical barriers between them and the distribution of food resources and predators,
one may envision the first tribes as regional entities whose constituent local
communities “exchanged” females and enjoyed various levels of peaceful relations
with one another.
Importantly, as between-group hostility markedly decreased within the tribe, it
remained at its prior level between tribes. This aspect of the present model helps
resolve the discrepancy between chimpanzees and human foragers with regard
to intergroup patterns of violence. Compared with chimpanzees, human huntergatherers are much more egalitarian and display substantially lower levels of
intergroup competition and violence. So striking is the difference that some authors
2
The deep structure of human society: Primate origins and evolution
43
spoke of a phylogenetic discontinuity between chimpanzees and human foragers as
far as patterns of violence were concerned (Knauft 1991; Kelly 2000). But as
argued by Rodseth and Wrangham (2004), the local band of hunter-gatherers is
not the right social unit for a meaningful comparison with chimpanzees the tribe
is (see also Crofoot and Wrangham, this volume).
2.11
The Evolution of Residential Diversity
Although its boundaries were relatively loose and its structure primitive, the
nascent tribe likely brought about some profound changes in the social relationships
of hominids. It notably rendered the diversification of postmarital residence feasible. I have hitherto been concerned with human patrilocality and its phylogenetic
antecedent, male philopatry. But human groups exhibit several other residence
patterns: matrilocality (spouses live with or near the wife’s parents), bilocality
(spouses live near the husband’s parents or the wife’s parents), neolocality (both
spouses leave their natal home to live elsewhere), and avunculocality (males live
with their maternal uncles, wives move to their husbands’ location, and their sons
return to live with the mother’s brothers). Moreover, each broad category constitutes only an ideal or modal type, allowing a fair degree of residential flexibility.
For example, several hunter-gatherer societies may be classified as “patrilocal with
a matrilocal alternative” (Ember 1975).
In the evolutionary scheme presented here, between-group pacification is a
prerequisite for the evolution of novel residence patterns involving the transfer of
males between groups, namely, matrilocality, bilocality, and avunculocality. Stated
otherwise, the tribal level of organization was a necessary condition for residential
diversity. In chimpanzees and bonobos, male territoriality is incompatible with
males transferring freely between groups, and this was presumably the case in the
ancestral hominid society: a male could hardly move to a group that was collectively defended by males. But after the tribe had evolved, males could move
between nonhostile groups of the same tribe though not between tribes. From
then on hominids were in a position to adjust residence patterns to various conditions; for example, to resource distribution, subsistence activities, and patterns of
cooperation.
One might object that the occurrence of female philopatry in nonhuman
primates male emigration coupled with female localization indicates that
human matrilocality does not require a tribal level of organization and therefore
that matrilocality might well have evolved earlier in the hominid line. But this
argument misses the point. First, one must take into consideration the phylogenetic
constraints acting on the evolution of residence patterns in the hominid lineage.
Ancestral male philopatry and its correlate, male cooperation in territorial defense,
had to be circumvented before male transfer and matrilocality became possible.
This is precisely what the tribal level of organization achieved. Second, female
philopatry is not simply the structural equivalent of human matrilocality. The two
44
B. Chapais
patterns differ in some important respects and it is likely that female philopatry is
not the evolutionary antecedent of human matrilocality. In female philopatric
primates, females cease to interact with their male relatives once the latter have
moved out of the group. In marked contrast, human matrilocality consistently
comes along with the political control of women by their kinsmen despite their
being away (Schneider 1961). Indeed, an important correlate of matrilocality and
avunculocality for that matter is matrilineal descent, in which the line of authority
runs from men to their sister’s sons, rather than from women to their daughters. The
localization of women is, thus, associated with their maintaining lifetime bonds
with their kinsmen married in other groups, hence with the tribal level of organization. This suggests that a prerequisite for the localization of women was that men be
in a position to exercise control over their kinswomen despite physical separation
from them, a state of affairs that was possible only after the tribal level of organization had evolved.
2.12
The Origins of Exogamy Rules
As described earlier, Lévi-Strauss’s theory of reciprocal exogamy features kinshipconstrained marriage rules, the most basic of these being sister (or daughter)
exchange, the levirate, the sororate, and cross-cousin marriage. Where do these
rules come from? The answer proposed here is that they were ultimately derived
from the atom of between-group alliances and the ensuing familiarity biases that
affected mate selection in the nascent tribe. As illustrated in Fig. 2.10, upon pairbonding with male B1 and moving into group B permanently, female Ego underwent long-lasting familiarity biases with her husband’s close kin, including her
brothers-in-law and sisters-in-law. Such biases were likely to translate into amicable relationships between them for reasons already given. In this context, if Ego’s
husband were to die, Ego might well form a pair-bond with her husband’s brother
(B3), in which case one obtains the structural equivalent of the levirate a widow
marrying the brother of her deceased husband. Similarly, upon pair-bonding with
Ego, male B1 experienced long-lasting familiarity biases with his wife’s close
kin, including his brothers-in-law and sisters-in-law. If Ego were to die, male B1
could form a pair-bond with his wife’s sister (A2), this producing the structural
equivalent of the sororate
a widower marrying his deceased wife’s sister.
If, however, male B1 were to form a pair-bond with Ego’s sister A2 while Ego is
still alive, this produces a form of sororal polygyny, another widespread practice in
human societies.
Interestingly, simple processes akin to those described here have been invoked
by cultural anthropologists to explain the levirate and the sororate. Citing figures
based on 250 societies, Murdock described the closely related phenomenon of
privileged relationships between siblings-in-law of opposite sex, “within which
sexual intercourse is permitted before marriage and frequently afterwards as
well.” Murdock argued that both permissive sex and preferred marriage between
2
The deep structure of human society: Primate origins and evolution
45
Fig. 2.10 Mating biases stemming from disproportionate levels of familiarity between affines, and
their relations with known exogamy rules in humans. The individuals pictured are the same as in
Fig. 2.1, except that siblingships include one more individual. (1) Initial pair bond between female
Ego and male B1. (2) Pair bond between B1 and his wife’s sister, the equivalent of sororal
polygyny if Ego is still alive, or of the sororate if Ego is dead. (3) Pair bond between Ego and
her husband’s brother, the equivalent of the levirate if the husband is dead. (4) Pair bond between
Ego’s brother and B1’s sister. In conjunction with (1), this produces the structural equivalent of
sister exchange
siblings-in-law “are explicable as extensions of the marital relationship” and
reflect the attraction of people to persons who most closely resemble their
spouse. “The persons who universally reveal the most numerous and detailed
resemblances to a spouse” he wrote, “are the latter’s siblings of the same sex” . . .
“who are likely to have similar physical characteristics” . . . and “almost identical
social statuses since they necessarily belong to the same kin group” (Murdock 1949,
pp 268 269). Murdock’s explanation has much in common with the present model.
Both conceive of mate selection as being affected by informal regularities, whether
the latter stem from familiarity biases, physical similarities, or social compatibilities. To Lévi-Strauss, in contrast, exogamy rules were normative and part of
reciprocity agreements. But viewed from an evolutionary perspective, the two
types of explanations are compatible: informal regularities paved the way for
normative rules; in the same manner, incest avoidances paved the way for incest
prohibitions.
These considerations apply to another marriage rule: sister exchange (or daughter exchange, depending on one’s viewpoint). Structurally speaking, sister
exchange is simply bilateral marriage between two groups of affines. In all likelihood, the exchange dimension of the phenomenon is a further and more recent
aspect of it, an aspect involving the control of sisters by their brothers (or of
daughters by their fathers). As for cross-cousin marriage, it is the extension of
sister exchange to the following generation, as described earlier (Fig. 2.3). Thus,
from an evolutionary perspective, the relevant question for both sister exchange and
46
B. Chapais
cross-cousin marriage concerns the origin of bilateral marriage between affines and,
again, the answer proposed here lies in familiarity differentials affecting mate
selection. As illustrated in Fig. 2.10, if female Ego is already pair-bonded with
male B1, bilateral marriage between affines ensues from B1’s sister (B2) pairbonding with Ego’s brother (A1). From male A1’s viewpoint, female B2 is a
familiar affine, the sister of his brother-in-law. Reciprocally, from female B2’s
viewpoint, male A1 is also a familiar affine, the brother of her sister-in-law. In
short, the most basic of exogamy rules levirate, sororate, sister exchange, and
cross-cousin marriage might have originated in mate selection biases stemming
from disproportionate levels of familiarity between affines.
2.13
Conclusion
If reciprocal exogamy is the deep structure of human society, the configuration of
elements listed in Table 2.1 may be seen as the most sophisticated form the
structure had reached prior to the evolution of the symbolic capacity. Some crucial
elements are still lacking at that stage, notably the actual exchange of kinswomen
by men, an aspect which probably required language. Table 2.1 may also be read as
the list of human universals that stemmed from the presymbolic and prenormative
state of human society.
I began this article by proposing that Lévi-Strauss’s concept of reciprocal
exogamy was a strong candidate for humankind’s deep social structure. I based
that hypothesis on the observation that from the outset Lévi-Strauss’s characterization of reciprocal exogamy met the formal criteria of such a structure. The present
phylogenetic analysis provides further critical evidence for that claim by showing
that reciprocal exogamy breaks down into evolutionarily meaningful building
blocks. Indeed, a number of components of the exogamy configuration listed in
Table 2.1 are observable in nonhuman primates while several others that are not
agnatic kinship, exogamy, postmarital residence, and so on appear to be the byproducts of the combination of building blocks that do exist in nonhuman primates.
Hence, it can be said that whether the constituent elements of the exogamy
configuration are visible in other primate species or not, they make sense evolutionarily speaking. Had reciprocal exogamy not broken down into phylogenetically
meaningful elements, one could not propose that it embodies the deep structure of
human society. Correlatively, the phylogenetic reconstruction of the exogamy
configuration readily fits with our knowledge about some of the most basic events
in the evolutionary sequence that led to human society, namely, an ancestral Panlike society and the subsequent evolution of stable breeding bonds. Had it been
problematic to figure out how the exogamy configuration came about in the
hominid lineage, there would be more grounds to question its significance.
In sum, Lévi-Strauss’s concept of reciprocal exogamy, although issued from
an asynchronic theoretical framework, is basically a primate-like, or primatecompatible, structure. The reason it is so is that it centers around two factors of
2
The deep structure of human society: Primate origins and evolution
47
cardinal importance in all primate social structures, sex (mating system) and kinship,
and that it hinges on the issue of outbreeding through dispersal from one’s group
(exogamy). Unknowingly, then, Lévi-Strauss characterized human society in terms
of a primate society. It is remarkable that two approaches as distinct as comparative
primatology and Lévi-Strauss’s structuralism one avowedly excluding the evolutionary paradigm, the other issuing from it should converge independently on the
issue of the most essential factors that organize human society. This lends further
credence to the exogamy model of human origins.
Acknowledgments I am grateful to Robin Fox, Peter Kappeler, and Joan Silk for their helpful
comments on the manuscript, and to Julie Cascio for technical assistance with the figures. I also
thank several people who provided invaluable comments on my book Primeval Kinship: How
Pair Bonding Gave Birth to Human Society, on which the present chapter is based, namely Peg
Anderson, Bernard Bernier, Annie Bissonnette, Carol Berman, Robert Crépeau, Michael Fisher,
Michel Lecomte, Martin Muller, Jean Claude Muller, Robert Sussman, Shona Teijeiro, and
Richard Wrangham.
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Chapter 3
Conflict and Bonding Between the Sexes
Ryne A. Palombit
Locked together by their need for partners in sexual reproduction, the sexes undergo an
antagonistic dance to the music of time.
Tracy Chapman and Linda Partridge (1996)
Abstract The derivation of human universals from nonhuman data is complicated
by the immense diversity of patterns exemplified by both human and nonhuman
primates. One approach is to elucidate processes that may operate universally,
though the particular phenotypic patterns that result may differ, depending upon
the distinctive features of species biology. Below, I argue that sexual conflict and its
corollary, sexually antagonistic coevolution, are of central importance for understanding the evolution of reproductive strategies in nonhuman primates. Because
sexual conflict is a relatively new area of theory and research, and because primate
life histories limit the kinds of data that can be collected, relevant primate data are
limited (with the possible exception of one form of conflict: infanticide). Consequently, I review sexual conflict theory, relevant comparative data from nonprimates, and preliminary evidence from select primate studies. Theoretical
considerations and empirical evidence suggest significant potential for the widespread action of sexual conflict in nonhuman primates, in both precopulatory and
postcopulatory domains of reproduction, and affecting characters ranging from
morphology and physiology to sociosexual behavior. Female counterstrategies to
male-imposed costs are diverse, but male female association has been argued to
forestall sexual conflict both in the form of precopulatory coercion and of infanticide. In light of evidence for pervasive and diverse effects of sexual conflict in
nonhuman primate biology, it is likely that it also constitutes a universal process
R.A. Palombit
Department of Anthropology, Center for Human Evolutionary Studies, Rutgers University,
New Brunswick, NJ, USA
e mail: rpalombit@anthropology.rutgers.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 3, # Springer Verlag Berlin Heidelberg 2010
53
54
R.A. Palombit
underlying human reproduction. I briefly explore several potential sources of
human sexual conflict suggested by the nonhuman data.
3.1
An Approach to Universals
Universals are “mechanisms of human behavior held in common among people all
over the world. . .” (Boyd and Silk 2006: 590). The variability of human behavior
has always bedeviled the search for universals, prompting Fox (1989: 116) to ask
how we get beyond the “ethnographic dazzle” to the universals of general,
biological importance? The problem is only magnified when we expand the taxonomic context of the analysis to include nonhuman primates, a mammalian order
famous for immense diversity in behavior, reproduction, life history, morphology,
and physiology. One might say that ethological dazzle threatens to obscure this
comparative analysis: how can one discern anything about human universals from
this extraordinary variety? There are two solutions to this problem of deriving our
family resemblances (sensu Fox 1989).
One approach is to search for specific patterns of behavior shared between
human and nonhuman primates. This orientation towards substantive universals
necessarily concentrates our attention on a relatively small number of species
most closely related to us, notably the chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), or perhaps the African great apes, or the great apes, generally.
To remain useful, however, this approach, focused as it is on elucidating homologous patterns, cannot extend too far beyond this group of primates. This method
offers advantages and insights (e.g., Goodall 1971; Wrangham and Peterson 1996;
de Waal 2005).
An alternative approach, however, is suggested by Wittgenstein’s (1953) theory of
universal family resemblances, as captured by the “Churchill face” metaphor (Aaron
1965). Among members of a family, such as the Churchills, there is a distinctive
Churchill face, which is recognizable as the same, in some sense, but which cannot be
said to have any one feature common to all faces. In other words, there is no shared
pattern per se. The crucial aspect of this view is its emphasis on a process generating
predictable patterns not necessarily defined by any one feature or character. The
particular patterns will depend upon distinctive features of a species’ biology or a
population’s conditions. It is the process that constitutes the universal.
It is this second perspective on behavioral universals that frames this chapter’s
examination of nonhuman primates. Here, I focus on one process that I believe is
paramount for understanding primate reproductive strategies: sexual conflict. Sexual conflict has attracted increasing attention over the last decade, and the studies of
this process have now come to outnumber investigations of the conventional forms
of sexual selection (intrasexual selection and mate choice) (Pizzari and Snook
2003). Most of this research has focused on invertebrates particularly insects
although there have also been studies of sexual conflict in some vertebrates, such as
fish, birds, and an occasional mammal (e.g., Arnqvist and Rowe 2005). In spite of
3
Bonding and Conflict Between the Sexes
55
an early landmark article (Smuts and Smuts 1993), research on sexual conflict in
primates has not progressed dramatically.
3.2
What is Sexual Conflict?
As with any relatively new field, there is considerable debate over the definitions,
assumptions, and models of sexual conflict (Hosken and Snook 2005; Tregenza
et al. 2006). Of course, the notion that male and female reproductive styles do not
always coincide perfectly has a long history in evolutionary thinking, beginning
with Darwin’s (1871) exposition of sexual selection, demonstrated by Bateman’s
(1948) study of Drosophila reproduction, and elaborated by Williams’s (1966)
“battle of the sexes” metaphor. But it was Trivers (1972) who spotlighted the
potential for sexual conflict with an ostensibly simple point: sex differences in
parental investment, originating with anisogamy, but amplified in mammals by
gestation, lactation, and postnatal care, will generate different reproductive strategies for the males and females, maximizing quantity vs. quality of offspring,
respectively. The implication is that reproductive strategies of the sexes not only
diverge, but may comprise elements that are incompatible. This incompatibility is
crucial because different fitness optima for males and females will not generate
conflict if they can be achieved simultaneously (Parker 2006). Sexual conflict
emerges when strategies among members of one sex impose fitness costs on the
other sex. In the resulting evolutionary dialectic, each sex attempts to mitigate these
Fig. 3.1 A comparison of average fitness profiles of reproducing males and females over evolu
tionary time under “conventional” intersexual selection (female choice) (left) and sexual conflict
(right). Under intersexual selection, male fitness (dashed line) and female fitness (line) often
(though not invariably) increase to an asymptote set by natural selection. Under sexual conflict,
mutations conferring a net mating benefit to males reduce female fitness, thereby selecting for a
female counter adaptation decreasing male fitness, etc. It is important to note that the figure does
not represent the average fitness of males and females in a population, which will coincide with
one another (Fisher 1930), but rather the average fitness profiles of reproducing individuals (see
Arnqvist 2004; Pizzari and Snook 2004). Figure modified from Pizzari and Snook (2003)
56
R.A. Palombit
costs and move members of the other sex closer to its own optimum (Gowaty 1997).
This coevolutionary dynamic of sexually antagonistic strategies positions sexual
conflict as a potential third form of sexual selection, in addition to intrasexual
selection and mate choice (Smuts and Smuts 1993; Clutton-Brock and Parker 1995;
Chapman et al. 2003; Zeh and Zeh 2003; Tregenza et al. 2006) (Fig. 3.1). It is
female avoidance of male-imposed costs that drives sexual conflict, rather than the
acquisition of benefits from preferred mating. Parker’s (1979) ESS analyses gave
rise to the current theoretical framework recognizing two general forms of sexual
conflict as sexually divergent optima for either (1) alleles determining a specific
trait intralocus conflict as in the evolution of sexual dimorphism (e.g., Lande
1987; Lindenfors 2002; Cox and Calsbeek 2009); or (2) the outcome of male
female interactions interlocus conflict. This chapter is concerned only with the
outcome of male female interactions.
3.3
Approaches to Studying Sexual Conflict
There are three general approaches to studying sexual conflict. The first method is
exemplified by the now classic study of seminal proteins in the fruit fly (Drosophila
melanogaster) (Rice 1996; Holland and Rice 1999). These proteins originate in
accessory glands, are transferred (with sperm) to female mates, and influence
females in a number of ways that benefit males, such as: (1) increasing the rate of
female egg-laying (Chen 1984); (2) decreasing female receptivity to additional
matings (Ravi Ram and Wolfner 2007); and (3) improving sperm competition by
displacing the sperm of previous copulators (Harshman and Prout 1994; Clark et al.
1995). Seminal fluids are apparently toxic, such that prolonged exposure to them
elevates female mortality (Chapman et al. 1995; Clark et al. 1995; Lung et al.
2002). In order to test the prediction that monogamous mating systems engender
less sexual conflict than polygynous systems, Holland and Rice (1999) randomly
assigned individual D. melanogaster to one of two population treatments: imposed
monogamy versus the (control) polygynous ancestral condition. After 47 generations, the monogamous lineage was characterized by lower toxicity of male seminal
fluids and lower female resistance to seminal fluids (see also Rice et al. 2005).
These data exemplify a key (though not inevitable) corollary of interlocus sexual
conflict: sexually antagonistic coevolution. This historical approach, tracking
changes over evolutionary time, can provide particularly compelling evidence of
sexual conflict and sexually antagonistic coevolution, but it is feasible primarily
with relatively short-lived animals that can be manipulated in the laboratory.
A second approach, based on quantitative genetics, defines sexual conflict as
negative covariance between the sexes in fitness, particularly over generations
(Rice and Chippindale 2001; Shuster and Wade 2003; Pizzari and Snook 2003,
2004). For example, red deer (Cervus elaphus) stags with greater lifetime reproductive success sired less successful daughters and more successful sons than stags
with lower lifetime fitness (Foerster et al. 2007). The negative correlation between
3 Bonding and Conflict Between the Sexes
57
the fitness of males and females suggests opposing optimal genotypes for males and
females, i.e., sexually antagonistic coevolution. Again, this method is impractical
for primates because we know relatively little about lifetime reproductive success,
particularly for males, and even less about the selection coefficients and heritability
of characters related to fitness.
A third approach considers how certain behavioral, anatomical, or physiological
aspects of reproductive strategies among members of one sex impose costs on the
other sex, and how phenotypic features of the second sex may function to mitigate
those costs (as coevolutionary counterstrategies). The relevant data are collected
over relatively short time periods, rarely long enough to demonstrate the effects of
sexual conflict on the lifetime reproductive success of individuals. These kinds of
analyses can reveal the extent and form of sexual conflict, but they can only
indirectly imply the action of sexually antagonistic coevolution. This approach is
the only one that is now tractable for studies of nonhuman primates.
3.4
3.4.1
Pre- and PostCopulatory Conflict over Mating: Sexual
Coercion
Sexual Coercion: A Conceptual Framework
Aggression between the sexes surrounding mating is termed “sexual coercion”
(Smuts and Smuts 1993). Clutton-Brock and Parker (1995) distinguish three forms
of sexual coercion: forced copulation, sexual harassment, and sexual intimidation.
Although few nonhuman primate studies explicitly differentiate these three categories of sexual coercion, there is evidence that all three may operate in primates.
3.4.2
Forced Copulation
This form of sexual coercion involves the physical restraint and forcible insemination of resistant females. Among primates, forced copulation has been noted
occasionally in several species (chimpanzees Tutin 1979; patas monkeys Chism
and Rogers 1997; spider monkeys Gibson et al. 2008), but it is regularly observed in
only two species, the orangutan (Pongo pygmaeus) (van Schaik and van Hooff
1996) and Homo sapiens (Smuts 1992; Goetz et al. 2008).
Although forced copulation occurs in a number of different taxa (Table 3.1), it is
a less common form of sexual conflict than harassment or intimidation. This may be
because forced copulation is only possible under a restricted set of conditions, such
as when males are much larger than females (Clutton-Brock and Parker 1995) or
when females are isolated and unable to obtain social support. However, neither of
these factors provides an entirely satisfactory explanation for the distribution of
forced copulation in primates. Although the orangutan is a strongly dimorphic
Male dominance displays
Sexual intimidation/
punishment
Postcopulatory
(Prezygotic)
Seminal fluid proteins
Non-fertile sperm
Reproductive tract injury
Referencea
Thornhill and Alcock 1983;
Gowaty and Buschhaus 1998;
Bertin and Fairbairn 2005;
Siva-Jothy 2006; Vahed and
Carron 2008, Knott in press
Insects, Fish, Anurans,
Howard 1980; Clutton-Brock and
Parker 1995; Réale et al. 1996;
Snakes, Artiodactyls
Microcebus murinus?
Arnqvist and Nilsson 2000;
Shine et al. 2000; Eberle and
Kappeler 2004a; Bowcock et al.
2009
Papio hamadryas
Henzi et al. 1998; Kitchen et al. in
griseipes, P h ursinus
press
Males target females in aggressive
displays that function in acquisition
and/or maintenance of intragroup
dominance status or intergroup
spacing
Aggression to (estrus) females that refuse Primates
to associate or copulate with male, or
that associate or copulate with other
male(s)
Proteins beneficially affect outcomes of
sperm competition for males, while
imposing costs upon female viability
and/or reproduction
Anucleate sperm reduce female
receptivity to subsequent mating
Male-induced changes/injury of female
genital tract, typically during
copulation, results in decreased sexual
interaction
Seminal coagulates may: improve sperm
transport, reduce sperm loss, physically
block intromission by other males,
and/or physiologically induce female
refractory period
Smuts and Smuts 1993; CluttonBrock and Parker 1995, see text
Insects, Nematodes
Gems and Riddle 1996; Holland
and Rice 1999, see text
Insects
Cook and Wedell 1999
van der Schoot et al. 1992,
Crudgington and Siva-Jothy
2000; Blanckenhorn et al. 2002;
Stockley 2002; Low 2005
Insects, Rodents, Primates Matthews and Adler 1978;
Simmons and Siva-Jothy 1998;
Dixson and Anderson 2002
Insects, Rodents,
Strepsirrhines?
H sapiens?
R.A. Palombit
Genital plugs, coagulates
Example taxaa
Pongo pygmaeus, Homo
sapiens, Anseriform
birds, some insects
58
Table 3.1 Potential forms of (interlocus) sexual conflict
Context
Category
Nature of sexual conflict
Precopulatory
Forced copulation
Catch and physically restrain female
followed by forced insemination; incl.
anatomical specializations to grasp
and prevent escape of female prior to
forced insemination
Harassment, indirect costs Repeated, persistent courtship or
copulation (attempts), by single or
of mating or mate
especially multiple males; physical
guarding
aspects of courtship or copulation
(e.g., posture, inexperienced males)
Sexual intimidation/
punishment (mate
guarding)
Egg-sperm interaction
Postcopulatory
Feticide
(Postzygotic)
Prevention of penis removal from female Insects, Galago
reproductive tract for prolonged period
crassicaudatus,
following ejaculation; due to genital
Macaca arctoides
clasping structures or partial
enlargement of penis and vaginal
adhesion
Temporary male association with an
Insects, Primates
inseminated female for a prolonged
period following ejaculation to
aggressively prevent subsequent
mating by female
Genes of sperm and egg differentially
Invertebrates, Fish
influence processes surrounding
capacitation, penetration of egg, and
fertilization
Eberle and Kappeler 2004b; Sato
and Kohama 2007
Rice and Holland 1997; Levitan
2008; Martin-Coello et al. 2009
Equids, Primates
Berger 1983; Pereira 1983; Sommer
1987; Agoramoorthy et al. 1988;
Pluháček and Bartoš 2000
Primates, Fissiped
carnivores,
Toothed whales
Rodents
van Schaik 2000a
Gorilla gorilla
subspecies, Pan
troglodytes
Boehm 1994; Watts 1997; Sicotte
2002; Stokes 2004; Harcourt and
Stewart 2007
Keverne 2001; Roulin and Hager
2003
59
Male harassment (or forced copulation) of
pregnant female promotes or induces
spontaneous abortion of implanted zygote
or fetus
Sexually selected
Killing of dependent infants to prematurely
infanticide
end lactational amenorrhea and return
females to fertilizable (estrus) state
Parental investment &
Activity of genes depends upon sex of parent
genomic imprinting
from which inherited (e.g., paternally
derived genes induce disproportionately
greater maternal investment in offspring)
“Policing”
Male intervenes to curtail female-female
aggression, mitigating or eliminating
benefits a “winner” could derive via
individual or coalitionary competitive
superiority
a
Taxa and references list are not exhaustive, but rather represent illustrative examples
Thornhill and Alcock 1983; Dixson
1998; Werner and Simmons
2008
3 Bonding and Conflict Between the Sexes
Genital lock
60
R.A. Palombit
species, forced copulation is frequently done by small males, who are either subadults
or “unflanged” adults with arrested development of secondary sexual characters
(Knott 2009). Moreover, in many strongly dimorphic monkeys, males do not exhibit
the behavior at all. Social isolation may increase vulnerability to forced copulation. In
contrast to the vast majority of highly gregarious anthropoid primates, female orangutans are often alone (Rodman and Mitani 1987). Humans are not solitary, but
Emery-Thompson (in press: 361) argues that college-age women experience the
highest rate of rape in the United States partly because “they are the group most likely
to be living away from natal kin but not yet with a domestic partner.” However, social
vulnerability does not explain why forced copulations are so rare in chimpanzees
(0.2% of the copulations observed by Tutin (1979)) even though females typically
disperse from their natal communities and spend much time alone. Possible explanations for the rarity of forced copulation in chimpanzees are female influence on male
dominance relations (Stumpf and Boesch 2006) or simply the effectiveness of male
sexual coercion in generating mating opportunities (see below), which reduces the
benefits of physical restraint and forcible insemination.
Forced copulation in orangutans is commonly considered part of an alternative
reproductive strategy of unflanged adult males. The males avoid direct mating
competition with large, flanged males by retarding development of secondary
sexual traits and relying on force to copulate with uncooperative females that
generally prefer flanged males as mates (van Schaik and van Hooff 1996; Atmoko
and van Hooff 2004; Maggioncalda et al. 1999). Knott (2009) argues, however, that
since forced copulation is not restricted to unflanged males, it is better viewed as a
general male orangutan strategy to overcome female resistance. Both models are
consistent with sexual conflict arguments that forced copulation in nonhuman
animals is an alternative mating strategy (Table 3.1).
Thornhill and Palmer (2000) have similarly proposed the controversial hypothesis that human rape reflects an alternative strategy of low-status, socially disadvantaged males to obtain conceptions. Emery-Thompson (2009) rejects this argument
on several grounds, including observations that a substantial majority of rapes are
perpetrated by men casually or intimately known to their victims (acquaintance
rape) and that women often continue their relationships with these attackers. Thus,
she contends instead that rape is one of several forms of sexual aggression used by
men to maintain long-term reproductive access to female mates. Emery-Thompson
has shifted the functional focus from immediate copulatory benefits (as in orangutans) to prospective reproductive gains via intimidation and punishment (see
below). Again, both hypotheses are based on sexual conflict.
It is important to recognize that forced copulation in humans is an extremely
heterogeneous phenomenon (Travis 2003). Some cases of rape may originate in
pathological behavior (such as “stranger rape”) (Emery-Thompson 2009) or in male
tactics of terror and control (e.g., violent rape in the context of warfare; Swiss and
Giller 1993). Thus, although a comprehensive understanding of rape in humans will
no doubt involve an array of processes and factors, sexual conflict theory seems
likely to improve understanding of some forms of the behavior (Emery-Thompson
2009).
3 Bonding and Conflict Between the Sexes
3.4.3
61
Sexual Harassment versus Sexual Intimidation
Sexual harassment refers broadly to the fitness costs of mating to females (sensu
Daly 1978), particularly costs connected with persistent male courtship, repeated
intromission attempts, or the nature of copulation itself. Sexual intimidation refers
to situations in which “males punish females that refuse to associate with them or
that associate with other males,” and is thus designed to reduce female resistance or
promiscuity (Clutton-Brock and Parker 1995, p 1353). When males use sexual
intimidation tactics, females learn to modify their behavior to minimize the costs of
male aggression. This definition is directly similar to Smuts and Smuts’s (1993)
original definition of sexual coercion. To illustrate the distinction between sexual
harassment and sexual intimidation, consider the following examples:
1. During the rut, female sheep (Ovis spp.) may be pursued by up to 11 rams at a
time, whose repeated attempts to charge, sniff, and mount result in exhaustion
and injury to females (Réale et al. 1996) as well as increased mortality, as
females evade male suitors on precipitous terrain (Festa-Bianchet 1987).
2. When a female dung fly (Scatophaga stercoraria) lands on a dropping occupied
by several males, their struggles to copulate and exclude rivals from mating may
trample her into the patty, covering her with dung that impairs her ability to fly
and sometimes even drowns her (Parker 1970).
3. A male chimpanzee severely attacks an estrous female for “no obvious reason,”
i.e., in circumstances unrelated to ongoing mating, and when the female’s sexual
swelling is small or flat; many days later, during the period of maximal swelling
and mating, she restricts copulations to this male (Goodall 1986: 341).
The various costs imposed on female sheep and dung flies are classified as sexual
harassment because they are the indirect by-product of female mate discrimination
and male competition, which are particularly relevant when mating attempts are made
repeatedly or by multiple males (or both). The chimpanzee example highlights
aggression designed to reduce female resistance or promiscuity, in this case, to
promote future female mating compliance. Harassment and intimidation can operate
in either pre- or postcopulatory contexts. For example, mate guarding is a common
manifestation of coercion that can precede or follow copulation. It may comprise
threats and attacks on the female herself (sensu intimidation) or aggression directed at
rival males, thereby imposing indirect mating costs on females (sensu harassment).
Harassment and intimidation are behavioral examples of a general distinction in
sexual conflict theory between negative pleiotropic side effects and adaptive harm
to females, respectively (Partridge and Hurst, 1998). Many students of sexual
conflict maintain that the costs accrued by females are incidental (pleiotropic) byproducts of male mating strategies, selected for not because of, but in spite of the
harm to females (Hosken et al. 2003; Morrow et al. 2003; Arnqvist 2004). Conversely, proponents of the adaptive harm hypothesis posit that males benefit from
directly harming females, if an existing system of phenotypic plasticity promotes
female responses that benefit males (e.g., a female injured by a male may increase
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R.A. Palombit
her resting time, thereby limiting copulation with other males, or she may invest
more in her current offspring due to the harm-induced reduction in her reproductive
value) (Lessells 1999; Lessells 2005; Johnstone and Keller 2000).
Of crucial importance for understanding these forms of sexual coercion are data
addressing not just the magnitude of costs to females, but also the nature of those
costs. There are few such data for primates, but studies of other animals reveal costs
in the form of reduced foraging efficiency (Rubenstein 1986; Magurran and Seghers
1994; Pilastro et al. 2003; Erez et al. 2005; Heubel and Plath 2008), increased
exposure to predation (Magellan and Magurran 2006), injury and associated
increased mortality (Hiruki et al. 1993; Miller et al. 1996; Réale et al. 1996;
Mühlhäuser and Blanckenhorn 2002), and physiological stress (Moore and Jessop
2003; Shine et al. 2004). These costs are in addition to those associated with
suboptimal reproduction due to fertilization by a lower quality male or to lost
opportunities for polyandry.
3.4.4
Is Sexual Coercion Beneficial to Females?
It is possible that sexual coercion may actually enhance female fitness by providing
a behavioral filter for higher quality males as mates or guaranteeing that females’
sons will carry sexually antagonistic traits that enable them to achieve higher
reproductive success (Eberhard 1996; Cordero and Eberhard 2003). If the net effect
on female fitness is therefore positive, then sexual conflict becomes a mechanism of
female choice, which Eberhard (2005) contends explains most male mating aggression to resistant females. This hypothesis has not been supported by some mathematical models (Kirkpatrick and Barton 1997), but there is some related evidence
for benefits of coercion to females (Valero et al. 2005).
Most primate researchers assume that sexual coercion reduces the effectiveness of
female mate choice and that female preference for less aggressive males is a likely
counterstrategy to sexual coercion (Smuts and Smuts 1993). This view derives in part
from the intensity of both male aggression and toward females and female resistance,
which seems to impose high costs on the victims (e.g., chimpanzees: Goodall 1986;
Muller et al. in press). Moreover, for most anthropoid primates, group life may provide
females with less costly means of evaluating mates than provoking male attacks upon
themselves. An arguably more relevant variant of this hypothesis, however, is that
females prefer to mate with high-quality males (e.g., dominant males), who also
happen to be more aggressive generally (which constitutes an indirect cost of mating).
3.4.5
Sexual Harassment and Intimidation in Non-Human
Primates
Three conditions promote sexual harassment that occurs when multiple males
attempt to mate simultaneously with a single female (Réale et al. 1996; Head and
Brooks 2006; Smith and Sargent 2006): (1) a male-biased operational sex ratio;
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63
(2) asynchrony in female estrous; and (3) weak dominance among males (i.e., reduced or incomplete male ability to control sexual access to females).
All three conditions prevail in nocturnal mouse lemurs (Microcebus murinus)
studied at Kirindy, western Madagascar: reproducing males tend to outnumber
estrous females; females breed on only one night each year, but are receptive on
individually different nights of the mating season; male male competition sometimes involves contests, but scramble competition via extensive roaming behavior
is more common (Eberle and Kappeler 2004a, b). On her night of receptivity, a
female is typically approached by 2 15 males and mates with almost all of them up
to 11 times. Notably, the usual social dominance of females wanes during the
mating season, prompting Eberle and Kappeler (2004a: 97) to interpret the high
rates of mating with multiple males as “harassment” stemming from a temporary
female inability to reject suitors. Postcopulatory mate guarding does occur occasionally, raising the possibility of sexual intimidation. But this mate-guarding is
based less on aggression directed at the female than on chasing rival males away.
Attacks on females occurred in only 4 of the 55 cases of mate guarding and were
also largely ineffectual in light of the fact that three of the four females succeeded in
deserting the male. These patterns of sexual coercion are generally more consistent
with multi-male harassment than with sexual intimidation, as predicted by the
demographic, social, and reproductive conditions.
The gregarious (diurnal) strepsirrhines are of comparative interest for distinguishing between harassment and intimidation because intimidation relies particularly on learned cooperation in explicitly gregarious contexts (Clutton-Brock and
Parker 1995). Unfortunately, few relevant new data have become available since
Smuts and Smuts (1993) to address this question. Brockman’s (1999) description of
sexual aggression by male sifakas (Propithecus verreauxi) suggests harassment
rather than intimidation. Multiple males attempt simultaneously to mate with most
estrous females during the mating season. Intersexual sexual aggression increases
significantly at this time, but the vast majority of it is female aggression to males
(not vice versa). Harassment typically takes the form of disrupting an ongoing
copulation, and can be perpetrated by either males or females. Although interfering
females direct aggression at either copulating partner, males virtually always focus
exclusively on the rival male instead of the female. These patterns are collectively
inconsistent with the definition of sexual intimidation. Indeed, the data support
Smuts and Smuts’s (1993) hypothesis that female dominance in some lemurs
effectively deters coercion in the form of sexual intimidation. Even so, indirect
costs via sexual harassment apparently persist for female sifakas. The nature and
magnitude of these costs for female fitness remain unclear, however. Limitation of
female choice seems likely, but this possibility needs to be clarified quantitatively
(do less harassed females achieve their preferences more often?) as well as tested
against the alternative that female resistance functions as mate choice (see below).
Moreover, the mating benefits of harassment for the males remain obscure.
A quasi-experimental anecdote concerning ring-tailed lemurs (Lemur catta)
further supports the notion that female dominance limits sexual intimidation
(Parga and Henry 2008). Partly due to the effects of provisioning, a young female
reached sexual maturity at an earlier age than usual, but before she had developed
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social dominance over males. This young estrous female subsequently became
the target of direct aggression and even forced copulation attempts by a particular
adult male.
Data on the diurnal, group-living strepsirrhines also provide a relatively rare
primate example of support for the mate choice hypothesis for coercion. In ruffed
lemurs (Varecia variegata), female conspicuously resist male sexual overtures,
even resorting to physical aggression against them. Although males do not typically
retaliate with aggression of their own, both Foerg (1982: 119) and Morland (1993)
suggest that this sexual antagonism ensures that a female copulates with higher
quality (“strong”) males who “are more likely to overcome her beating” long
enough to achieve insemination.
Studies of anthropoid primates have made little effort to test between indirect
and direct costs to females. Japanese macaques (Macaca fuscata) were among the
first primates to provide data on sexual coercion, primarily in the form of chases of
estrous females or “possessive following” (Carpenter 1942; Itani 1982; Enomoto
1981). As Huffman (1987) points out, these patterns were often interpreted as
incidental components of male courtship or “precourtship” behavior (Itani 1982:
362), thereby implicating sexual harassment. Likewise, a key form of sexual
harassment the costs of mating with multiple males is reflected in the decreased
foraging efficiency of females on days they mated polyandrously, compared with
days they consorted with the alpha male only (Matsubara and Sprague 2004). Soltis
et al. (1997, p 725; 2001, p 486) conclude that male aggression to estrous females is
primarily a “side effect” of a general mating season increase in overall male
aggressiveness and female-maintained proximity to males. Although sexual intimidation does occur, it accounts for a minority of instances of sexual coercion.
Subsequent studies of mating-related aggression in this species, however, have
emphasized sexual coercion in Clutton-Brock and Parker’s (1995) sense of intimidation (Jack and Pavelka 1997; Soltis 1999; Soltis et al. 2001).
Indeed, this interpretation tends to emerge from many recent studies of male
aggression over mating in primates (e.g., Kuester et al. 1994; Perry 1997; Reed
et al. 1997; Boinski 2000; Colmenares et al. 2002; Arlet et al. 2008; Table 3.1 and
references above). This is partly because comparatively few investigations have
addressed the Clutton-Brock and Parker (1995) distinction between harassment and
intimidation (Soltis et al. (1997) being a notable exception) and have focused on the
processes of intimidetion implicit (or explicit) in (Smuts and Smuts 1993). But this
emphasis may also reflect the fact that many of the species studied are characterized
by gregariousness and male contest competition, which are conditions especially
likely to promote sexual intimidation.
One of the more compelling demonstrations of intimidation is provided by the
10-year study of the Kanyawara population of chimpanzees, Kibale, Uganda. It is
striking as well as suggestive of the biological significance of sexual intimidation that in a species well-known for male male aggression, male female
aggression occurs at roughly the same rate at Kanyawara (Muller et al. 2009).
The majority of this aggression involves male charging displays and chases, but
approximately 35% of it entails physical attacks on females (often in coalition with
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Bonding and Conflict Between the Sexes
65
other males). Muller et al. (2007) provide data by directly testing three predictions
of the Smuts and Smuts (1993) sexual coercion hypothesis:
Prediction 1: Sexual coercion is costly to females. The intensity of male aggression is difficult to quantify, but assaults on females can involve flailing with
branches, pummeling with fists, pulling of hair, and inflicting injuries (Goodall
1986). These attacks are typically assumed to carry costs, such as risk of infection
from wounds, but Muller et al. (2007) clarify potential costs with evidence that
cycling parous females, who are the primary targets of male coercion, have elevated
cortisol levels. The data cannot demonstrate that male coercion directly causes
hormonally mediated stress in females. A causal connection is suggested, however,
by the fact that, compared with parous females, nulliparous females copulated at
equivalent rates, spent similar (if not more) time in the company of males, but
received relatively less coercion from them (as less preferred sexual partners) and
had cortisol levels that were not only lower but that did not differ significantly on
estrous versus nonestrous days.
Prediction 2: Male mating success is improved by sexual coercion. Previous
primate studies had rejected this prediction based on the lack of a positive correlation between overall rates of male aggression to females and male mating success
(Bercovitch et al. 1987; Soltis 1999; Stumpf and Boesch 2006). Muller et al. (2009)
provide a more direct assay of the selective impact of sexual coercion by demonstrating that male chimpanzees copulated at significantly higher rates with females
that they were more aggressive to, than with females that they were less aggressive
to (Fig. 3.2).
Fig. 3.2 Median dyadic rates of aggression for each of 13 male chimpanzees (Pan troglodytes)
with 15 parous females. For each male, the median copulation rates were calculated with females
who received above (white) or below (black) the median amount of aggression for that male. The
difference was significant (Wilcoxon signed rank test, p 0.002). Data from Muller et al. (2007)
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R.A. Palombit
Prediction 3: Coercion is not simply an indirect cost of female choice. One of the
noteworthy aspects of this study is its test of the alternative hypothesis that male
aggression reflects female choice. Muller and colleagues marshal several lines of
evidence to reject the possibility that intersexual aggression is a by-product of
female mating preferences for aggressive males. First, male rank was uncorrelated
with aggression directed at females. Second, the relationship between male coercion and mating success with targeted females also held for low-ranking males as
well, who are arguably less preferred sexual partners. Finally, a matrix partial
correlation analysis revealed a significant association between male aggression
directed at individual females and the copulation rate with those females,
controlling for time spent together.
3.5
PostCopulatory Sexual Conflict: Prezygotic
Sexual conflict after copulation may involve processes occurring at or prior to
fertilization (prezygotic) or thereafter (postzygotic) (Table 3.1). The postcopulatory
manifestation of intrasexual selection is sperm competition (along with associated
factors such as genital locks, penis morphology, etc.), which has attracted much
study (e.g., Birkhead and Møller 1998). Postcopulatory intersexual selection is
cryptic female choice (Eberhard 1996), which primarily concerns the differential
treatment of sperm in the reproductive tracts of polyandrously mating females
(as well as associated phenomena, e.g., abortion). The important question here,
however, concerns the potential for conflict between these two postcopulatory
processes: how do the benefits to females of cryptic mate choice via multimale
mating compare with the costs incurred from male adaptations for sperm competition? Current data are too few to answer this question for primates. Although sperm
competition is relatively well investigated (Gomendio et al. 1998), cryptic female
choice remains virtually unstudied (Reeder 2003), with the possible exception
of H. sapiens (Baker and Bellis 1995; Thornhill and Gangestad 1996). Therefore,
I highlight below two areas where postcopulatory-prezygotic sexual conflict might
occur in primates.
3.5.1
Genital Coagulates
One possible source of conflict concerns enzymes acting on seminal vesicular
proteins to congeal ejaculates into structures ranging from a soft coagulum to a
more substantial, rubbery plug. Seminal coagulation is more pronounced in primates with multimale mating patterns (compared to unimale systems), suggesting a
male strategy to block rival sperm access to the cervical Os (Dixson and Anderson
2002). What is not known, however, is whether these coagulates impose costs on
females. Plugs can be dislodged by subsequent male partners or by inseminated
3 Bonding and Conflict Between the Sexes
67
females in L. catta (Parga 2003) and P. troglodytes (Dixson and Mundy 1994),
suggesting low potential for sexual conflict (at least over remating) or the existence
of an effective female counterstrategy to male manipulation. Intersexual conflict
may be more relevant in taxa where females cannot remove plugs, such as
M. murinus (Eberle and Kappeler 2004a). But even in these cases, conflict cannot
be assumed as plugs potentially confer benefits to females, such as facilitating
fertilization via sperm retention or transport. This could be valuable in a species like
M. murinus, in which females are in estrous for only a few hours on a single night
each year.
3.5.2
Penis Morphology and Female Injury
In strepsirrhines, keratinized penile spines, plates, or papillae are so conspicuous,
widespread, and variable as to have long informed taxonomy (Bearder et al. 1996).
Similar, but generally simpler, anatomical features are also found in a few platyrrhines and catarrhines (Dixson 1998). Spines develop upon sexual maturity (Perkin
2007), suggesting testosterone mediation and a mating-related function, but the
precise nature of that function remains obscure. Adaptive hypotheses include tactile
facilitation of ejaculation, removal of sperm or copulatory plugs, genital locking of
partners, stimulation of reproductive readiness in females or of synchrony between
partners, and Fisherian female choice (Dixson 1989; Eberhard 1990; Harcourt and
Gardiner 1994).
Comparative data from insects suggest an alternative explanation: sexual conflict. In the cowpea weevil (Callosobruchus maculatus), the penis is equipped with
spines that damage the female genital tract during copulation, reducing her likelihood of subsequent mating, and thereby enhancing sperm competition outcomes
for the male (Crudgington and Siva-Jothy 2000; Hotzy and Arnqvist 2009). In
primates, the magnitude of spinosity is negatively correlated with the duration of
female sexual receptivity during the ovarian cycle (Stockley 2002), suggesting that
penile spines similarly improve male sperm competition success by restricting
female mating. The precise mechanism underlying this association is unclear,
however. Penile spines could stimulate ovulation or associated neuroendocrine
reflexes, but they could also cause “short-term local damage to the female genital
tract, making continued sexual activity painful or aversive” (Stockley 2002, p 130).
Correspondingly, sexual conflict theory may shed light on the function of
human practices of genital modification (e.g., Wilson 2008). The patterns and
frequency of female genital cutting vary substantially across populations, and the
effects on female (and male) sexual behavior and reproduction are strongly debated
(Gruenbaum 2001). Reason (2004) argues that in one West African population, the
practice enhances female reproductive success because it is a virtual prerequisite
for marriage and because men invest significantly more in the offspring of wives
who are circumcised. Both patterns are consistent with a sexual conflict interpretation, but clearly more study of human behavioral ecology in the context of relevant
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cultural influences is needed to test this hypothesis against alternative explanations.
As Low (2005, p 76) concludes, although current data on genital modification “may
not prove [sexual] conflicts of interests, they are suggestive.”
3.6
Postcopulatory Sexual Conflict: Postzygotic
Precopulatory intimidation by male chimpanzees can only be fully understood in
the context of postcopulatory sexual conflict in the form of infanticide. Muller et al.
(2009) argue that sexual coercion, particularly as practiced by high-ranking males,
is a counterstrategy to limit female promiscuity, and that promiscuity is itself a
counterstrategy to male infanticide (see also Stumpf et al. 2008). This scenario
highlights the nature of sexually antagonistic coevolution: male infanticide favors
female promiscuity, which favors male sexual coercion, etc.
Infanticide figures prominently in Smuts and Smuts’ (1993) original discussion
of sexual coercion, but it does not fit easily within Clutton-Brock and Parker’s
(1995) more general harassment-intimidation dichotomy. It is initially difficult to
appreciate that male infanticide might constitute incidental harm to females, since
an infant’s death seems so directly detrimental to the mother’s fitness. But this
proposition becomes clearer when we consider that the specific “problem” that
lactating females pose to reproducing males is a straightforward consequence of
primate biology: a nursing infant is, in the words of Altmann et al. (1978: 1029), a
“perfect contraceptive.” The function of sexually selected infanticide, then, is to
disrupt this contraceptive system, not to harm the mother (or reduce her fitness)
per se. Thus, following the broader theoretical logic of Partridge and Hurst (1998)
and Lessells (2005), if, speculatively, males possessed other means of effectively
counteracting the contraceptive e.g., by manipulating the mother’s hormonal state
or accelerating weaning and if the costs of such a strategy did not exceed the
costs of infanticide, then males would not be selected to kill infants (but could
still achieve the same reproductive benefit). Under such conditions, the death of
infant, if it occurred, would be an incidental by-product of the male manipulative
strategy, not a necessary harmful component of that strategy. This is not to say that
male attacks on infants can not, in principle, function as sexual intimidation, if
their mothers’ mating compliance forestalls further aggression directed at them.
As Clutton-Brock and Parker (1995) point out, however, this mechanism of
indirect sexual intimidation predicts that male threats and attacks will also
extend to juveniles, which is neither predicted by the sexual selection hypothesis
nor is a common correlate of infanticidal behavior (Hrdy 1974; van Schaik and
Janson 2000).
Male infanticide is still, however, a drastic form of sexual conflict. It reflects
adaptive harm (sensu Johnstone and Keller 2000) insofar as infanticidal males
exploit a preexisting feature of female reproductive plasticity, such that infant
loss often accelerates resumption of ovulatory cycling. Although the adaptive
significance of infanticide in primates continues to be debated, the available
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Bonding and Conflict Between the Sexes
69
evidence is more consistent with the sexual selection argument (Borries et al. 1999;
Soltis et al. 2000; van Schaik 2000b) than with competing hypotheses, such as the
generalized aggression model (Bartlett et al. 1993) and the social pathology argument (Dolhinow 1977). Thus, infanticide appears a likely manifestation of postcopulatory sexual conflict in primates, as well as, arguably, the most studied form of
sexual conflict (Hausfater and Hrdy 1984; van Schaik and Janson 2000).
3.7
A Counterstrategy: Male–Female Association
The counterstrategies to sexual conflict are as diverse as the manifestations of
conflict itself. They may be morphological, such as the thick skin of female blue
sharks (Prionace glauca) vulnerable to bites from “courting” males (Pratt 1979), or
the large body size of some female lemurs, which is argued to limit sexual coercion
during the mating season (Foerg 1982; Taylor and Sussman 1985; Richard 1992;
Morland 1993; Brockman 1999).
Female sexual behavior particularly promiscuity can limit sexual conflict,
both in the form of precopulatory coercion and postcopulatory infanticide. The
convenience polyandry hypothesis holds that conceding copulations allows females
to avoid the costs of resistance to coercive males (Thornhill and Alcock 1983;
Mesnick and le Boeuf 1991; Blyth and Gilburn 2006). This explanation is less often
invoked as an anticoercion counterstrategy in primates than in other animals, but
one example is Eberle and Kappeler’s (2004a, p 97) argument that the multimale
mating of female mouse lemurs reflects “ ‘making the best of a bad job’ in the face
of male harassment.” The counteractive value of convenience polyandry is improved when it is supplemented with postcopulatory mechanisms of cryptic female
choice (e.g., spermicides) (Holman and Snook 2006), but this remains unstudied in
nonhuman primates. In the postcopulatory domain, both theoretical models and
empirical evidence suggest that female promiscuity offers significant potential to
limit infanticide by confusing paternity (Hrdy 1979; Ebensperger 1998; van Schaik
and Janson 2000; Wolff and MacDonald 2004; Pradhan and van Schaik 2008).
Association with males is a hypothesized female counterstrategy to sexual
conflict, again in both the form of sexual coercion and of male infanticide. Sustained proximity to a large, dominant male reduces estrous female exposure to
male harassment and intimidation in Japanese macaques (Matsubara and Sprague
2004) and chimpanzees (Wrangham 1986), as well as in many other taxa (insects:
Thornhill and Alcock 1983; fish: Pilastro et al. 2003; Dadda et al. 2005; birds:
Gowaty and Buschhaus 1998; bighorn sheep: Réale et al. 1996; elephant seals:
Mesnick and le Boeuf 1991). This function has also been suggested for the
temporary consortships of female orangutans at risk of forced copulation (Mitani
1985; Fox 2002; Setia and van Schaik 2007). Thus, protection from sexual coercion
is an alternative functional hypothesis for consortships, independent (though not
mutually exclusive) of mate guarding, and female choice hypotheses (Manson
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1997). The relevance of this hypothesis for understanding variation in consortships
has not been explored thoroughly.
Reducing the costs of precopulatory sexual harassment may similarly underlie
sexual swellings. Previous analyzes have suggested that sexual swellings might
benefit females because they incite male male competition, which then facilitates
insemination by high-quality males (Clutton-Brock and Harvey 1976) or copulation
with many males (Hrdy and Whitten 1987). Alternatively, sexual swellings might
serve to reduce the costs of harassment or intimidation by ensuring mate guarding
by a dominant male who keeps other males away. The adaptive value of this
counterstrategy, however, must be measured against the (coercion) costs of advertising estrous, the benefits of multimale mating, and the benefits of the alternative
counterstrategy of reducing coercion via concealment of receptivity.
Male female association is also a proposed counterstrategy to postcopulatory
conflict in the form of infanticide (Wrangham 1979; van Schaik and Dunbar 1990;
van Schaik and Kappeler 1997). Empirical evidence supports this hypothesis in
numerous taxa, including insects, birds, and rodents, and a few primate species
(reviewed by Palombit 2000). Mountain gorilla (Gorilla beringei) groups have long
been viewed as associations of females with a male protector, but whether he deters
infanticide or predation is debated. A recent mathematical simulation supports
the antiinfanticide hypothesis (Harcourt and Greenberg 2001), but Harcourt and
Stewart (2007) argue that rejection of the antipredation hypothesis is premature.
Recently, this argument was extended to orangutans with Setia and van Schaik’s
(2007) suggestion that lactating females use male long calls to stay loosely
associated with adult male protectors.
Van Schaik and Dunbar’s (1990) hypothesis that social monogamy is an antiinfanticide strategy remains one of the most interesting versions of this hypothesis.
Evidence that infanticide has selected for social monogamy is strong in some
nonprimate taxa such as burying beetles (Nicrophorus spp.) and tropical house
wrens (Troglodytes aedon), but interpretations of the gibbon data have generated
divergent conclusions (Palombit 1999, 2000; Sommer and Reichard 2000; Fuentes
2002; van Schaik and Kappeler 2003). Recent tests of the hypothesis in prosimians,
such as fork-marked lemurs (Phaner furcifer), avahis (Avahi occidentalis), and
spectral tarsiers (Tarsius spectrum), have not consistently supported the hypothesis
(Schülke and Kappeler 2001; Thalmann 2001; Gursky 2002). However, this
intriguing hypothesis awaits further direct testing in the taxa it primarily addresses:
the gibbons.
One population in which long-term data continue to suggest an antiinfanticide
function of male female bonding is the chacma baboon (Papio hamadryas griseipes)
of the Okavango Delta, Botswana (see also Weingrill 2000). Like yellow baboons
(P. h. cynocephalus) and olive baboons (P. h. anubis) of east Africa, these baboons
live in relatively large, multimale, multifemale groups, with female philopatry and
dominance relationships in both sexes. In contrast to its east African cousins,
however, the chacma baboon exhibits comparatively high rates of infanticide
(Palombit 2003). Infanticide is the primary source of mortality for infants, and
accounts for at least 38% of infant mortality, though this rate may be as high as 75%
3 Bonding and Conflict Between the Sexes
71
in some years (Cheney et al. 2004). The patterning of infanticide in this population
is more consistent with the sexual selection hypothesis than with alternative
hypotheses (Palombit et al. 2000). Infanticide is generally committed by males
that have recently immigrated into a group and attained alpha status. The relatively
short tenure of alpha males (approximately 7 months, on average) combined with
their apparently greater monopolization of matings (Bulger 1993) creates conditions that enhance the potential benefits of infanticide. In other words, a new alpha
male is confronted with a short period of relatively exclusive sexual access to
females. Conversely, since loss of an infant significantly accelerates resumption of
fertile cycling in females, lactating mothers are confronted with a significant threat
of infanticide.
Unsurprisingly, lactating females exhibit conspicuous and aroused aversion to
newly immigrated alpha males, including continual retrieval of infants, screaming,
and tail-up displays (Busse 1984). They almost always establish a “friendship” with
an unrelated, adult male shortly after parturition (Busse 1981; Palombit et al. 1997).
Friendships can be unambiguously differentiated from a female’s relationships with
other males in the group on the basis of spatial association, grooming, infant
handling, and vocal interaction (reviewed by Palombit 2009). Ad libitum evidence
suggests that friendship status increases a male’s defense of infants during potentially (or actual) infanticidal attacks. Although several males may rush to the scene
of such attacks, it is primarily the male friend of the infant’s mother who provides
direct, apparently costly forms of defense, such as fighting or threatening the alpha
male, or carrying the infant. Experimental playback experiments further showed
that male friends were more likely to respond to their female friends’ screams than
to the screams of other females, and females’ screams were more likely to provoke
responses form their male friends than from other males (Palombit et al. 1997).
These experiments also revealed that the solicitude of male friends was tied closely
to the presence of infants: playback of female screams shortly after infants died
elicited similarly weak responses from all males, regardless of their friendship
status. Alternative benefits of friendships to females, such as protection from
harassment from higher-ranking females, lack empirical support (Palombit 2009).
Since these original observations, a series of hormonal studies in this population
have further supported the antiinfanticide function of heterosexual friendships.
Following the immigration of a new male, glucocorticoid levels rise in females
generally, but remain high over subsequent weeks only among anestrous females,
not among cycling females (Beehner et al. 2005; Wittig et al. 2008). This is a
striking difference because cycling females are the primary targets of the protracted, aggressive chasing that seems to facilitate a new male’s rise to alpha status
(Kitchen et al. 2009). Thus, hormonal patterns suggest that it is females at risk of
infanticide (not simply of aggression) from the new male who experience greater
stress upon his arrival in the group. This is further substantiated by additional
increases in glucocorticoids among lactating females when a new alpha actually
commits an infanticide (Engh et al. 2006) or among the (few) lactating females who
lack male friends at the time of male immigration (Beehner et al. 2005).
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A final indication of the potential importance of friendships is that females
compete with one another for them (Palombit et al. 2001). This is reflected partly
by the positive correlation between the dominance ranks of male and female
friends, and partly by observations of high-ranking females displacing subordinate
rivals from friendship with a particular male. Competition among females for
males is relatively rare in mammals (Berglund et al. 1993; Andersson 1994), and
in this case, it suggests that males provide a service with important fitness consequences for females. It is not immediately obvious why male protection is not
shareable among multiple lactating females, but since friendship status appears so
crucial, females may compete for social access to males in order to develop this
relationship.
Sexual conflict hypotheses for male female bonding are potentially relevant to
understanding human pair-bonding, although space precludes a thorough treatment
of this question here. Early models argued that a durable pairbond between the
sexes was part of an adaptive suite of traits including reproductive monogamy and
a division of labor between females and provisioning males (Murdock 1949;
Washburn and Lancaster 1968; Lovejoy 1981). An alternative hypothesis emphasizes the importance of male protection of females from sexual conflict in the form
of sexual coercion and/or infanticide (Betzig 1992; Smuts 1992; Mesnick 1997;
Hrdy 1999; Hawkes 2004). A recent cross-cultural analysis rejected the male
protection hypotheses partly because pairbond stability (overall divorce rates in a
society) was uncorrelated with general male aggressiveness (overall rates of male
homicides and assaults) (Quinlan and Quinlan 2007). However, this conclusion is
limited in the same way that the lack of a correlation between overall male
aggressiveness and mating success in chimpanzees may overlook the fact that
sexual coercion significantly increases a male’s mating success with the particular
females he targets (see above). Thus, the hypothesis must be tested with human data
addressing specifically how risk of sexual coercion or infanticide to individual
women varies with the nature of their pair bonds. Since male partners are themselves sometimes a source of sexual coercion to women (Rodseth and Novak 2009),
these analyses must differentiate between the costs of pair-bonding with men and
the protective benefits of pair bonds from other men. The variety of current
evidence suggests the possibility that the different selective pressures proposed
may each promote pair-bonding under different conditions (Quinlan 2008). This
proposition merits greater scrutiny.
3.8
Conclusions and Future Directions
Sexual conflict is inevitable and ubiquitous: the question is not whether it occurs,
but how and when, and to what degree sexually antagonistic coevolution has acted,
compared with other mechanisms of sexual selection (Hosken and Snook 2005: S1,
Andersson and Simmons 2006). Sexual conflict theory situates explanations in the
“arms race” perspective previously reserved for more conventional coevolutionary
3 Bonding and Conflict Between the Sexes
73
adversaries, such as predators and prey (Dawkins and Krebs 1979). The Red Queen
hypothesis, that any gain in fitness by one unit of evolution is balanced by equivalent
losses in fitness by others (van Valen 1973), may provide the most appropriate
framework for analyzing reproductive strategies as a zero-sum game between
opposing males and females (Chapman and Partridge 1996; Rice and Holland
1997). This does not mean, however, that conflict universally characterizes the
phenotypic expression of male and female interaction. Affiliation and intersexual
cooperation may be one outcome of this coevolutionary conflict, as suggested for
chacma baboon friendships. Indeed, the chacma baboon and chimpanzee together
highlight the view of universals as process, rather than as pattern. Current
evidence suggests that sexually selected infanticide has generated two distinct
modes of female counterstrategy in these species: promiscuity and association
with males. The patterns are different, but the underlying process that generates
the patterns is the same: sexual conflict. This chapter has focused mostly on sexual
conflict over mating, but it may also occur at the level of sex chromosomes, gamete
interaction, parental investment, group size and composition, and group dynamics
(Table 3.1).
Sexual coercion via intimidation/punishment is likely to be a common, if not
universal feature of life among animals that live gregariously and modify their
behavior through learning (Clutton-Brock and Parker 1995). The attention following
the publication of the Smuts and Smuts (1993) model has enlarged the data base for
male mating aggression to females. Somewhat surprisingly, however, relatively few
studies have rigorously tested the full set of constituent predictions (but see Muller
and Wrangham 2009) or differentiated analytically between sexual harassment and
intimidation. Costs to females are often an assumed rather than measured consequence of overt aggression, or are assessed qualitatively (e.g., as an “injury”). A key
goal for future studies is quantitative measurement of these costs (as Muller et al.
[2009] do). These data will help address some other questions: do females do worse
reproductively when mating with more coercive or persistent males, as predicted by
theory? The hypothesis that females may derive benefits from coercion also merits
greater study. Likewise, the costs of coercion to males are virtually ignored, but may
be significant. For example, the seminal fluids of bushcrickets inhibit receptivity of
females to further mating in a manner similar to D. melanogaster, but males that
deliver greater quantities of these fluids also experience longer sexual refractory
periods themselves (Vahed 2007). Information on costs to males, combined with data
addressing covariation in male coercion and fitness, will help to clarify the trade-offs
of coercion or manipulation of females versus alternative mating strategies. Most
primate studies of sexual conflict have focused on sexual coercion, but male manipulation in the form of antagonistic seduction, and concomitant females resistance
(Holland and Rice 1999) merits more attention.
The life history of primates, as well as the practical constraints on an experimental study of them, significantly limit the kinds of data that can be collected.
Nevertheless, there are compelling reasons to study sexual conflict in primates.
Until fairly recently, much of the research on sexual conflict was conducted on
(invertebrate) taxa that conform more or less to the Bateman (1948) principle that
74
R.A. Palombit
males are selected to mate and females not (Partridge and Hurst, 1998, Tregenza
et al. 2006). Our understanding of the full significance of sexual conflict will be
improved by greater study of systems violating this assumption, i.e., taxa in which
remating is potentially beneficial to females. Additionally, as Clutton-Brock and
Parker (1995) emphasize, models of sexual conflict have generally focused on
relatively simple social contexts. The study of highly social species promises to
reveal important and subtle influences of social relationships on the economic
trade-offs of sexual coercion and resistance. In spite of the methodological difficulties they pose, primates are excellent subjects to achieve all of these goals.
In summary, conflict among genes is “a universal feature of life” (Burt and
Trivers 2006, p 3). This is true not only for genes within a genome, but also for
genes residing in the genomes of the interacting entities we call “male” and
“female.”
Acknowledgments I am extremely grateful to Peter Kappeler and Joan Silk for their generous
cooperation and advice in the preparation of the manuscript. Field research was funded by NSF
(BCS 0117213), the Leakey Foundation, the Wenner Gren Foundation and the Center for Human
Evolutionary Studies (Rutgers University).
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Part III
Politics & Power
Chapter 4
The Unusual Women of Mpimbwe: Why Sex
Differences in Humans are not Universal
Monique Borgerhoff Mulder
Abstract Parental investment theory provides a strong basis for generalizations
about how male and female mating strategies might vary, and has generated a large
number of successful predictions regarding gender differences in human reproductive strategies. There are, however, many situations in which traditional sex roles
are not observed, and behavioral ecologists are beginning to determine how and
why this might be. In this chapter, I explore the implications of generalizations
about universal sex differences for our understanding of gender differences in
sexual and reproductive strategies of humans. First, I examine recent work within
behavioral ecology on the status of parental investment as a determinant of sex
differences in reproductive strategies. Second, I summarize analyses of reproductive strategies in a rural forager-horticultural population in western Tanzania where
variance in women’s reproductive success is not significantly different from that of
men and where women use serial matings rather more effectively than do men to
outcompete their competitors, to show that key sex differences predicated on the
mammalian pattern of parental investment are not necessarily observed. Third, I
broaden this discussion of an obvious ethnographic exception to examine the
relationship between human pair bonds and parental investment, to show again
that sex differences in parental investment provide only a partial story. The
implications of these observations for claims of universal sex differences and the
gap between studies of human and nonhuman reproductive strategies are discussed
in the conclusion.
M.B. Mulder
Department of Anthropology, University of California at Davis, Davis, CA, USA
e mail: mborgerhoffmulder@ucdavis.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 4, # Springer Verlag Berlin Heidelberg 2010
85
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4.1
M.B. Mulder
Introduction
Much of the legitimacy of applying evolutionary approaches to the study of human
behavior has been predicated on the existence of universal sex differences. In our
species, men are on average taller (Alexander et al. 1979) and stronger than women,
and die earlier and from different causes than women (Teriokhin et al. 2004).
Additionally, they are generally thought to show higher variance in reproductive
success than women (Barrett et al. 2002). These apparently universal sex-varying
traits can be attributed to the common mammalian pattern of reproduction, in which
gestation and lactation fall exclusively to women, paternity certainty is never
assured, and even small amounts of paternal care are provided facultatively (Trivers
1972). As such, male fitness has, for a long time, been seen as limited by competition over access to females, and female fitness limited by access to resources that
can often be acquired through males (Emlen and Oring 1977; Wrangham 1980).
Starting in the 1980s, predictions derived from parental investment theory
sparked an evolutionary literature addressing human reproductive and mating
strategies (reviewed in Cronk et al. 2000; Low 2000; Dunbar and Barrett 2007).
Several findings emerge that suggest (or at least are interpreted as) human universals. For example, rich ethnographic and comparative studies demonstrate the
prevalence of competition among men over women (Irons 1979; Betzig 1986;
Chagnon 1988; Daly and Wilson 1988; Hawkes 1991), although mating competition is mediated through diverse avenues such as political office, murder, wealth
accumulation, or the provision of public goods. Similar kinds of work explore how
women (or their parents on their behalf) choose and compete for desirable mates
(Dickemann 1979; Buss 1989; Gangestad and Simpson 2000), again through a
variety of means, including cognitive preferences, dowry payments, and olfactory
cues. Sex differences in mating preferences are also evident, with men tending to
favor health and fecundity in their mates whereas women look for ambition and
resources, as evidenced both by reported preferences (e.g., Buss 1989; Cashdan
1993) and actual behavior (e.g., Borgerhoff Mulder 1989, 1990). The exquisite
sensitivity of mechanisms underlying such preferences to ecological and social
circumstances (reviewed in Gangestad 2007) have helped to bring the study of
human behavior into mainstream evolutionary theory, as well as to promote popular
awareness of humans as yet another uniquely evolved species (e.g., Ridley 1994).
With the success of this work, there nevertheless emerged a dangerous tendency to
generalize from specific observations to universal sex differences. Such generalizations are problematic for several reasons (e.g., Smith et al. 2001). First, recognizing
the importance of culturally transmitted norms Boyd and Silk (2005, using Buss’s
1989 data) demonstrate how cultural factors explain a great deal more of the crosscultural variation in mate choice preferences than does gender. Second, objecting to
the stereotypic portrayal of women as at the mercy of their biology and the antics of
men, Hrdy (1986), Smuts (1992) and Gowaty (1997) provide cogent qualitative and
quantitative support for the view that women can and do operate with agency and
employ a wide array of strategies to subvert and counter the strategies of men. Third,
4 Sex Differences in Humans
87
anthropologists and others emphasize the importance of the social and ecological
environment in generating variability not fixity in sex roles according to principles
well established in behavioral and cultural evolutionary theory (e.g., Laland and
Brown 2002).
In recent years, studies of sex differences have become more nuanced. In part,
this reflects a growing awareness among psychologists and feminist scholars that
gender differences have often been inflated, even derived from poor science
(Shibley-Hyde 2005). Evolutionary social scientists too are using ethnographic
data to emphasize the flexibility in gender roles, and the overlap in gender differences (e.g., Bliege Bird and Bird 2008). In fact, it is now indeed time to ask “are
men and women really all that different?” (Borgerhoff Mulder 2004; Brown et al.
2009), and to reevaluate the extent to which sex differences in human reproductive
strategies are contingent on sex differences in postzygotic investment.
In this chapter, I scrutinize the notion of universal sex differences in the
reproductive strategies of men and women. My goal is cautionary. I do not argue
that parental investment theory is wrong, but rather that other factors need to be
taken into consideration, factors that may be of particular importance in humans. To
demonstrate this point, I present empirical data on a horticultural-hunter-fisher
population in Tanzania (Pimbwe) where the variation in fitness among women
equals the variance in fitness of men and, quite contrary to the normative pattern,
women benefit more from multiple marriages than do men. Finally, I consider how
anthropologists think about the relationship between pair bonding and parental
care. I finish by considering what my conclusions mean for the gap between
human and nonhuman studies, the theme of this volume.
4.2
Parental Investment Theory and Beyond
Models predicated on the differential postzygotic investment of males and females
(Trivers 1972) have dominated the study of sexual and reproductive strategies in
most mammals, and provided a theoretical context for the classic finding that
males benefit more from multiple matings than do females. Key to this discussion
has been the regression of reproductive success on mating success, known as
“Bateman’s gradient (Bateman 1948). Whichever sex has the steepest gradient is
the sex that experiences the stronger sexual selection pressure on traits that enhance
mating success (Andersson and Iwasa 1996).
In mammals, gestation and lactation fall exclusively to females, paternity certainty is never assured, and paternal care is provided facultatively. Therefore male
fitness is seen as limited by competition over mates, and female fitness by access to
resources that can in some but not all cases, be acquired through males (Emlen and
Oring 1977; Wrangham 1980). Thus, the reproductive strategies of each sex, in
particular decisions over mating effort and parenting effort, are analyzed as a
product of sex differences in parental investment. Trivers’ model (in an expanded
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M.B. Mulder
form that deals more explicitly with the operational sex ratio and potential reproductive rates, Clutton-Brock and Vincent 1991) can in fact predict much of the
variation in sexual selection across taxa and has important implications for sex
roles. As noted earlier, its successes in predicting human sex differences in reproductive strategies brought prominence to the new discipline of evolutionary social
science (Borgerhoff Mulder et al. 1997).
In the intervening years, theoretical and empirical work in behavioral ecology has
taken a richer and more dynamic approach to sex roles (reviewed in Borgerhoff
Mulder 2009a). First, there has been a rethinking of the internal logic and consistency of Trivers’, and specifically Maynard Smith’s (1977) model (Queller 1997;
Houston and McNamara 2005; Kokko et al. 2006). These revisions do not change
the basic prediction that the caring sex is more likely to be choosier and the object of
more competition, but fundamentally alters the evolutionary sequence. In the conventional sequence, differences in prezygotic investment determine potential reproductive rates which then shape payoffs to postzygotic care. In the revised sequence,
prezygotic investment generates the conditions for sexual selection as numerically
abundant male gametes compete for access to rare female gametes. This lowers the
confidence of males in paternity and, given male-male competition for access to
females (and/or female choice), creates an elite subset of males that are more eligible
to mate (Kokko and Jennions 2003). This revised logic gives more salience to sex
differences in competition over mates and less to sex differences in parental care.
Second, and independent of these revisions, both theoretical and empirical work
shows that anisogamy does not always produce classic sex roles (Gowaty 2004) and
that competition and choice are not mutually exclusive (Kokko et al. 2006), as
indeed long recognized in empirical studies of nonhuman primates (Hrdy 1986). In
other words, choosiness is not simply a function of operational sex ratios, with the
limiting sex enjoying the luxury of choice; it is also dependent on variance in
quality among potential mates (Owens and Thompson 1994; Johnstone et al. 1996),
the costs of reproduction (Kokko and Monaghan 2001; Maness and Anderson
2007), and extrinsic survival rates (Gowaty and Hubbell 2005).
Third, there is evidence that there are some species in which females are the
principal caregivers, but compete more frequently and more intensively with each
other than do males. In meerkats (Suricata suricatta, Clutton-Brock et al. 2006) and
many other cooperatively breeding vertebrates (Holekamp et al. 1996; Hauber and
Lacey 2005), females gain greater reproductive benefits from dominance than do
males (e.g., Engh et al. 2002, for spotted hyenas, Crocuta crocuta), and accordingly
are more competitive with one another, thereby demonstrating that sex differences
in parental investment are not the only mechanism capable of generating sex
differences in reproductive competition. Finally in some species, notably cooperative breeders with single breeding pairs, sex differences in fitness variances are
unrelated to differences in mate number, thus providing evidence that counters
Bateman’s gradient (the idea that males benefit more from multiple mates than do
females, Hauber and Lacey 2005). Higher female than male variance in fitness is
also observed in sex role-reversed species such as dusky pipefish, Syngnathus
floridae (Jones et al. 2000) and wattled jacanas (Jacana jacana) (Emlen and
4 Sex Differences in Humans
89
Wrege 2004). Recognition of these additional selective considerations generates a
much richer picture of how competition and choice can figure in the strategy of each
sex and how these may vary over the life time and across populations.
In short, contemporary perspectives within behavioral ecology provide a broader
framework within which to study the great diversity of sex differences in nature
than that afforded by the simple parental investment model that guided seminal
work in the evolutionary social sciences until the late 1990s.
4.3
The Unusual Women of Mpimbwe
The Pimbwe live in the Rukwa Valley of present day western Tanzania. Impacts
from German, Belgian, and British colonial escapades in this central African region
were indirect (Tambila 1981), but colonial wildlife policies had more severe
impacts, effectively displacing Pimbwe from parts of their traditional chiefdom
(Borgerhoff Mulder et al. 2007). In the socialist era (mid 1970s), Pimbwe families
were settled in government villages, but many have now returned to ancestral lands
that lie outside areas protected for wildlife. Modern Pimbwe rely primarily on a mix
of subsistence and cash crops, supplemented by foraged resources and poultry
keeping. Small enterprise activities, such as trading, traditional medicine, hunting,
fishing, honey production, carpentry, and beer brewing supplement farm income for
men and women. Livelihoods are unpredictable because of highly seasonal rainfall
that creates critical periods of food shortage and labor demand (Wandel and
Holmboe-Ottesen 1992; Hadley et al. 2007), poor infrastructure that makes cash
cropping risky, and very poor health services. Between 40 and 50% of households
in the district fall below the basic needs poverty line (United Republic of Tanzania
2005), and development initiatives are seriously jeopardized by prevalent beliefs in
witchcraft. These and following general observations are based on intermittent
fieldwork between July 1995 and February 2008, as well as previous studies in
the area.
The traditional marriage pattern, reported as clan controlled, monogamous, and
accompanied by bridewealth (Willis 1966), must have been seriously challenged by
the high rates of labor outmigration in the colonial period (Tambila 1981). Marriage
is now effectively characterized by cohabitation, initiated with a facultative transfer
of bridewealth and a celebration (Fig. 4.1). Polygyny appears never to have been
common. Nowadays, marriage can be defined as sharing in the production and
consumption of food and shelter, with the expectation of exclusive sexual relations.
Divorce is permitted and, like marriage, can be defined by the physical movement
of one or both partners out of the house, requiring no legal or formal procedures.
Divorces occur often when one spouse starts an extramarital relationship, with both
sexes tending to claim responsibility for abandoning the relationship. At divorce,
children under the age of 8 are supposed to stay with the mother (or the mother’s
kin), whereas older children should stay with their father. In practice, the fate of
children is quite variable. Sometimes fathers “kidnap” very young children from
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M.B. Mulder
Fig. 4.1 A longterm monogamously married husband and wife sitting outside their house in
Mirumba
their mothers, sometimes mothers leave a recently weaned child with a divorced
husband; older children may live with a range of maternal or paternal kin.
Given these residence patterns, parental care is highly facultative. Wives typically take primary responsibility for the direct care of their own small children, with
some assistance from older children and/or other kin, including their own mothers
or husband’s mothers. Regarding indirect care, the bulk of farming is done by
husbands and wives, but there is considerable variability within marriages as to how
the fruits of joint farm labor are allocated among family subsistence needs, joint
family benefits (like health and education), individual cash purchases, or capital for
individual economic enterprises (such as using maize for beer brewing). These
allocations prompt frequent spousal arguments, and one spouse may even place
locks on the family granary to exclude “inappropriate” use of resources by the other
spouse. There are no significant heritable resources in this population; men and
women get access to land and houses opportunistically from maternal or paternal
relatives (or from unrelated individuals) who happen to have unused land or living
sites available in the village. Commonly they clear land and build houses anew,
such that there is very little to inherit in the way of bequests.
Basic demographic data were collected in all households of a single village in
seven different study periods between 1995 and 2006 (for details see Borgerhoff
Mulder 2009a) and analyses include only individuals who are assumed to have
neared completion of their reproduction (>44 years), yielding 138 men with a mean
age of 60.3 years (range 45.3 92.7) and 154 women with a mean age of 59.2 years
(range 45.0 86.8) dropping younger men (<55 years), produced statistically equivalent results. Variables used in the analyses presented here are age, sex, number of
livebirths, reproductive success (measured as the number of offspring surviving to
4 Sex Differences in Humans
91
a
25
Men
Women
Frequency
20
15
10
5
0
0
5
10
15
20
0
5
10
15
20
Fertility
b
25
Men
Women
Frequency
20
15
10
5
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Offspring surviving to five years
Fig. 4.2 Variance in (a) fertility and (b) reproductive success for Pimbwe men and women
5 years of age, beyond which mortality is low), and number of spouses over the
lifetime, categorized as 1, 2, 3, or more.
Among men and women who had completed their reproductive careers, only 3
(2.2%) men and 2 (1.3%) women had never married, indicating that marriage is
virtually universal in this population. The mean values of fertility (men 8.41;
women 8.17) and reproductive success (men 5.99; women 6.14) are not statistically
different from each other, which suggests that there is no distortional sex bias to the
sample. While men show greater variance in fertility (16.16) than women (11.34,
Levene’s test for equality of variances F ¼ 5.87, p ¼ 0.016, Fig. 4.2a), there is
no significant difference in the variances in completed reproductive success (men
9.00; women 7.27, Levene’s test F ¼ 2.15, ns, Fig. 4.2b).
When fertility (Fig. 4.3a) and the numbers of offspring reaching 5 years of age
(Fig. 4.3b) are shown in relation to number of spouses (1, 2 and 3 or more), an
unexpected pattern emerges. Men fail to benefit in terms of fitness from multiple
marriages, but women who marry three or more times produce more surviving
children than do other women. Fertility and completed reproductive success are
regressed on age, sex, and the number of spouses in a number of different models
(Table 4.1a, b). Generally, across models, the number of spouses is negatively
92
M.B. Mulder
a
Mean+-2 SE
Fertility
11
Men
Women
10
9
8
7
6
5
1
Mean+-2 SE
Surviving offspring
b
9
2
>2
1
Men
2
>2
Women
8
7
6
5
4
3
1
2
>2
1
2
>2
Number of spouses
Fig. 4.3 The associations between number of spouses and (a) fertility and (b) reproductive success
for Pimbwe men and women. The mean is shown with a circle, and the standard error (*2) with a
bar. For statistics see Table 4.1
Table 4.1 Regression models for how sex, age, and number of spouses affect fertility and number
of surviving offspring (showing beta, standard error, and significance). (a) fertility, (b) number of
surviving offspring
(a)
Model 1
Model 2
Model 3
Model 4
Sex
0.244 (0.433)
0.172 (0.425)
0.285 (0.415)
1.862 (0.985)þ
Age
0.067 (0.020)*** 0.061 (0.020)** 0.067 (0.020)***
No. of Spouses
0.516 (0.284)þ
2.045 (0.912)*
Sex* No. of spouses
1.015 (0.575)þ
(b)
Model 1
Model 2
Sex
0.144 (0.333)
0.174 (0.332)
Age
0.028 (0.015)þ
No. of spouses
Sex*#Spouses
***p<0.001, **p<0.01, *p<0.05, þ 0.05<p<0.10
Model 3
0.117 (0.328)
0.022 (0.015)
0.223 (0.224)
Model 4
1.281 (0.776)
0.028 (0.015)þ
1.578 (0.718)*
0.899 (0.453)*
associated with fertility and the number of surviving offspring, and there are
interaction effects between spouse number and sex, which reflect the pattern
shown in Fig. 4.3, namely that men suffer reproductively from multiple marriages
in a way that women do not. The models also show that age is positively associated
with fertility and less consistently with reproductive success, suggesting (in this
postreproductive sample) that levels of fertility were slightly higher in cohorts that
4
Sex Differences in Humans
93
finished reproduction in the 1980s than in the 2000s, which is to be expected in a
community where some individuals are beginning to choose smaller family sizes.
In this population, sex differences in fitness variances are not pronounced. While
men do show greater variance in the number of live births, reproductive success is
equally variable across the sexes, suggesting that men with very high fertility raise
few of these “extra” children. A sex difference, or lack thereof, in variance does not,
however, shed much light on the operation of sexual selection (Sutherland 1985;
Hubbell and Johnson 1987). Much more illuminating is the relationship between
breeding success and physiological or behavioral phenotypes (Clutton-Brock
1988). Thus, the finding that men and women benefit differently from multiple
marriages is interesting. While the data are very variable (large standard errors),
women appear to gain more from multiple mating than do men; furthermore, the
same statistically significant effect is observed as a control variable in a different
sample of younger women (Borgerhoff Mulder 2009b). Note, however, that it
would be analytically more revealing to look at the probability of bearing (and
successfully raising) a child as a function of the marital status of the parents the
present analysis shows only that reproductive performance is correlated with the
number of spouses married over the lifetime. Note also that these findings take no
allowance of the marital status of the spouse. It is tempting to think that women who
have married many men should be married to men who have had many wives, but
this is not necessarily the case.
Possible reasons why men and women make multiple serial marriages are
discussed in more detail elsewhere (Borgerhoff Mulder 2009a). Ethnographic
observations suggest that lazy and heavy-drinking men are often divorced and
end up marrying postreproductive women. In some cases, these men have dependent children and clearly remarry to find help from a new wife in raising their kids.
For these men, multiple marriages therefore, rather counter-intuitively, reflect
parenting effort rather than mating effort, although why anyone would want to
marry them remains a puzzle.
There are a host of hypotheses to account for multiple mating in females (e.g.,
Jennions and Petrie 2000; Setchell and Kappeler 2003), entailing both direct and
indirect benefits. Women may gain direct benefits by mating with and marrying
multiple men if their husbands help them to obtain the resources needed to support
reproduction. Pimbwe women benefit from the farming activities of men, as well as
from the products of their hunting, fishing, honey production, and other enterprises,
but all of these are highly unpredictable, in part because of poor farming conditions
and in part because of the current illegality of entering protected areas where fish
and meat are plentiful (Borgerhoff Mulder et al. 2007). Given the potentially high
inter- and intraindividual variability in male provisioning, it is quite possible that
women switch mates to maximize economic income, the “musical chairs” hypothesis reviewed by Choudhury (1995). A similar argument has been made for the
instability of marriages among the poor in contemporary USA (Kaplan and
Lancaster 2003, see also Maness and Anderson 2007 for Nazca boobies, Sula
granti). Parallels can also be drawn with baboons (Papio cynocephalus) where
serial if nonexclusive pair bonds produce temporary male protectors for mothers
94
M.B. Mulder
whose success in raising offspring is heavily influenced by their social networks
and matrilineally inherited dominance rank (as reviewed by Silk 2007).
As regards indirect (or genetic) benefits, numerous mechanisms have been
proposed (including the maximization of male genetic potential, bet hedging,
prevention of inbreeding, and confusion of paternity certainty to avoid infanticide).
The most plausible in this context is the idea that a woman can afford to forego the
benefits of paternal care (and to risk the dangers of a stepfather in house) for mates
with high genetic potential. This argument has been made most forcefully for
humans by Gangestad and Simpson (2000) and is particularly plausible in environments with high disease loads where demonstration of heritable fitness is very
important (Hamilton and Zuk 1982). In support of this explanation is the fact that
Mpimbwe is beset by all of the health problems typical of rural tropical Africa
(Hadley and Patil 2006; Hadley et al. 2007) and has a minimal health care
infrastructure. If Pimbwe women were choosing genetically superior males and
keeping them we would expect once-married women to show highest fitness, but
they do not show this. In addition, if Pimbwe women were choosing genetically
superior males and then losing them to other women, we would expect multiply
married men to show elevated fitness, which is not observed.
In sum, the Pimbwe analysis, while provisional, provides clear evidence that a
key sex difference predicated on mammalian patterns of parental investment, the
Bateman gradient, is not observed. Whether this results from sex differences in the
range of quality in potential mates (Owens and Thompson 1994; Johnstone et al.
1996), costs of reproduction (Kokko and Monaghan 2001; Maness and Anderson
2007), extrinsic mortality rates (Gowaty and Hubbell 2005), or other factors is not
yet known. However, the cautionary tale here is that just because humans are
typical mammals with all the polygynous tendencies predicated on gestation and
lactation, conventional sex-differentiated reproductive strategies are not assured!
Therefore, if Pimbwe women enter into marital bonds to reap direct benefits,
does this mean that pair bonds are best thought of as a universal adaptation whereby
women trade sex for paternal care? The simplicity of this scenario is alluring, but
again the reality more complex.
4.4
Pair Bonds in Humans
Claims regarding the universality of human pair bonds are controversial but this is
so because they are often confused with statements about origins. The ethnographic
record displays a range of grouping patterns, from small two-adult “family” groups
to large multimale/multifemale bands (e.g., Pasternak et al. 1997), but within these
formations, specialized relationships emerge, between (usually) heterosexual individuals, typically glossed as “marriage.” Although it is widely recognized that these
bonds do not map precisely onto sexual relationships (Fox 1967), male sexual
access to females is key to the definition of marriage, even if it is given quite
different salience across cultures (Bell 1997). Furthermore, despite the well-known
4 Sex Differences in Humans
95
“double standards” in sexuality (Betzig 1989), Jankowiak et al.’s (2002) survey of
detailed ethnographic material shows that in all the 66 societies studied, men and
women actively mate guard, indicating that sexual propriety is a core component to
marital unions, even if much violated. Precise definitions of marriage may be
elusive, but in all cases, rights and responsibilities are exchanged (Needham
1962), concerning legitimacy of offspring (Gough 1959), property (Leach 1955)
and economics (Murdock 1949). In short, marital bonds are about sexual access,
and although additional rights and responsibilities are emphasized in different
cultural contexts, these bonds are always identifiable. Societies sanctioning total
promiscuity as the principle mating system do not exist (Bell 1997; Kunstadter
1963; Rodseth et al. 1991). Note that this is a claim about the universality of pair
bonding in the ethnographic record, not about its more contentious evolutionary
origins (as discussed in Knight and Power 2005), to which I now turn.
Several sources of evidence point, at least indirectly, to a long history of pair
bonding in our species, for example, relatively limited sperm competition (Birkhead
2000), little sexual dimorphism in size dating back in our lineage to 1.8 mya
(McHenry 1996), and a distinctive patterning of testosterone production with pair
bond status (e.g., lower levels in undergraduate men in well-established romantic
relationships: Gray et al. 2004). Furthermore, a notion of “romantic love” is
observed across the vast majority of human cultures (Jankowiak and Fischer
1992), mediated by various neuro-endocrinological systems (e.g., Carter 1998).
Much more controversial is the role of male provisioning in the evolution of pair
bonded behavior.
Pair bonds evolving from male provisioning were once central to narratives of
human origins (Washburn and Lancaster 1968). Bipedal hominins could carry meat
(Lovejoy 1981), opening up the possibility for a complementary division of labor in
which males provide resources to females encumbered with dependent offspring in
return for sexual exclusivity. Modern attempts to unravel the origins of pair
bonding attribute very different roles to paternal provisioning. In some formulations, males provision because both parents are assumed to have identical reproductive goals. Thus, Fisher (1989) suggests that divorce rates peak after 4 years,
because this is when a typical forager child is safely through the period of dependence, and both parents are free to look for new spouses. Others view the relationship as one in which both cooperation and competition exist. In a carefully argued
scenario that links intelligence, longevity, altriciality and diet, as coevolved traits,
Kaplan et al. (2000) posit that long lifespan and a cooperative division of labor
coevolved as humans moved into a foraging niche where food acquisition (hunting)
required great skill and knowledge, and partners could benefit from specialization
and exchange (Gurven et al. 2009). In other scenarios, marital bonds are believed to
be entirely independent of paternal provisioning. In one version, pair bonds are
deemed to have emerged from mate guarding, with males favoring pair bonds to
avoid incessant fighting over females (Symons 1979) and females to find refuge
from harassment (Blurton Jones et al. 2000) and/or infanticide (Hawkes 2004).
A different idea that again relies not on paternal provisioning (rather on female
provisioning) is that once females discovered how to increase the diversity and
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M.B. Mulder
density of food value through cooking, they were worth monopolizing (Wrangham
et al. 1999). In the latter kinds of scenarios, paternal care is more likely to have
evolved after the emergence of pair bonds in our lineage, and not be a necessary
condition for the evolution of pair bonding (Chapais 2008).
Phylogenetic analyzes of the relationship between mating systems and paternal
care shed light on this origins debate. In mammals, Brotherton and Komers (2003)
show that monogamy evolved more often in the absence of paternal care than in its
presence, and propose that paternal care most likely arose subsequently (although in
birds biparental care may have preceded avian pair bonds, Burley and Johnson
2002). Similar conclusions are being reached for nonhuman primates. Since direct
paternal care is present only in some species, it is most likely that monogamy is a
preadaptation facilitating the evolution of paternal care rather than a consequence
(Palombit 1999; van Schaik and Kappeler 2003). In short, despite the apparent
universality of pair bonds in contemporary human populations and cogent models
that these evolved to subsidize the high costs of reproduction and encephalization
(Kaplan et al. 2000; Gurven and Kaplan 2006), there is little clear comparative
evidence that pair bonds evolved in mammals to facilitate paternal provisioning,
nor is there much evidence from nonhuman primates to support such a claim.
Humans, however, with their exceedingly large brain and unusually long lifespan,
may be unique in this respect.
To gain more insight into the nature of the pair bond, anthropologists turn to
ethnographic materials, both within and between population analyzes. One question
pertinent to this debate is whether men’s economic activities are best characterized
as mating effort or parental effort. There are cases where men work particularly
hard when their mates are lactating (Hadza: Marlowe 2003), allowing the latter to
do less work at this energetically demanding time (Hiwi and Ache: Hurtado et al.
1992). These data suggest that men’s activities are a form of parenting effort.
Similarly, there are cases where men’s allocation of effort to food production fits
closely with expectations derived from the paternal effort model (Tsimane: Gurven
and Kaplan 2006); Although Tsimane men invest in mating effort through extramarital relations, they do so early in the marriage when there are few, if any,
children to care for, rather than later, when paternal contributions are most needed
(Winking et al. 2007). In addition, confronted with hypothetical scenarios, Ache
and Hadza men show preferences for hunting groups with good hunters that yield
high returns for provisioning but low returns for mating effort, rather than groups of
poor hunters where provisioning benefits are small and mating benefits large (Wood
and Hill 2000; Wood 2006). These patterns support the parental effort hypothesis,
as do demographic and economic data that show how brides are hard to find
(Borgerhoff Mulder 1990; Cashdan 1993) and quick to leave (Betzig 1989) when
men’s provisioning resources are not forthcoming, patterns also observed in the
modern US when marriage transitions are analyzed in relation to income (Nakosteen
and Zimmer 1997; Burgess et al. 2003). However, in other studies, men’s economic
activities are better characterized as mating effort, as with Hawkes’ (1993) analysis
of hunting in the Hadza and Smith et al.’s (2003) study of turtle feasts of Mer
islanders in the Torres Straits. In both these cases, it is argued that men specialize
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Sex Differences in Humans
97
on risky resources, ones that are better characterized as public goods to be ostentatiously shared (the show off hypothesis: Hawkes 1991) than as reliable streams of
paternal provisioning. Despite strong arguments on each side, it seems most likely
that men everywhere exhibit elements of both paternal and mating effort in their
economic exploits (Anderson et al. 1999a, b; Gurven and Hill 2009), for example in
the Marlowe’s analyzes of the Hadza (1999).
Cross cultural data can also be used to address, at least indirectly, the question of
whether pair bonding is associated with the provision of paternal care or with the
defense (or guarding) of mates. Quinlan and Quinlan (2007) find that marital
stability (a low divorce rate) is associated with substantial male contributions to
subsistence, an absence of alternative caretakers, and late weaning, all indices of the
potential value of male assistance. However, marital stability is also associated with
high levels of polygyny, a possible proxy for the difficulty men face in finding new
mates. The authors therefore conclude that pair bonds likely evolved in response to
multiple selection pressures a need for male care and a strategy for each sex to
deal with intense mate competition. In fact, there may even be two different kinds
of human pair bonds one geared to child rearing and one geared toward male
reproductive competition (Quinlan 2008).
All in all, studies of contemporary populations do little to resolve whether pair
bonds evolved for paternal investment or mate guarding. Why so little progress?
First, despite the wealth of empirical studies we have puzzling cases where
different data sets from the same population support different models. Second,
comparative studies raise multiple problems for interpretation for example is
divorce prevalence really a valid indicator of the importance of pair bonds? A
more appropriate variable might be the number of people who actually do marry in
the population. Third, contemporary correlates of a trait do not necessarily shed
light on its evolutionary origins. While human behavioral ecologists maintain that
studies of current behavioral diversity illuminate the flexibility of human nature to
different social and ecological triggers (e.g., Smith et al. 2001), extrapolating to
evolutionary sequences is much more tenuous. Now that pair bonding is in place,
paternal care might be very important in contemporary populations. However, to
claim that it was the original selective pressure for pair bonds is an interpretative
error, Chapais (2008: 169) dubs the “pitfall of modern family reference” (see also
Marlowe 2007). Finally, investigators often talk about men and women as if they
had a single sex-specific strategy. In the modern USA, for example, upper strata
parents use biparental care to invest in highly profitable education for their
children, whereas lower strata women raise children alone and both sexes have
serial and simultaneous relationships (Kaplan and Lancaster 2003); multiple
strategies characterize many other populations (see, for example, Dickemann
1982). In so far as an individual’s optimal mating and reproductive strategy
depend on the behavior of both same- and different-sexed conspecifics, multiple
strategic spaces emerge. Only recently have theoreticians begun to tackle this, as
greater computing power allows model parameters to be generated by individual
strategies rather than fixed at a priori levels (e.g., Cotar et al. 2008, and for more
general discussion see Kokko et al. 2006).
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M.B. Mulder
As such, it is perhaps more productive to think about pair bonds in terms of
sexual conflict (Borgerhoff Mulder and Rauch in prep). Sexual conflict theory does
not inherently avoid the pitfalls of making generalizations about sex differences,
but it provides a more dynamic framework for analyzing variable sex roles. For
example, it examines the question of how much a wife and husband should work
and when they should break their contract, in terms of broader market (supply and
demand) conditions. Economists focus on each spouse’s bargaining power, the
well-being a lady can expect without the cooperation of a gentleman, and vice
versa; differential bargaining power sets each spouse’s “threat point” (Manser
and Brown 1980), the resource sharing contractual arrangement at which the lady
(or gentleman) would be better leaving than staying. Anything that improves an
individual’s bargaining power with their spouse, such as relatively larger earnings,
gender-biased divorce laws, or greater chances of remarriage, increases that person’s share of the marital pot (e.g., Lundberg and Pollak 1996). Accordingly, the
benefits of a marriage are not expected to be shared equally, but in accordance with
bargaining power. The implications of these considerations for marital stability,
marital assortment, and equilibrial states of the marriage market can be explored
with game theory, as demonstrated in models of the “better options” hypothesis for
divorce in birds (e.g., McNamara et al. 1999) where divorce rates are shown to
decrease with individual quality (fewer “better options” assuming a good first
pairing), and age (no time to recoup the costs of divorce).
A key factor affecting these conflicts is whether spousal labor is complementary
or substitutable (Kaplan and Lancaster 2003). If mum feeds and dad protects the
baby, parental roles are complementary, in so far as each activity is valueless
without the other. If mum (or dad) uses salary to pay rent and school fees, parental
roles are substitutable and marriages become more brittle (and subject to corner
solutions). Changes in divorce rates and prevalence cross culturally might be
usefully examined from this perspective. Rather intriguingly, Quinlan and Quinlan
(2007) find that divorce is least common in populations where both sexes contribute
approximately equally to household production, suggesting (albeit indirectly) that
marriage is indeed a more stable institution where spouses’ work is complementary.
Thinking about the marriage as a complementary division of labor raises the
interesting question of when and how the benefits of specialization can be offset by
positive assortment of skills (Borgerhoff Mulder 2004). When are the public goods
produced by a breadwinner and homemaker eclipsed by two extremely successful
breadwinners who might bicker over homemaking? In traditional economies, where
most tasks are gender-specialized, the benefits of the sexual division of labor are
unlikely to be dwarfed by positive assortment for skills. One might expect, however, that in a modern economy, where most jobs can be done by men or women,
positive assortment counterbalances the division of labor (corporate executives
intermarry and hire an au pair). Positive assortment among mates for various skills
is found in many modern economies (e.g., Logan et al. 2008). Indeed a study testing
whether US university students show preferences to assort on similar traits (rich
men like rich women, and vice versa) or on reproductive potential (high earning
men prefer beautiful women, and vice versa) showed that the first pattern is much
4 Sex Differences in Humans
99
stronger than the second (Buston and Emlen 2003); this suggests that the complementary marital relationship is disappearing in some western societies. We see here
how far an evolutionary-based discussion of marriage in terms of sexual conflict
and bargaining theory has moved us away from the simple constraints of mammalian reproduction.
4.5
Mind the Crack: Concluding Observations
What are the implications of this discussion for universal sex differences and the
gap between studies of human and nonhumans? Early evolutionary studies identified predicted sex roles, but failed to consider variation. Nowadays, with more
sophisticated theoretical models and richer empirical evidence, we see that the roles
of men and women can be highly variable. The Pimbwe study shows how some
women, despite common mammalian constraints, can use multiple sequential pair
bonds to out-reproduce their monogamous counterparts. Similarly, the broader
discussion of the relationship between pair bonds and paternal care reveals limits
to the conventional view of marriage as a trade of sex for paternal resources. Men
and women have negotiable roles in marriage, for which models from behavioral
ecology (beyond conventional parental investment theory) and economics can be
brought to bear.
In what sense do we differ from nonhuman primates in this respect? Anthropologists once viewed regularly patterned family behavior (and particularly
marriage, e.g., Levi-Strauss 1949) as what divides us from the other primates,
contrasting the apparent promiscuity of our apelike cousins to, initially, our nice
nineteenth century stable monogamous families, and later the wide range of family
types evident in the ethnographic record. However, the gap is nowhere so big as
once thought and is better viewed as continuum (e.g., Foley and Lee 1989; Rodseth
et al. 1991) or just a crack. This is so in part because over the years so much has
been learned about the complexity of nonhuman primate kinship and social behavior. With long-term studies of known individuals, the sophistication of primate
kinship behavior is now well appreciated. The narrowing gap also reflects the fact
that the human pair bond, with dad provisioning mum in exchange for sex, is now
no longer viewed as necessarily the supreme human adaptation that sent us down
our distinct evolutionary route. In fact, as I have tried to argue, the contemporary
human pair bond is a highly variable trait which, while most likely universal in
some form, functions very differently in different social and economic contexts,
and has hotly contended evolutionary origins.
Another reason why crack (rather than gap) better characterizes the distinction
between sex-differentiated reproductive strategies in human and nonhuman
primates is that there are so many parallels between contemporary debates within
primatology and human behavioral ecology. I end with a consideration of some
of these.
100
M.B. Mulder
First, nonhuman primates generally exhibit rather low levels of direct paternal
care. Males can be important for protection, particularly from infanticide (e.g.,
Palombit et al. 1997), and males do in some cases provide direct care to their
offspring (Buchan et al. 2003), but with a few exceptions (e.g., Goldizen 1987),
paternal care is not extensive among primates. Furthermore, as we have seen in
humans, there is lengthy debate over whether male activities are designed for
improving offspring survival or enhancing access to mates (e.g., Smuts and
Gubernick 1992; van Schaik and Paul 1996). In short, as in mammals more generally,
male care is not extensive, and where it occurs, its relationship to paternity, and its
impact on offspring survival, is debatable (Woodroffe and Vincent 1994). The
situation is rather similar in the study of humans, as reviewed in this chapter. The
traditional view that paternal care is central to our evolutionary trajectory is now in
question, and suggestions that pair bonds originated for mate defense and avoidance
of harassment are gaining attention. Humans do, however, look different with respect
to the current function of pair bonds: the division of labor over the production and
consumption of food among spouses is particularly developed in our species, hence
the foray into economics for new theoretical tools (and see Noë et al. 2001, for similar
applications of economic theory to nonhuman primates).
Secondly, related to low paternal care is the significance of female-bonded kin
groups in nonhuman primates. If males are not helping, who is? In many species,
amongst them cercopithecine monkeys (Silk 2007), the fertility and reproductive
success of females is heavily influenced by social networks and matrilineally
derived access to resources. Such social systems are increasingly being used as a
model with which to think about human evolutionary origins and the importance of
cooperation among kin in subsidizing the costs of childbearing (Hrdy 2005a, b).
This marks a radical departure from seeing our origins in the social organization
of the male-bonded apes (e.g., Rodseth et al. 1991).
A final parallel trend lies in the prevalence of multiple mating by females.
Female primates commonly mate with multiple males to avoid infanticide, and
rather rarely mate for resources or good genes; indeed, preferences for dominant
individuals dissolve once the male loses dominance, suggesting that protection is
more prominent in this preference than genetic quality (Setchell and Kappeler
2003). Interestingly, human behavioral ecologists turned initially to birds for
models for mating systems (Flinn and Low 1986; Borgerhoff Mulder 1990) because
of the clear importance of paternal investment in many extent societies. This,
however, may have distracted us away from very different (and non paternal
care-based) arguments for the origins, if not the current function, of pair bonds in
our species.
In sum, there are several parallels between the arguments advanced in this
chapter and current debates within primatology that support the claim that we
should perhaps be minding a crack rather than a gap.
Acknowledgments Fieldwork was funded by the LSB Leakey Foundation, the Packard Founda
tion and UC Davis. Thanks to the editors and two reviewers for the helpful comments on the
manuscript.
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101
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Chapter 5
Dominance, Power, and Politics in Nonhuman
and Human Primates
David P. Watts
As long as politics is the shadow cast on society by big business, attenuation of the shadow
will not change the substance . . . Power today resides in control of the means of production
exchange, publicity, transportation, and communication. Whoever owns them rules the life
of the country. The machinery of power . . . is business for private profit . . . reinforced by
command of the . . . means of publicity and propaganda.
(John Dewey, quoted in Westbrook 1991: 440, 442)
Abstract Dominance is a common, although not universal, characteristic of social
relationships in nonhuman primates. One individual is dominant to another when it
consistently wins the agonistic interactions between them. Attainment of high
dominance rank can bring reproductive payoffs, mostly because it confers priority
of access to monopolizable food sources (for females) or to mating opportunities
(for males). For females in particular, a wide variation exists in the frequency of
intense aggression, the directionality of aggression within dyads, the tolerance of
high-ranking individuals, and other aspects of “dominance style.” This variation
reflects variation in ecology and is also influenced by phylogenetic history. Variation also exists in male dominance style, although it has not received as much
attention. Dominance is one component of power, which also encompasses other
sources of asymmetry in relationships that affect the relative ability of individuals
to carry out their goals against the interests of others. Leverage is an important
source of power in many nonhuman primates; an animal has leverage over another
when it controls a resource or service that cannot be appropriated by force, such as
agonistic support. Individuals behave politically when they try to increase or
maintain their power relative to that of others by manipulating social relationships,
both their own and those of others. The concept of power applies universally to
gregarious primates, although asymmetries do not occur between adults of all
D.P. Watts
Department of Anthropology, Yale University, New Haven, CT, USA
e mail: david.watts@yale.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 5, # Springer Verlag Berlin Heidelberg 2010
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species, but we should only ascribe politics to species in which the actors have
knowledge about the relationships between others in their groups. Some parts of the
literatures in political science and political anthropology not only provide useful
frameworks for a comparative investigation of power and politics in nonhuman
primates, but also highlight qualitative differences between humans and other
primates, especially regarding the importance of ideologies and political rhetoric.
5.1
Introduction
Dewey’s remarks seem incongruous in a chapter about dominance, power, and
politics in nonhuman primates. The influential philosopher of pragmatism was
writing about humans in the context of twentieth century industrial capitalism;
his ideas about politics, power, and the proper social role of “the means of publicity”
were grounded in and contentious in that context. They do not describe
human universals, and “politics” and “power” have many definitions in political
anthropology, a subdiscipline that includes multiple, often contrasting and sometimes complementary, explanatory paradigms that cover the entire range of current
and historically known human social arrangements (reviewed in Kurtz 2001, and
Lewellen 2003, among others). However, Dewey makes an implicit point about
nonhuman primates: without the capacity for language and for symbolically
mediated systems of meaning, they cannot engage in “publicity and propaganda”
or seek profit, nor can they invent, pursue, or argue about ideologies, whether
capitalism or any other: these are qualitative differences from humans. Can we
then justifiably ascribe politics and power dynamics to them and seek commonalities
with humans?
Any answer starts with the fact that individuals of most primate species maintain
long-term social relationships with conspecifics. These often have affiliative dimensions, but they also involve competition over food, mates, and other resources, the
outcome of which can have major effects on fitness. Competition may or may not
involve social interaction and may or may not lead to dominance relationships.
Competitive interactions, or contests, are crucial to the concept of dominance,
which becomes part of a social relationship when one individual can monopolize
resources at the other’s expense or usurp them from the others by using force or
threatening to do so, even though not all aggression concerns immediate access to
resources or contests over status, and some contests are decided by unprovoked,
unilateral submission rather than by aggression.
However, not all resources accessible via social interaction can be appropriated
by force, and dominance can be subsumed within a broader category of power
(Lewis 2002). Variation in power among individuals potentially allows for social
maneuvering that sometimes warrants the label of “politics.” Alliance formation
strategies illustrate these points well. Most contests are dyadic, but many primate
species stand out when compared with most mammals because of the frequency
with which they form coalitions, in which two or more individuals collaboratively
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direct aggression at joint targets (Harcourt 1992). Alliances develop when particular dyads repeatedly and consistently form coalitions; they feature prominently in
the competitive strategies of macaques, chimpanzees, and many other species. One
effect of coalitions is to help individuals to win contests they would otherwise lose.
Male baboons can sometimes forcibly take over consortships with estrous females
from higher-ranking males, for example, but require coalition partners to do so. Not
all potential partners are equally effective, and males may compete for access to the
best partners (Seyfarth 1977; Noë 1990, 1992). Males cannot coerce others to join
them; the ability to grant or withhold support gives potential partners power over
their would-be allies, and males whose value is higher than that of their partners are
likely to take disproportionate shares of the benefits of successful coalition formation (Noë 1990, 1992). Strategic pursuit of alliances and negotiation over the
distribution of their costs and benefits easily bring to mind human political maneuvering, as de Waal (1982) argued, for the ways in which male chimpanzees (Pan
troglodytes) use coalitions in their complex strategies of competition for status.
Other nonhuman primates that also engage in complex social maneuvering for
status by using alliances, exchanging social services like grooming, and testing
the strength of social bonds arguably also have politics (e.g., white-faced capuchins, Cebus capucinus: Perry and Manson 2008).
However, we should be wary of using “politics” too loosely (e.g., Boehm 1997)
and of anthropomorphizing chimpanzees, baboons, capuchins and other nonhumans
in the service of superficial and misleading extrapolations to humans (e.g.,
Fukuyama 1998). In the following section, I define dominance and briefly review
concepts of power. Differentials in power among individuals are nearly universal in
primates; dominance relationships and dominance hierarchies are not, although
they exist in the majority of taxa. I review competing explanations for variation in
dominance “style” and briefly summarize evidence concerning the relationship of
dominance rank to reproductive success. I also consider the aspects of social
relationships in nonhuman primates that involve politics; I argue that politics is
far from universal and only characterizes species capable of triadic awareness (i.e.,
knowledge about the social relationships between others in one’s social group).
Chimpanzees necessarily figure in large measure in my discussion of politics,
because they have been the main subject of relevant research and speculation.
Finally, I offer some comparisons between human and nonhuman primates.
5.2
What is Dominance?
Following Hinde (1976; cf. Dunbar 1988), I regard dominance as a property of
social relationships, not a characteristic of individuals, who are “dominant” or
“subordinate” only in the context of social relationships. This perspective differs
from the one common in social psychology, in which dominance is seen as a human
personality trait that varies quantitatively among individuals (e.g., Maner et al.
2008). By convention, we can determine whether dominance is part of a social
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relationship by quantifying the direction and outcomes of agonistic interactions,
i.e., those that involve aggressive and/or submissive acts and/or signals. For example,
supplants are agonistic acts in which a stationary individual moves away at the
approach of another, who takes its place, while “pant grunts” are formal vocal
signals of subordinate status in chimpanzees (Marler 1976) and “silent bared teeth
faces” are formal visual signals of subordinate status in and some (but not all)
macaque species (Thierry 2000). If one of two members of a dyad consistently
wins the agonistic interactions between them, it is dominant to the consistent loser,
which in turn is subordinate to the consistent winner. Since the same individual can
be dominant to some members of his or her social group and subordinate to others,
terms like “dominant individuals” or “dominance interactions,” while convenient
shorthand, are best avoided because they risk reifying dominance as an inherent
aspect of individual phenotypes.
Linear dominance hierarchies, of varying degrees of stability, form when most
or all group members or all members of one class of individuals (e.g., adult females)
have dominance relationships. These occur in many, but not all, group-living
primate species. For example, both macaques (Macaca spp.) and mountain gorillas
(Gorilla beringei beringei) live in multifemale groups; macaque females typically
form stable, linear dominance hierarchies (de Waal 1986, 1989; de Waal and
Luttrell 1988; Chapais 1992; Thierry et al. 2000; Thierry 2007), but mountain
gorilla females do not necessarily do so and many female dyads lack decided
agonistic relationships (Watts 1994).
5.3
Functions of Dominance
Group-living primates have many means to manage the tensions that arise from
conflicts of interest among individual group members (de Waal 1986, 1989, 2000).
The argument that establishing decided dyadic dominance relationships and (when
these occur) linear dominance hierarchies limits aggression, and thus mitigates such
tensions and reduces the costs of aggression notably injury risk has a long
history (e.g., Bernstein 1976; Chapais 1991). De Waal (1986) argued that formal
signals of subordinate status and “conditional reassurance” from higher to lowerranking individuals limit the use of force to express competitive tendencies, and
stated (ibid.: 475) “a well recognized hierarchy promotes social bonds and reduces
violence,” as evidenced by data from macaques, chimpanzees, and other species.
But this begs the question of why status striving should so often be all that
prominent. The standard answer is that high rank, by conferring priority of access to
resources, can lead to high reproductive success. A sex difference in the key
resource over which individuals compete exists, with females competing mostly
for food (although safety can also be important; van Schaik and van Noordwijk
1986) and males mostly for mating opportunities. The literature on the association
between rank and reproductive success is too vast to summarize here. In brief, the
weight of the evidence generally supports the hypothesis that rank is positively
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113
correlated with reproductive success for both sexes. Data on some species show the
expected positive relationship for females (e.g., yellow baboons, Papio cynocephalus: Altmann and Alberts 2005; long-tailed macaques, Macaca fascicularis: van
Noordwijk and van Schaik 1999). Exceptions exist (e.g., chacma baboons, Papio
hamadryas ursinus: Cheney and Seyfarth 2007), and the strength of social bonds
can influence female reproductive success independently of rank (Silk et al. 2003),
but rank effects may only become discernable during times of prolonged food
shortfalls and thus only become evident in very long-term studies (Cheney and
Seyfarth 2007). Harcourt (1987) noted that no cases were known in which female
reproductive success was inversely related to rank; this is still true. Accumulating
genetic evidence also supports the “priority of access” model for males in many
species (reviewed in Di Fiore 2003), including yellow baboons (Alberts et al. 2006),
long-tailed macaques (van Noordwijk and van Schaik 2004; Engelhardt et al.
2006), mandrills (Mandrillus sphinx: Setchell et al. 2005), and chimpanzees
(Boesch et al. 2006), although the number of males per group and the degree of
estrus synchrony among females also influence reproductive skew among males.
Behavioral variation linked to differences in personality (or “behavioral syndromes”) also can influence male reproductive success independently of dominance
rank (Bergman et al. 2008).
5.4
Sources of Variation in Female Dominance Style:
Ecology, Phylogeny, and Self-Structuring
Relationships between females vary widely among primate species that form multifemale groups, as do those between males and females and, in multimale groups,
those between males. Females in some species may not form dominance hierarchies
(e.g., mountain gorillas: Watts 1994; blue monkeys, Cercopithecus mitis: Cords
2000). Aggression directionality and intensity varies among other species (e.g.,
aggression goes unidirectionally down the hierarchy in rhesus macaques, Macaca
mulatta, but is common up the hierarchy in stump-tailed macaques, M. arctoides:
de Waal 1986, 1989; de Waal and Luttrell 1988). Dyadic agonistic asymmetries can
vary, sometimes consistently, across contexts, and in some species, they are susceptible to the influence of third parties. Kawai (1958) used the term “dependent
rank” to refer to the attainment of dominance by young female Japanese macaques
(Macaca fuscata) over larger adolescent and adult females subordinate to their
mothers, with the help of coalitionary support from kin and, sometimes, nonkin.
White (1996) described bonobos (Pan paniscus) as having female “feeding priority,”
because females usually win contests with males over food, and males sometimes
avoid patches where females are feeding, but males are not necessarily submissive
to females in other contexts. Similar variation occurs in tolerance of higher-ranking
individuals for their subordinates, dynamics of conflict management and resolution,
and other behavior related to preserving social bonds in the face of potentially
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serious competition. De Waal (1986, 1989; cf. de Waal and Luttrell 1988)
introduced the concepts of “formal dominance” (based on the directionality of
formal signals of relative status) and “real dominance” (based on the actual outcomes of agonistic interactions) to clarify understanding of this variation and
referred to particular patterns of variation as the “dominance style” of a species or
group of species.
Feeding efficiency can crucially influence female reproductive success; thus,
variation in the relative strength and intensity of scramble and contest feeding
competition may lead to predictable variation in female dominance styles (van
Schaik 1989). The socioecological model (Sterck et al. 1997; cf. Koenig 2002)
accounts well for much of this variation by considering the effects of competitive
regimes; by incorporating sexual conflict, it accounts for much variation in male
female relationships and helps to explain why in almost all species that form stable
social groups, males associate permanently with those females. Briefly, this model
holds that when within-group scramble competition predominates and contest
competition is inconsequential, females in multifemale groups should either form
weak and unstable dominance hierarchies or not form them at all, and females can
transfer between groups to minimize scramble competition and/or to choose mates.
In contrast, when feeding on clumped resources monopolizable by single individuals or by coalitions that include only some group members is important i.e.,
when contest competition for food is important and monopolization confers
nutritional advantages, linear dominance hierarchies occur. Given associated
female philopatry, nepotism is the main basis for coalition formation, and related
females help each other to acquire and maintain ranks. High within-group contest
competition should result in strong (“despotic”) hierarchies, stabilized by nepotism.
When between-group contest competition also strongly influences female fitness,
however, high-ranking females those able to win within-group contests against
most or all others
should be tolerant of lower-ranking females, and hence
hierarchies should be weaker, so that high-ranking females can retain their subordinates’ support in contests with other groups. Finally, high between-group
contest competition combined with low within-group contest competition should
lead to egalitarian relationships (no dominance hierarchies or weak ones) combined
with female philopatry.
Much evidence supports the model, although accounting for intertaxon diversity
in female social relationships minimally also requires other consideration of overall
food abundance and variation in food nutritional quality (Isbell 1991; Pruetz 2009).
For example, within-group contest competition is relatively weak, female agonistic
relationships are egalitarian, and female transfer is common in Thomas langurs
(Presbytis thomasi), and within-group contest competition is relatively high in
“despotic” long-tailed macaques (Sterck and Steenbeek 1997). Likewise, variation
in the strength of within-group contest competition accords with variation in the
strength of dominance hierarchies among populations of hanuman langurs (Semnopithecus entellus: Koenig et al. 1998) and among three species of squirrel monkeys
(Saimiri spp.: Boinski et al. 2002; cf. Mitchell et al. 1991). Blue monkeys at
Kakamega face high between-group contest competition, but within-group contest
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competition is weak; correspondingly, females are philopatric, but do not form
linear dominance hierarchies (Cords 2000). Such egalitarianism in species with
high between-group contest competition (e.g., some guenons) may be a general
solution to a collective action problem that female superior competitors face, but
whether this is actually so depends on how much they could gain by winning
within-group contests. If potential gains rarely outweigh the costs of aggression
perhaps because most food patches can accommodate all group members (but not
more than one group), egalitarianism or tolerance requires some other explanation.
Lions provide a valuable comparative example of female egalitarianism: females in
the same pride cooperatively defend carcasses against females of neighboring
prides and defend cubs against infanticidal males, and they engage in communal
care of cubs; they do not form dominance hierarchies despite high potential for
within-pride contests over food (Packer et al. 1990; Heinsohn and Packer 1995).
Despite the considerable success of the socioecological model, its generality and
functional logic have been strongly challenged, notably with respect to variation in
female dominance style among macaque species. Macaques fall into semi-discrete
grades along a spectrum in which the directionality of aggression, the relative
frequency of high-intensity aggression, the extent of kin biases in social behavior,
conciliatory tendencies, and tolerance co-vary (Thierry 2007). This co-variation
seems to have a strong phylogenetic signal (ibid.; Thierry et al. 2000). In despotic
species like rhesus and Japanese macaques, rigid hierarchies co-occur with
relatively high rates of intense aggression, unidirectional aggression down the
hierarchy, strong nepotism and consistent operation of the “youngest ascendancy
rule” in rank acquisition (maturing females assume ranks immediately below those
of their mothers and above any older sisters), and low conciliatory tendencies. At
the other end of the spectrum, high tolerance is associated with weaker nepotism
and less kin-bias in social relationships, low rates of intense aggression, less
consistent operation of the youngest ascendancy rule, and high conciliatory tendencies. The extreme despots belong to the fascicularis lineage; the most tolerant
species belong to the silenus-sylvanus lineage; and other members of these lineages
and species in the sinica-arctoides lineage occupy various intermediate positions
(Thierry 2007), although not all variation sorts neatly by phylogeny (e.g., Tibetan
macaques (M. thibetana) belong to the relatively “tolerant” sinica-arctoides lineage, but have despotic female dominance; Berman et al. 2004). Moreover, limited
field data indicate that variation in hierarchy strength may not correspond to
variation in the strength of within-group contest competition and that high between-group contest competition does not obviously characterize the more tolerant
species (Cheney 1992; Berman et al. 2004; Ménard 2004; Thierry 2007).
Thierry (2007; cf. Matsumara 1999; Matsumara and Kobayashi 1998) has
proposed that these grades are different evolutionarily stable outcomes of selection
on a collection of traits that are linked because all are mediated by the same
underlying neurobiological and hormonal mechanisms (e.g., the effects of serotonin
on anxiety and aggression intensity). Contrary to the “collective action” explanation for tolerance among female macaques, high-ranking females are not forced to
restrain their competitive tendencies to induce cooperation from low-ranking
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D.P. Watts
females, because their competitive tendencies are already low. Shallower dominance gradients in tolerant species than in more despotic ones also could mean that
for a female of a tolerant species, proportionately more of the other females in her
group are potentially valuable allies, although alliances would have less importance
for her if the co-evolved system involves low competitive tendencies. Alternatively,
Preuschoft and van Schaik (2000) proposed that low asymmetries in fighting ability
and less consistent or predictable agonistic support in tolerant than in despotic
species means that females of tolerant species need to probe others repeatedly to
assess their current willingness to act as allies. This leads to high tolerance for
spatial proximity regardless of rank difference, relatively infrequent retaliation
against females that direct mild aggression at higher-ranking partners, more need
for reconciliation, and more frequent grooming between nonkin. In their view,
tolerance is really “calculated generosity.”
Problems with the sociological model are not restricted to macaques. For
example, female hanuman langurs at Ramnagar engage in high within-group
contest competition for food and, as expected, form despotic dominance hierarchies, but are not tolerant despite also facing high between-group feeding competition (Lu et al. 2008). Between-group contest competition seems to be generally a
poor predictor of female dominance style.
Hemelrijk has used a series of agent-based simulation models to argue that selfstructuring could account for much of the observed variation in dominance styles
(e.g., Hemelrijk 1996, 1999a,b, 2000a,b, 2002; Hemelrijk et al. 2003). Model
entities are assigned initial “dominance” values for which the gradients vary across
simulations. They then follow one of various alternative sets of rules that determine
how they move and how they behave on encounters with others moving in the same
space. They may estimate their capacity to win agonistic interactions with others
based on their past histories of interaction; alternatively, they may assess the risk of
conflicts by directly evaluating their own fighting ability relative to that of the
entities they have encountered. Group cohesion also varies, along with the probability that more than two entities meet simultaneously and “coalitions” form.
Entities are sometimes divided into species in which aggression intensity is typically high and others in which it is low, and sex differences in attack intensity can
vary. Crucially, contest outcomes are probabilistically determined, and both winning and losing are self-reinforcing (winning reinforces an agent’s dominance
value, whereas losing decreases it), with effects of winning greater for the subordinate of two interactors and the losing effect stronger on the dominant. The selfstructuring effects of these simulations produce dominance hierarchies that vary
along the axes of “despotism-egalitarianism” and “tolerance-intolerance”, in the
degree of nepotism, and in the extent to which male and female dominance
hierarchies overlap. Inclusion of variation in food clumping and in sexual attraction
between males and females also influences variation in male female hierarchy
overlap (Hemelrijk et al. 2003).
Hemelrijk states that these models are “caricatures” that do not reflect the
complete behavior of real monkeys and apes. For example, dyads with established
social relationships almost certainly use memory-based assessment (Silk 2002),
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coalition formation is nonrandom, and triadic awareness, which depends on
learning and memory, leads to strategic decisions about intervening in conflicts
and soliciting coalition partners. As Preuschoft and van Schaik (2000) state,
nonhuman primates often have accurate information about relationships in their
groups. Clear evidence that monkeys classify others on the basis of kinship (e.g.,
Bergman et al. 2003) argues against the possibility that apparent nepotism is simply
a byproduct of variation in the intensity of aggression (Hemelrijk 1999b).
It remains an open question whether we should jettison the socioecological
model in favor of game theoretic analyses of alternative equilibria that result
from selection on linked traits (Thierry 2007) or can improve it by incorporating
data on other aspects of food distribution and quality (Pruetz 2009), better data on
the extent to which low-ranking females participate in contests between groups, and
consideration of other potential sources of leverage for low-ranking females (e.g.,
willingness to engage in cooperative defense of infants against infanticidal males;
Lu et al. 2008). Ultimately, alternative social equilibria can only be stable within
bounds set by species’ ecology, and we need much better quantitative data on
feeding competition in many species, notably macaques. However, in looking for
bivariate associations between competitive regimes and categorical distinctions
like despotic versus egalitarian or tolerant versus intolerant, the model’s advocates
fail to acknowledge that these probably are parts of coevolving complexes that
cannot be atomized (Thierry 2007; cf. Thierry 2000; Thierry et al. 2000). Hemelrijk’s nonadaptationist models can help to provide proximate explanations for
dominance style variation. Superficially, their outcomes strongly resemble the
spectrum of dominance styles evident in macaques and some other nonhuman
primates. However, they may reproduce the surface structure of dominance styles
precisely because in the real world, the variables that lead to self-structuring
winning and losing effects, aggression intensity, group cohesion, sex differences in
agonistic power help to determine developmental outcomes within ranges of
possible phenotypic space set by varying histories of evolutionary response to
competitive regimes, and whatever explanatory power they have is not independent
of phylogeny and ecology.
Occasional references to winner and loser effects in the real world occur; for
example, Preuschoft and van Schaik (2000) note that individuals that repeatedly
lose contests to various opponents can become “trained losers,” who defer to most
or all opponents, whereas those that repeatedly win may show the opposite pattern.
Any such effects could be hormonally mediated, perhaps by testosterone. An
extensive literature on humans (reviewed in Archer 2006) indicates that the relationship between testosterone and competition in both sexes is complex. One
apparent generalization based on a meta-analysis of relevant studies (ibid.) is that
male testosterone levels rise slightly in anticipation of sports competition; they also
increase from before to after the competition, with the increase greater in winners
than in losers, although variation in personality, in attribution of causality for the
outcome, and other factors can influence the magnitude of change. Additionally,
losing sometimes decreases testosterone, although this effect may occur only in
individuals that attribute their losses to intrinsic factors (e.g., Mehta and Josephs
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D.P. Watts
2006), and drops in testosterone in association with high social anxiety can make
losers less willing to compete again (Maner et al. 2008). Differential effects of
winning and losing like these, if persistent, could help to produce the kind of selfreinforcement that Hemelrijk envisions.
Attempts to monitor short-term fluctuations in testosterone induced by competitive interactions in wild nonhuman primates face formidable logistical hurdles, but
longer-term increases or decreases in testosterone respectively associated with
winning or losing contests for alpha male status in mandrills (Setchell et al. 2008)
and for control of one-male units in geladas (Beehner and Bergman 2009) suggest
that such effects occur. Further circumstantial support comes from the evidence that
testosterone influences decisions by male chacma baboons in Botswana (Beehner
et al. 2006; Bergman et al. 2006) whether to engage in contests. Testosterone was
highest in males rising in rank, and current levels predicted male rank and mating
success over the subsequent eleven and seven months, respectively (Beehner et al.
2006). Males avoided others with high testosterone more often than those with low
testosterone; relative rank had little effect, and adjacently ranked males (probably
each other’s most serious current competitors) with high combined testosterone
were most likely to avoid each other.
5.5
Variation in Male Dominance Style
Sex differences in dominance style are common, as expected given that variation in
female fitness depends mostly on the outcome of competition for food, whereas
variation in male reproductive success depends mostly on the outcome of mating
competition, and given that dispersal is often male-biased. For example, in some
chimpanzee communities, many female dyads lack dominance relationships (e.g.,
Ngogo: Wakefield 2008), and females usually do not form linear hierarchies.
Instead, they reduce feeding competition by adjusting gregariousness to food
availability. In contrast, males typically form steep linear hierarchies (e.g.,
Ngogo: Watts and de Vries 2009), although male male social relationships are
also highly affiliative. Male dominance ranks in nonhuman primates usually depend
on individual fighting ability and show an inverse-U shaped relationship to age,
even in cercopithecine species in which females form stable hierarchies in which
ranks depend on predictable nepotistic and mutualistic support, not on fighting
ability (Chapais 2001). Nepotistic effects on male ranks prior to dispersal are more
likely in those cercopithecines in which adult size dimorphism is relatively low than
in those in which adult males are much larger than females (Pereira 1988). Postdispersal nepotistic effects are unlikely, and nepotism may not be consistently
important even in species with male philopatry. For example, while some maternal
brothers form alliances in chimpanzees, most allies are not maternally related
(Langergraber et al. 2007). Bonobos (P. paniscus) may be exceptional, in that
high-ranking females may form alliances with their sons that enable the sons also
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to attain high rank (Kano 1992), although how pervasive such effects are is
unknown.
In general, the extent of intersexual overlap in dominance ranks varies inversely
with sexual dimorphism in body size in cercopithecines, presumably because the
risk of female aggression to males increases as dimorphism in body size and canine
size increases (Packer and Pusey 1979; Thierry et al. 2000); these relationships help
to explain the inverse relationship between the sex difference in initial “dominance”
(i.e., fighting ability) and male female rank overlap in Hemelrijk et al.’s (2003)
self-structuring model. But in macaques, rank overlap among males and females
seems to depend on the extent of kin-biases in female behavior and the strength of
female alliances against males; thus, it varies along phylogenetic lines rather like
variation in female dominance style and is somewhat independent of variation in
sexual dimorphism (Thierry et al. 2000). Variation in male dominance style in
macaques also partly mirrors that among females. For example, tolerance between
males is lower, serious aggression is more common, and the directional consistency
of aggression higher in rhesus macaques than in more tolerant species (Thierry
2007). However, the influence of male rank on mating tactics and reproductive
success depends on variation in female mating synchrony in a manner independent
of female dominance style variation (reviewed in Thierry 2007). Rank strongly
affects reproductive success in nonseasonally breeding species, including highly
tolerant ones (e.g., Tonkean macaques, Macaca tonkeana), in which low-ranking
males must win aggressive challenges against high-ranking males to have good
prospects for siring offspring (cf. van Noordwijk and van Schaik 1985). In contrast,
reproductive skew is low in seasonally breeding species, including despotic ones
(e.g., rhesus macaques); severe aggression is correspondingly uncommon except in
small groups, and rank tends to increase with age and length of group residence.
Variation in population density and demography can strongly influence the costs
and benefits of dispersal and of alternative mating tactics for males (e.g., yellow
baboons: Alberts and Altmann 1995; macaques: Soltis 2004). Such variation may
not fundamentally affect male dominance style; for example, male yellow baboons
in low-density populations may delay dispersal, but their ranks, and thus to important extents, their reproductive success, still depend on their fighting ability (Alberts
and Altmann 1995). Nevertheless, the potential for such effects bears further
investigation, as does variation in the steepness of male hierarchies.
5.6
Power and Politics
Variation in dominance style reflects variation in power and raises questions about
political maneuvering. The term “power” is used widely, but often without explicit
definition. For example, Datta (1983a,b) implied that power in rhesus macaques
(M. mulatta) comprises individual fighting ability plus any competitive advantages
gained from coalitionary support. Likewise, “politics” is often not explicitly
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defined. De Waal (1982) characterized the tactics of alliance formation (including
competition for partners and disruption of potential alliances between rivals),
strategic and sometimes conditional use of reconciliation, and other ways in
which chimpanzees manipulate their social partners and manipulate relationships
between other individuals, as political. Subsequently, de Waal (1989) specified that
the need for allies in within-community competition for status and the need for all
males in a community to cooperate in aggression against neighboring communities
are “internal and external ‘political reasons’” why male chimpanzees require
effective mechanisms to cope with within-community competition. In describing
challenges for the alpha position among male chimpanzees at Mahale, Nishida and
Hosaka (1996) also stressed the complexity and flexibility of alliance formation
strategies and the varied ways in which males use social resources like grooming
and meat sharing to maintain alliances and referred to such behavior as political (cf.
Mitani and Watts 2001; Watts 2002; Mitani 2006). Duffy et al. (2007) characterized
the Kanyawara alpha male’s selective tolerance of his allies mating behavior in
exchange for coalitionary support as a political tactic from which all parties
benefited, despite potential tradeoffs (less than maximum mating monopolization
vs. prolonged tenure as alpha; additional current mating opportunities vs. foregone
attempt to challenge for the alpha position).
A review of the many ways in which political anthropologists have characterized
power is beyond the scope of this chapter, but brief attention to some of this
literature provides a useful context for considering power and politics in nonhumans. Some explanatory paradigms (e.g., postmodernist ones) provide little or no
basis for comparative analysis (sometimes deliberately), but others, such as “processualism,” are more amenable. Processualists see politics as the processes
involved in determining and implementing public goals and the differential use of
power by group members concerned with those goals (Swartz et al. 1966; Adams
1977; Kurtz 2001; Lewellen 2003; Swartz et al. 1966; Box 5.1). Power has many
dimensions, not all political; for example, it is embedded in healing rituals (Kurtz
2001). Weber’s (1965 [1947]) definition of power as the relative ability of one actor
to carry out his or her own will despite resistance underlies the relevant notion of
political power, and many anthropologists extend politics to the pursuit of individual goals as well as group goals. For example, Kurtz (2001: 21) writes:
“Politics is all about power: about how political agents create, compete for, and use power
to attain public goals that, at least on the surface, are perceived to be for the common good
of the political community. Yet just as open and more covertly, political power is used to
attain private goals for the good of the individuals involved.”
Processualists divide power into several broad categories (Box 5.1). Independent
power is based on individual capabilities. Dependent power is granted, delegated,
or allocated to others by someone who has independent power, but that individual in
turn is subject to consensual power (i.e., the assent of the people).
Anything that contributes to or maintains power counts as support; two basic
supports are coercion and legitimacy (Box 5.1).
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Box 5.1. Selected examples of how “power” and “politics” have been
conceptualized in the literature on political anthropology.
Politics: “The processes involved and determining public goals . . . and the
differential use of power by the members of the group concerned
with those goals.” (Swartz et al. 1966: 7)
Power: “. . . a quality ascribed to people, and often also to things, that
concerns their relative abilities to cope with the real world or their
potential effects on it . . . [power cannot exist alone,] but must be
recognized by others and by the individual possessing it.” (Adams
1977: 389)
“. . . the probability that one actor in a social relationship can carry
out his will despite resistance.” (Weber 1965 [1947])
An ability to control the behavior of others and/or to gain controlling
influence over others; the ability of one individual to bend another to
his or her will. (Lewellen 2003)
Components of Power in the “Processualist Paradigm” (Adams 1977;
Lewellen 2003):
l
l
l
l
l
l
l
l
Independent power: “A relation of dominance based on the direct capabilities of an individual, such as knowledge, skills, or personal charisma.”
(Lewellen 2003: 231).
Dependent power: Power that is granted, allocated, or delegated, either
by someone who has independent power over its recipient or to a more
powerful individual by his or her supporters.
Granted power: Decision-making rights that one individual gives to
another.
Allocated power: Power given by a group of people to a certain individual
(e.g., a “Big Man,” a shaman).
Delegated power: Decision-making rights given to a number of different
people by a single individual who has a concentration of individual power.
Consensual power: Leadership that arises from the assent of the people
(based on tradition, faith in the personal qualities of the leader, etc.) rather
than from force alone.
Support: A broad concept that includes virtually anything that contributes
to or maintains political power. Two basic supports are coercion and
legitimacy.
Legitimacy: A primary basis for power that derives from the people’s
expectations about the nature of power and how it should be attained (e.g.,
by election, by holding redistributive feasts) and used.
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D.P. Watts
Box 5.1. continued
Power resources (Kurtz 2001):
l
Material resources:
1. Tangible resources: Culturally defined goods (e.g., money) that individuals compete for and use to attain their goals.
2. Human resources: allies and supporters that any political agent
requires to be a leader.
l
Ideational resources:
1. Symbolic resources: material objects, mental projections, actions,
ideas, or words that humans infuse with ambiguous, multiple, and
disparate meanings; political symbols may be anything in the social
or physical environment that leaders can use to convince people to
support them; fluid, changeable, respond to shifting social, cultural, and
political conditions.
2. Ideological resources: a political ideology is “a system of hypotheses,
principles, and postulates that justify the exercise of authority and
power, asserts social values and moral and ethical principles, sets
forth the causal connections between leaders and the people they
govern, and furnish guides for action. . .around a set of beliefs and
ideas.” (Kurtz 2001: 35).
3. Informational resources: Information both includes and produces
knowledge; to the extent that leaders can produce and harness the
flow of information, it becomes a source of political power.
Kurtz (2001) presents a rather different perspective, in which political power
depends on the ability of agents to acquire and maintain control over resources;
this resonates with the emphasis on resource competition in the primatological
literature, but redistribution of resources often accompanies control in humans. In
political struggles, those who control more resources tend to win against those who
control less, but agents who control less can win if they use resources more wisely
and skillfully. Some resources are material; these include tangible objects and
goods as well as human supporters (Box 5.1). Other crucial resources are ideational.
These include ideological, symbolic, and informational resources (Box 5.1). Political rhetoric the deliberate exploitation of eloquence in public speaking or writing
by leaders to persuade others is the most common source of information as
political power and is a pervasive and extremely important alternative to coercion.
Independent power thus depends on the ability to control culturally constructed
ideational resources as well as material resources; a leader must be good at using
ideational resources to attract supporters, but maintaining their support ultimately
depends on providing them with sufficient tangible resources.
Chapais (1991) and Lewis (2002) explicitly have characterized power in nonhuman primates by drawing on literature in political science. This literature mostly
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concerns modern nation states and lacks the comparative ethnographic focus of
political anthropology, but shares certain themes with it. Chapais (1991) adapted an
explanatory framework developed by Bacharach and Lawler (1980). Echoing
Weber, power in this framework is defined as the capacity of an actor to carry out
its own will despite resistance. It can be “aggression-based” that is, based on
aggression or threat and thus coercive, although this category also includes power
based on control of resources, services, or knowledge or “dependence-based,”
with the power of actors derived from support that others give them. Acquisition
and maintenance of dominance rank in nonhuman primates provide good examples
of both categories (Chapais 1991). It generally involves aggression-based power,
but sometimes depends largely on agonistic support from third parties (notably
among females in macaques, baboons, and vervets; Chapais 1992) and is thus
dependence-based. Dependence-based power in this sense is in keeping with
Kawai’s (1958) “dependent rank.” It contrasts with the processualists’ dependentpower subcategories of granted and delegated power (Adams 1977; Box 5.1), in
which the arrow of dependency points in the opposite direction and those who have
granted or delegated power to others can withdraw it; it is closer to allocated power,
although without the collectively agreed conferral of status that this term implies.
But dependence by leaders is a major theme in political anthropology, and recurring
emphasis on the need to attract and retain supporters (“the single biggest problem
that any leader confronts”; Kurtz 2001: 34) indicates that dependency is bidirectional in humans. When one individual, human or nonhuman, has dependencebased power over another and has some ability to manipulate outcomes for it, their
relationship can take on a political dimension. Chapais (1991) noted that many of
the sources of power (whatever allows one to control its basis) that Bacharach and
Lawler (1980) identified in humans do not apply to nonhuman primates. For
example, “normative power,” a form of dependence-based power in which one
individual or group can bestow symbolic rewards on another, is exclusively human,
as is most, and perhaps all, power based on possession of knowledge that cannot be
acquired simply by observational learning. Control of information beneficial to
others is an important source of power in humans (Bacharach and Lawler 1980;
Chapais 1991) and a potential source in nonhuman primates, but nonhuman primates do not appear to bargain over information (Chapais 1991), and manipulating
others by withholding information is uncommon and may be limited to a few taxa
(e.g., chimpanzees: Hare et al. 2006). More generally, nonhuman primates make
little, if any, use of ideational resources.
Lewis (2002) applied “power” to asymmetries in social relationships that can
originate in individual differences in resource holding potential (i.e., fighting
ability) or in differences in the strength of alliances, but also in the possession by
some individuals of inalienable resources, broadly defined, that can influence the
fitness of others. Thus, it comprises both dominance and leverage. Dominance is the
agonistic component of power; it is based on force or threat of force. “Intrinsic”
dominance depends solely on interindividual differences in fighting ability and is
thus roughly equivalent to Chapais’ (1991) “aggression-based power.” “Derived”
dominance (cf. Datta 1983a, b) depends also, or largely, on the relative strength of
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coalitionary support and resembles Chapais’ (1991) “dependence-based power.”
When individuals could benefit from resources that others control or services that
they could provide, but cannot take these by force, the resource holders and service
providers have leverage; implicitly, this idea is also included in dependence-based
power.
If politics is all about creation of, competition over, and use of power, it
necessarily involves social maneuvering, and not all potential sources of leverage
are necessarily available for political use. For example, Lewis (2002) considers
possession of fertilizable eggs as a source of leverage that can allow estrous females
to gain temporary social advantages (e.g., increased receipt of grooming from
males) without changes in dominance, but females may gain such advantages
simply because others respond to signals of fertility or receptivity or to proceptive
behavior, not because they use these signals to manipulate others socially. In
contrast, when individuals have alternative options for distributing allogrooming
or coalitionary support, strategic deployment of these options may qualify as
political, especially given variation in partner quality. One individual has leverage
over another when the second depends on it in some way, and such dependence can
be mutual; their interactions can become political when one or both partners can
manipulate socially determined outcomes that the other values (Chapais 1991).
Leverage can either increase or decrease power asymmetries, as illustrated by
classic examples of male alliances in baboons and chimpanzees. Noë (1990)
documented the formation of coalitions by two or three mid-ranking male yellow
baboons against higher-ranking males in several contexts, notably in attempts to
separate high-ranking males from estrous females with whom those males were
consorting. In Lewis’ (2002) terms, coalitions temporarily increased the allies’
power relative to that of their opponents without changing their dominance ranks
(they still behaved submissively to their opponents in dyadic encounters; Noë
1990), but the highest-ranking of the three allies had leverage over the other two
(and thus increased his relative power over them) because they had little chance of
succeeding without his participation. This presumably explained why he took over
the consorts in all observed cases in which the coalitions succeeded. Noë characterized coalition formation by these males as a “veto game” with the highest-ranking
ally acting as the “veto player,” and pointed out that variation in the value of
potential partners should lead to shopping for, and bargaining over, their services
(cf. Noë 1992; Noë and Hammerstein 1994). Variation in partner value means that
dependence, while bidirectional, is not always symmetrical, but bargaining can also
occur in established alliances, and asymmetries can be constrained, when weaker
partners have enough leverage (Noë 1990, 1992). Baboon males obliged to negotiate with weaker allies bear some resemblance to “weak leaders” among humans,
who “have allies whose commitments to them are transactional. . .based on what
they can get for their support, which therefore is tenuous” (Kurtz 2001: 45).
Derived power is important in status competition among male chimpanzees, and
alpha males usually depend on allies to attain and maintain their positions. The
third-ranking of three males involved in a status struggle in the Arnhem Zoo
community mated more often than expected, based on his rank, while another
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male depended on his support to consolidate his newly attained status as alpha (de
Waal 1982). However, his mating frequency declined when the new alpha’s
position was secure. Nishida (1983) described a similar case in a wild community
that had only three adult males. Duffy et al. (2007) found broader leverage effects in
a larger chimpanzee community at Kanyawara, in which mating success for males
other than the alpha was positively correlated with the frequency with which they
gave the alpha coalitionary support, independently of their own dominance ranks.
The alpha male exerted some control over others’ mating success by disrupting
many copulation attempts; the frequency with which he disrupted those of individual males was inversely related to the amount of support they gave him.
Lewis (2002; see Chapais 1991, for a somewhat different categorization) defined
four proximate characteristics of power: its base, or source (e.g., fighting ability); its
means, or the way in which individuals negotiate it in relationships (e.g., by using
force); its amount, which can vary with context and can be expressed as a probability
of winning contests; and its scope, or the range of behavior that an individuals can
invoke in others by using dominance or leverage. This framework can help to
resolve several longstanding debates and to clarify some terminological confusion.
Lewis (ibid.) gave de Waal’s (1986) “formal” and “real” dominance as one
example: formal dominance involves the use of formalized status signals that
are consistent across contexts; thus, its scope differs from that of real dominance,
which can vary contextually and which also involves derived dominance and
leverage.
The issue of “female dominance” in lemurs also benefits from reframing as a
question about power. Male female asymmetries are pronounced in some lemur
species; notably, female ring-tailed lemurs (Lemur catta) win all contests against
males and evoke formalized submissive signals from them, and aggression is
unidirectional from females to males (Pereira et al. 1990; Pereira and Kappeler
1997). However, females are less powerful in other species, and the scope of power
differs; for example, aggression is bidirectional in brown lemurs (Eulemur fulvus),
males win many contests, and formal dominance does not exist (ibid.). Researchers
have sometimes used “co-dominance” to refer to species in which neither sex
consistently wins contests against the other (e.g., gibbons: Leighton 1987) or
“female feeding priority” to refer to those in which females typically win contests
over food, but not always in other contexts and in which aggression is bidirectional
and formal dominance between males and females absent (e.g., bonobos, P. paniscus: White 1996). Specifying whether sex differences in the amount of power exist
would be more productive than arguing about whether a given species has “female
dominance” or just “female feeding priority” and about whether these are different
phenomena. As Lewis (2002: 154) notes, “female dominance to males occurs only
when female fighting ability is superior to that of males in intersexual dyadic
interactions.” Likewise, if no such dyadic asymmetry exists between males and
females, no dominance exists, even if a leverage asymmetry means that the sexes
differ in relative power; “co-dominance” is a meaningless term in this context.
Flack and de Waal (2004) have made the most elaborate attempt to define power
in nonhuman primates and to link it to politics, on the one hand, and to dominance
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D.P. Watts
style, on the other. In their scheme, dominance style operates on the level of social
relationships and refers strictly to the discrepancy between the inherent agonistic
asymmetry between individuals (roughly, fighting ability) and the degree to which
they express this asymmetry, as indicated by the directional consistency of aggression, the typical intensity of aggression, and the types of signals used to indicate
dominance or subordination. When inherent asymmetries are high and are routinely
expressed in social interactions, for example, dominance relationships are despotic;
when they are high to moderate, but only weakly expressed, relationships are
tolerant. They did not explicitly distinguish intrinsic from derived dominance, but
included alliance formation as a “contextually and temporally stable factor”
(p. 169) that makes agonistic outcomes more predictable and thus influences
agonistic relationships.
Flack and de Waal (ibid.) linked dominance style to politics via “social power,”
which they defined (p. 167) as “the degree of implicit agreement among group
members that an individual is capable of using force in polyadic social situations.”
In turn, force “leads to the reduction or elimination of the choices of others”
(p. 168). They restricted social power to species that use formal signals of dominance
or subordination (e.g., macaques, chimpanzees), arguing that “implicit agreement”
can only occur if individuals consistently acknowledge status differentiation in
nonaggressive contexts. They proposed that we can operationalize social power by
comparing, for each individual, the number of others from whom it receives and to
whom it gives signals of subordinance or dominance, the frequency of these signals,
and the way in which they are distributed among other group members. Such
operationalization provides a basis for interspecific comparison, but, as Flack and
de Waal acknowledge, their concept of social power is far narrower than those of
Chapais and Lewis, and it would exclude many species (e.g., ring-tailed lemurs and
gorillas have social power, but brown lemurs and chimpanzees do not). When social
power in their sense exists, groups have power structures, which lead to “political
systems” at the societal level that reflect “the interplay between the power structure
and conflict management” (p. 157). They classified political systems (Table 5.1)
based on four main factors: how much social power is concentrated in single
individuals versus distributed among all group members, who intervenes in conflicts
and what intervention strategies they follow, the extent to which interventions in
conflicts reinforce or reduce social power differentials, and how equally resources
are distributed.
5.7
Politics and Cognition
The claim that some nonhuman animals engage in politics has been criticized. For
example, Schubert (1991) argued that nonhuman primate “politics” is mostly a
metaphor and particularly criticized Hrdy (1977) for referring to “regimes” and
“usurpation of power” by extra-group males that lead to “regime changes” in
hanuman langurs. Boehm (1997) was more sympathetic when comparing power
5 Dominance and Politics
127
Table 5.1 Dominance style, social power, and political systems (after Flack and de Waal 2004)
Political system
Dominance Distribution of social power
style
Despotic
Uniform; increases as social power rank Hierarchy: resource allocation
(SPR) increases
determined mostly by SPR; conflict
interventions reinforce system
Informal oligarchy: some resource
Tolerant
Concentrated in a few individuals and
allocation by SPR; powerful 3rd
distributed uniformly among others
parties intervene in conflicts
so as to increase with SPR
impartially or to favor least powerful
participants, others intervene to
reinforce hierarchy
Concentrated in a few individuals; others Constrained: Leveling coalitions,
have approximately equal power
policing by powerful individuals, and
mediation maintain system
Relaxed
Temporally stable, but small, differences Equal outcome system: maintained by
in social power
coalitions against individuals
intolerant of subordinates and
mediation by powerful individuals;
policing can lead to equal resource
distribution; may be institutional
roles
Egalitarian No temporally stable differences in
Equal opportunity system: maintained by
social power, but some individuals
punishment of norm breakers and of
may temporarily be more powerful
nonpunishers; can lead to division of
than others
labor among arbiters and impartial
policing to mediate conflicts between
coalitions
asymmetries in chimpanzees and human hunter-gatherers. He defined “political
intelligence” as “the decision making capacity that enables social animals to
further their self-interests in situations that involve rivalry and quests for power
and leadership.” He ascribed political intelligence to chimpanzees and other nonhumans, but both too broadly and too narrowly, regarding all forms of agonistic
behavior involved in such decisions (e.g., bluffing, appeasement, aggression, deference) as its manifestations. By implication, leverage that involves social manipulation also represents political intelligence. This perspective risks conflating politics
with power. Attempts to manipulate relationships between others (e.g., separating
interventions by male chimpanzees; de Waal 1982) may well be political, but
invoking political intelligence adds little to the understanding of, for example,
how opponents assess each others’ fighting abilities or why targets of aggression
might sometimes appease their attackers by directing nonaggressive acts or signals
to them that reduce the probability of further attack. Conversely, restricting political
intelligence to decisions about “whether to try to dominate or submit to” others
(Boehm 1997: 354) neglects how coalition formation can influence power dynamics by reinforcing dominance relationships, by providing successful partners with
temporary advantages (e.g., takeovers of consorts with estrous females by male
baboons), or by conferring derived dominance in some relationships (e.g., alliances
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D.P. Watts
that allow male chimpanzees to attain and maintain alpha status). Correspondingly,
it omits the importance of leverage and of market effects in competition for allies
(Noë 1990, 1992; Noë and Hammerstein 1994).
Boehm (1997) argued that political decisions do not necessarily require complex
cognition. However, we should turn this around to argue that decisions that enable
individuals “to further their self-interests in situations of rivalry and quests for
power” are only political if they involve cognitively complex social manipulation.
This begs the question of what qualifies as complex cognition, but at least any
species capable of triadic awareness can behave politically in this sense. Individuals
who can acquire knowledge about the social relationships between others in their
groups can use this knowledge in calculated, sometimes opportunistic, ways to gain
extrinsic dominance and to exert leverage over allies and potential supporters
(Preuschoft and van Schaik 2000) and to avoid or mitigate the costs of conflicts.
Flack and de Waal (2004) seem to imply that “political systems” require such
knowledge. It is unclear whether their restriction of social power to “polyadic
situations” means that it operates only in polyadic interactions; I take it to mean
that it can only operate in groups of three or more individuals, in which two or more
can “agree” that another can use force against them. But they state that conflict
mediation by socially powerful individuals, which occurs in “constrained” and
“equal outcome” political systems (Table 5.1), requires cognitive empathy (the
ability to take others’ perspectives), and list several macaque species and chimpanzees as possible exemplars of these systems. Cognitive empathy would certainly
allow for triadic awareness. Additionally, their scheme applies specifically to social
variation in macaques, all of which are presumably capable of such awareness.
The list of species in which triadic awareness has been formally demonstrated
either experimentally or through statistical modeling is short, but further formal
investigation would undoubtedly lengthen it. Chacma baboons show triadic awareness in many ways (reviewed in Cheney and Seyfarth 2007), including avoiding
close maternal kin of higher-ranking females from whom they have just received
threats and responding more strongly to playbacks that simulate dominance rank
reversals between adult females belonging to different matrilines (which are rare
and threaten to disrupt the entire female dominance hierarchy) than to those
simulating within-matriline rank reversals (also rare, but with much less potential
to disrupt other dominance relationships; Bergman et al. 2003). Playback studies
also showed triadic awareness in vervets (reviewed in Cheney and Seyfarth 1990).
Perry et al. (2004) showed that white-faced capuchins at Lomas Barbudal solicited
coalition partners who had better-quality social relationships with themselves than
with their opponents more often than expected by chance, and also solicited
partners who outranked their opponents more often than expected by chance,
although they might have done this simply by preferentially soliciting partners
higher-ranking than themselves (cf. Range and Noë 2004, for mangabeys, Cercocebus torquatus). Nonrandom solicitation of potential coalition partners on the
basis of relative relationship quality, relative rank, and/or relatedness to opponents
has also been demonstrated in bonnet macaques (Macaca radiate: Silk 1999) and
Japanese macaques (M. fuscata: Schino et al. 2006). Conflict management and
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129
resolution tactics apparently based on the recognition of relatedness between
opponents and third parties (e.g., kin-redirected reconciliation and aggression;
reviewed in Das 2000 and Watts et al. 2000) provide indirect evidence that triadic
awareness is widespread among cercopithecines and perhaps other primates, and
use of separating interventions is one of many forms of circumstantial evidence for
triadic awareness in chimpanzees.
Triadic awareness combined with comparative knowledge of partner value
allows for politics, which is partly a strategic use of such knowledge. A female
chacma baboon who opportunistically forms a bridging alliance or a revolutionary
alliance (Chapais 1991, 1992) to challenge another female to whom she is subordinate, but who has just lost a challenge to a third individual (Engh et al. 2006;
Cheney and Seyfarth 2007) is behaving politically. Likewise, while he was alpha
male in the Mahale M group of chimpanzees, Ntologi behaved politically by
regularly directing separating interventions at other males, mostly at his main
rivals, Nsaba and Kalunde (who nevertheless formed an alliance that allowed
Kalunde to defeat Ntologi, although his tenure as alpha was then short; Nishida
and Hosaka 1996). Politics also encompasses the strategic use of knowledge about
variation in partner value and the corresponding strategic use of leverage, such as
the decision by Yeroen, the third ranking male in the Arnhem Zoo chimpanzee
colony, to ally himself with Nikkie, then a weak alpha, rather than Luit; Nikkie
needed the derived dominance provided by a strong alliance more (de Waal 1982).
Several primate species tested in lab settings can solve tasks that require coordinated efforts by two partners (e.g., brown capuchins, Cebus apella: Mendres and
de Waal 2000). This suggests that they recognize the necessity of acting jointly and
the value of partners and the services they can provide; his would facilitate political
exploitation of variation in partner value and political negotiation over services. An
alternative explanation is that they simply learn contingencies between their actions
and obtaining rewards (ibid.). However, this seems inadequate to explain differential recruitment of partners on the basis of their task-solving skills by chimpanzees
(Melis et al. 2006).
Politics may require triadic awareness, but it can occur at the level of dyads. One
possible political tactic would be to induce a loser effect in a potential rival by
targeting him or her sufficiently to prevent, or at least forestall, any competitive
challenge. Unpredictable attacks independent of direct contests over resources
might be particularly effective (Silk 2002). Rank changes were common in the
baboon group studied by Bergman et al. (2006) and males in that population did not
form alliances. Induced loser effects, if they occurred, might thus have often been
short lived, but this does not preclude the possibility that they form part of individual competitive strategies. Male chimpanzees are good candidates for such effects
because male philopatry means that adolescent males will become rivals of those in
older age cohorts, who could benefit by delaying challenges from the adolescents
and who could increase the delay by forming coalitions against them. This might
help to explain why adult males direct aggression at adolescents at high rates
often higher than those for adult dyads and sometimes single out particular
individuals for persistent attacks (Pusey 1990, Watts unpubl. data). Aggression
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D.P. Watts
sometimes produces direct benefits
e.g., adults sometimes steal meat from
adolescents but much of it may be punishment, in that it has an immediate
energetic cost, but changes the target’s future behavior in favor of the aggressor
(Clutton-Brock and Parker 1995), essentially via negative conditioning. Thinking
of punishment as a manifestation of political intelligence in chimpanzees seems
reasonable, but the common occurrence of age-related loser effects in ungulates
suggests that we should not always assume that political intelligence is involved.
Female dominance rank in many ungulates increases with age (reviewed in Côté
2000), and age is the main influence on rank among female mountain goats
(Oreamnos americanus), which form stable hierarchies in which neither the body
size nor the horn length influences rank and in which attacks by adult females lead
to persistent subordination younger females even after the younger females attain
adult size (ibid.)
5.8
Politics in Human versus Non-Human Primates
Power and politics in human and nonhuman primate share some similarities and
contrast in many ways, only a few of which I will briefly consider. First, politics and
power in nonhuman primates revolve around social relationships (or, in Flack
and de Waal’s (2004) view, politics arises from social relationships via the mediating effects of social power), and political anthropologists sometimes also stress that
power is a component of human social relationships (e.g., Adams 1977). Face-toface interactions in which individuals or small coalitions either directly assert their
own interests, with at least an implicit threat of force, or use social persuasion to do
so form part of human politics and would have characterized most human political
behavior during our evolutionary history in small-scale societies (Archer 2006). In
some respects, the forms and outcomes of such interaction resemble chimpanzee
political interactions and other aspects of power in nonhuman primates (reviewed in
Chapais 1991). Humans use visual and vocal threats, engage in physical aggression
and contest access to resources as individuals or members of small coalitions, and
compete over social partners. However, even at the interpersonal level, humans
have sources of power unavailable to other primates. An obvious example is the use
of weapons in intraspecific aggression, a uniquely human source of aggressive
coercion that distinguishes power relations in our species from those of other
nonhuman primates (Gat this volume). But another obvious example, one that
highlights human cognitive uniqueness and is probably more important than physical coercion, is the use of political rhetoric (Kurtz 2001).
Aggression-based dominance hierarchies like those in nonhuman primates,
whether they result solely from intrinsic dominance or also involve derived dominance, are uncommon in humans, at least among adults (Chapais 1991). Nor should
the association of outstanding hunting skill with high reproductive success, common among human foragers, be equated with the positive relationship between
dominance rank and male reproductive success in many nonhuman primates.
5 Dominance and Politics
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Hunting skill may provide leverage, but without conferring coercive power or any
formal political authority. Thus, Ache men known to be good hunters can get
preferential treatment from others in their bands regarding decisions about group
movements, but because they can threaten to transfer to other bands and thereby
depriving others of their hunting returns, not because they have any formal political
power (Hill and Kaplan 1988). Leaders in hunter-gatherer societies and other smallscale human groups are “episodic” or “weak” (Kurtz 2001) and must lead by
example and by persuading others (e.g., Tsimane village chiefs: Gurven and
Winking 2008). Regardless of the extent to which power ultimately rests on
coercive ability, humans have many more sources of dependence-based power
than do any nonhuman primates, thus many more ways to control and manipulate
the needs of others (Chapais 1991) and many ways to create needs. As Chapais
(ibid.: 216) notes, “the most powerful individuals (the ones most able to control
the behavior of others) are those with the greatest number of individuals depending
on them for the satisfaction of needs.” The obverse of manifold sources of
dependence-based power is the existence of far more sources of leverage than are
available to any nonhumans, such as possession of specialized skills or knowledge
and even influence over others’ prestige and self-esteem.
Coalitions in nonhuman primates are often conservative or polarizing, in that
they accentuate dyadic power asymmetries (Chapais 1992, 2001; Preuschoft and
van Schaik 2000). When female macaques follow the “support the high born” rule
by forming coalitions with other females whose mothers outrank their opponents,
they help the females they support to attain or maintain dominance over the
opponents; given that females intervene mostly against targets that they also
outrank, such support reinforces the existing dominance hierarchy and is best
regarded as mutualism (Chapais 1992, 2001). Likewise, male chimpanzee coalitions at Ngogo mostly include partners who both outrank their targets (Watts and de
Vries 2009). But leveling coalitions also occur in some primates. In these, either
coalition partners compensate for dyadic power asymmetries by collaborating
against higher-ranking opponents, or high-ranking individuals intervene in conflicts
on behalf of subordinates against opponents that they outrank and thereby suppress
within-group competition, (e.g., Barbary macaques, Macaca sylvana: Preuschoft
et al. 1998; Preuschoft and Paul 2000; yellow baboons: Noë 1992; Noë and
Hammerstein 1994; chimpanzees: Nishida and Hosaka 1996). At an extreme,
individuals can reverse dyadic power asymmetries by forming revolutionary alliances (Chapais 1992); in these, partners collaborate to reverse dominance ranks
with targets to which they are subordinate (e.g., male chimpanzees: de Waal 1982;
Nishida and Hosaka 1996; female chacma baboons: Engh et al. 2006).
But leveling coalitions in cercopithecines and among male chimpanzees
co-occur with linear dominance hierarchies and, although they may limit the ability
of high-ranking individuals to monopolize resources (e.g., they may reduce reproductive skew among males), they do not lead to the “egalitarian” politics of human
hunter-gatherer societies known from the ethnographic record (Boehm 1997).
Recognition of individual merit and accordance of limited authority to certain
individuals occurs alongside tracking of people’s behavior and linguistic
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D.P. Watts
communication about whether this stays within bounds acceptable to culturally
variable moral communities. Humans also have broad means to constrain powerful
individuals; some of these, like imposition of moral sanctions, are unavailable even
to chimpanzees (Boehm 1997; Kurtz 2001). Likewise, a growing body of evidence
indicates that people often engage in third-party punishment at some cost to
themselves, although considerable cross-cultural variation in willingness to do so
exists (Henrich et al. 2006; Gurven and Winking 2008) and people’s behavior in
economic games may not match that in the real world (Gurven and Winking 2008,
Plourde this volume).
In experiments, captive chimpanzees retaliate against others who have stolen
food from them (Jensen et al. 2007a) and respond negatively to situations in which
they have received unfairly small rewards (Brosnan et al. 2005). De Waal and
Luttrell (1988) documented negative reciprocity (or “bidirectionality”) in agonistic
interventions against others among the Arnhem Zoo chimpanzees that is, individuals often supported others against third parties who in turn often intervened
against them; male chimpanzees at Ngogo show similar bidirectionality (Watts
unpubl. data). De Waal and Luttrell (1988) labeled this bidirectionality as a
“revenge system,” and referred to aggression by food possessors toward others
who were trying to obtain food from them in a food sharing experiment, but were
reluctant to share when they possessed the food, as “moralistic aggression.” De
Waal (1989) subsequently argued that revenge and moralistic aggression “introduce
powerful sanctions to a social system.” However, such behavior appears to be
entirely egocentric. Chimpanzees seem to be unconcerned with whether others
have been treated fairly (Jensen et al. 2007b), and although retaliation could be
considered a form of punishment, currently available evidence indicates that they
do not engage in costly third party punishment (Jensen et al. 2007a). Such findings
seriously question Flack and de Waal’s (2004) categorization of political systems in
which, for example, they consider chimpanzees as possible representatives of the
“equal outcome” category (Table 5.1), in which policing by powerful individuals
leads to an equal distribution of resources; how this could occur when powerful
individuals are unconcerned with whether others achieve fair outcomes and are
perhaps incapable of such concerns (but see de Waal 2008, for a contrary view), is
unclear.
Absent the ability to form moral communities, chimpanzees could also not
control the cost and efficiency of punishment by delegating authority to enforce
social norms to individuals and institutionalized subgroups at different levels of a
hierarchical society: an ability essential for the formation of large scale, hierarchical societies in the first place (Dubreuil 2008). As Boehm (1997) argued, politics
among male chimpanzees remain individualistic, even when males try to achieve
social goals as members of small coalitions. Although Flack and de Waal (2004)
note that “equal opportunity” political systems (Table 5.1) occur only in humans
and, implicitly, only humans construct institutional roles of the sort that can
characterize “equal outcome systems” (Table 5.1) they present their classification
of political systems as if it is a continuum, when in fact it incorporates these and
other qualitative disjuncture between humans and nonhuman primates. More notably,
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not even chimpanzees can engage in symbolically mediated leverage, competition,
and manipulation, and their lack of language stringently limits their ability to use
information as a source of power. In his comparison of power in humans and
nonhuman primates, Chapais (1991) listed many similarities; virtually all of the
contrasts involve semiosis or otherwise derive from cognitive differences. Aggressive coercion in nonhuman primates never involves moral justification or attribution of blame to victims, for example, nor does “normative power,” based on
symbolically based group norms, occur in nonhumans. Primatologists who write
about politics in apes and monkeys need to recognize the crucially important
semiotic and ideational dimensions of human politics struggles over meaning
and ways in which language, including political rhetoric, mediates these and to
pay attention to corresponding human cognitive uniqueness. We should acknowledge that to paraphrase Plotkin’s (2003) summary comment on calling socially
learned behavioral traditions in chimpanzees “culture” it is not politics as we know
it in humans, but it is politics of a kind.
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Chapter 6
Human Power and Prestige Systems
Aimée M. Plourde
Abstract Prestige is a major source of social power in human societies, and one
that is often much more important than physical dominance. This observation,
while obvious, gains significance in light of the fact that prestige does not exist in
other primates and so, must be a derived property of our species. Understanding
prestige systems will, therefore, further our understanding of social power and
inequality in human societies, and contribute to anthropological and archeological
theories of the evolution of social structures and institutions. Here, an evolutionary
theory for the emergence of prestige in the human lineage is discussed, and a model
to explain the reason why material items may have begun to be used in competition
for prestige presented, in which I propose that they initially functioned as costly
signals of an individual’s skill and expertise, aimed at learning individuals. It is then
hypothesized how and why prestige became involved in the emergence of sociopolitical and economic ranking through the increasing importance of leadership and
collective action, with the result that the signal content of prestige goods became
linked not simply to prestige but also to coercion and dominance through the
possession of wealth, elevated social class, and positions of authority.
6.1
Introduction
Like many other primates, humans live in social groups with complex status
hierarchies, are deeply motivated to strive for higher status, and spend a great deal
of time and energy in doing so. But in this, we see a significant difference between
humans and other primates; while social power in nonhuman species is generally
based on dominance, in human societies both dominance and prestige are important
A.M. Plourde
AHRC Centre for the Evolution of Cultural Diversity, Institute of Archaeology, University
College London, London, England
e mail: aimee.plourde@ucl.ac.uk
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 6, # Springer Verlag Berlin Heidelberg 2010
139
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determinants of social power. Prestige, from an evolutionary perspective, is a funny
thing; it is a source of social power that is freely conferred on an individual by others
as a result of the respect and admiration in which they hold that person.
The quest for prestige has led humans to do many things that, at first pass, seem
quite puzzling. For instance, people spend enormous amounts of money on items,
from Gucci handbags to Mont Blanc pens, which, from a functional stance, are
hardly (if at all) better than the ones available at a fraction of the price. Some
prestige items, including some designer sunglasses and jewel-encrusted wristwatches are, indeed, known to perform worse than economy models. Alternatively,
many prestige items may be of higher quality or functionality than nonprestige
equivalents, but it is often unclear whether the difference is “worth” the extra cost;
for example, a Ferrari may, in fact, outperform an economy car, but its owner is
unlikely to be able to take advantage of that fact in the course of daily life. Instead,
displaying them to others appears to be at least as important a reason for the
purchase of these kinds of objects as their purported function. And the importance
of prestige display is not limited to Western, or contemporary, societies. The
Potlatch ceremony performed in Native American groups located along the Pacific
Northwest Coast, in which not only the display but the destruction of both prestige
and utilitarian items occurred, is a classic case demonstrating the importance of
prestige goods in social dynamics, while some “Big Man” societies of Highland
New Guinea have actually been termed “prestige economies” because of the critical
role played by prestige goods in socioeconomic dynamics there. Further, archeological data from cultures as diverse as the Han empire of China, Phaeronic Egypt,
and the Inca empire of South America suggest that the display of luxury goods was
as important, if not more so, in these societies as in modern ones.
This chapter presents an evolutionary model to explain how such seemingly
wasteful and perplexing behavior may have emerged in humans. I will begin by
exploring the unique aspects of the operation of prestige in human power systems,
particularly in comparison with the power systems in other primate species, and
theories for the emergence of prestige in the human lineage. I will then present an
evolutionary model to explain why material items may have begun to be used in
competition for prestige, in which I propose that they initially functioned as honest
advertisements of an individual’s skill and expertise, aimed at learning or unsuccessful group members. Following from this model, I hypothesize how and why
prestige became involved in the emergence of sociopolitical and economic ranking
through the increasing importance of leadership and collective action, with the
result that the signal content of prestige goods became linked not simply to prestige
but also to coercion and dominance through the possession of wealth, elevated
social class, and positions of authority.
6.2
The Basis of Social Power in Primates
Many nonhuman primate species form societies characterized by social ranking,
where an individual’s place within the dominance hierarchy is determined by their
ability to defeat others in agonistic conflicts (Ellis 1995; Henrich and Gil-White
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141
2001; Watts, this volume). Size, strength, kin relations, and political alliances all
play parts in how dominance relationships are determined in nonhuman primate
groups.
Humans’ status hierarchies differ from those of other primates in two important
ways. First, the repertoire of coercive behaviors is greatly expanded. In addition to
personal qualities (e.g., strength, fighting ability), social qualities (e.g., affiliation
with kin, political alliances), humans can also coerce others through economic
strength, and through the possession of an office of authority. Individuals who
control resources or hold offices of authority can coerce others by offering positive
incentives for preferred behavior or threatening negative consequences for deviating from preferred courses of action. This expansion of the ways in which an
individual can dominate others via coercion is the product of the rise of complex
social organization and institutions, which emerged over the course of the last
10,000 years and is a relatively recent addition to human power relations. Perhaps
an even more striking difference between humans and other primates, and other
social species more generally, is that humans also possess a noncoercive means to
increase social status that is based on prestige.
6.3
Prestige: A Unique Source of Social Power
“Prestige,” as used in everyday speech generally refers to having the respect of
others, even deference from them, which is freely conferred, rather than being
compelled through violence, threat, or coercion. Prestigious people are honored by
their peers and wield influence over them. The source of this respect and influence
is the possession of skill and/or knowledge in some domain of activity, particularly
one that is valued (Henrich and Gil-White 2001, p 167). An important ramification
of this is that a person may be highly skilled in one domain, and thus have a large
amount of prestige in that arena, but have little prestige in other areas. The number
of independent, simultaneous prestige hierarchies of this kind could, in theory, be
as numerous as the number of recognized skills (Henrich and Gil-White 2001,
p 170), which in contemporary societies is quite a large number, but in more
traditional societies, including most societies in the past, was probably much more
restricted.
Prestige is likely to be even more important than dominance in human status
hierarchies (Barkow 1975, p 554). Ethnographic and historical evidence suggests
that prestige systems constitute a “human universal” (sensu Brown 1991), and there
are differences in the extent of prestige conferred on individuals within groups of all
sizes. In contrast, skews in power based on dominance do not exist in all societies.
Given its ubiquity, any complete explanation of prestige will necessarily include an
evolutionary account for its appearance in humans, addressing the questions of
what adaptive function the psychologies and behaviors associated with prestige
dynamics could serve, and what in the evolutionary past of humans led to strong
selection for prestige that was not present in all other species, including those most
closely related to us.
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6.4
A.M. Plourde
The Evolutionary Origins of Prestige
Henrich and Gil-White (2001) outlined a theory for the origins of prestige in human
social dynamics as a result of changes in the nature of cultural transmission. They
suggest that selection favored new strategies to augment social learning, as culturally
transmitted knowledge became increasingly complex and important for success.
Young or migrant individuals, who have a need to learn new skills, would benefit
more from learning from successful individuals than from other possible types of
models. Selection should, therefore, favor psychological mechanisms that (1) assist
the learning individual to determine which members of her group are successful, (2)
generate admiration for and attraction to successful individuals as motivation to
maintain proximity to them and to copy their behaviors, and (3) cause the learning
individual to treat successful individuals with respect and deference, in order to gain
proximity. Ethnographic observations suggest that prestigious individuals may receive other kinds of positive benefits in social relationships, such as leniency after a
transgression of group norms or a failure to reciprocate in dyadic relationships, and
more support for self and family members following injury (see Bateson 1958, p 91,
Hawkes 1991; Henrich and Gil-White 2001, pp 182 183). Henrich and Gil-White
further suggest that paying attention to the distribution of respect and deference by
group members could provide an efficient means for determining which group
members are most successful, reducing the amount of time spent by learners in
personal observation of others’ success rates and reducing error.
6.4.1
Competition for Prestige
Henrich and Gil-White’s theory provides an explanation for why learning individuals and successful models would benefit from prestige dynamics. The model
assumes that there is some cost associated with gaining access to successful models;
that is, learners literally “pay deference” to skilled models. Such deference is
advantageous to learners as long as the net benefits that they gain from what they
learn provides a level of success that is higher than they would have gained from
learning from parents or other close relatives without cost. As Shennan and Steele
(1999) have observed in a survey of ethnographic data, when the transmission of
skills and knowledge is strictly vertical (from parents or other close relatives), as
might be the case when variance in skill levels is low, the opportunity to acquire
prestige benefits from learning individuals should be limited.
Successful individuals should welcome the social perks and favors that they
obtain from learning individuals, provided that these benefits outweigh the costs
associated with the presence of admirers. Moreover, if the benefits derived from
prestige are sufficiently high, then, over time, psychologies that motivate individuals to desire prestige and to strive for it should be favored. These motivations
may be built upon the more ancient motivational systems that encourage
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individuals to strive for high dominance status, and competition for prestige should
be expected to emerge as a result.
How then might a successful individual act, given that prestige is freely conferred by others, to attract learning individuals and the social benefits that they
might confer? Henrich and Gil-White suggest that when competition for prestige is
high, successful individuals should modify their behavior to attract and retain the
admiration and benefits from learning individuals, such as being less arrogant and
more approachable (Henrich and Gil-White 2001, pp 171, 178 179). Another
strategy would be to signal her degree of skill in order to convince prospective
admirers that her skills are better than those of other potential models. The
performance of the skill in question is, perhaps, the most obvious way to accomplish this, but I have argued elsewhere (Plourde 2006, 2008, in press) that the
production of a signal with physical items would work particularly well because
goods are more durable than actions.
The idea that material goods (as well as behavioral displays) could function as
signals of personal qualities, including social standing, is an old one; Veblen
((1899) 1994) put forward the idea that people used “conspicuous consumption”
and display of costly luxuries to advertise their degree of wealth over a century ago.
Signaling theory has a substantial literature in economics and evolutionary biology.
Applications of signaling theory to human behavior from evolutionary psychology
and human behavioral ecology were initially focused on sexual selection (for
example, see Miller 2000) but interest in other kinds of signals has been increasing.
For instance, several scholars have proposed that contributions to public goods may
be costly signals made in an attempt to gain prestige and related personal benefits.
The contributions to public goods may include hunting large game animals for
public consumption (Bliege Bird et al. 2002; Smith et al. 2003) and the punishment
of transgressors for violations of group norms (Gintis et al. 2001). Archeologists
have also begun to draw on costly signaling theory in relation to status striving by
political elites (Neiman 1997; Boone 2000).
6.4.2
Prestige Goods as Costly Signals of Skill and Expertise
Physical objects that accurately reflect some aspect of an individual’s skills or
knowledge could act as advertisements in prestige competition. These prestige
goods could signal the kind of technical expertise involved in hunting and craft
production, levels of knowledge about environmental information, or the quality of
an individual’s relationships with people outside of his/her local group. Of course,
goods could also be used to signal success at a general level, providing a measure of
the amount of “extra” time and labor that an individual has. This kind of signal
would be of interest to many different kinds of audiences, including potential mates
and rivals, as well as individuals looking for a skilled tutor. The specific reference to
a skill is what distinguishes prestige goods from other kinds of signals of wealth and
success that probably did (and do) exist; mates’ and rivals’ primary interest would
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be in the level of success held by the signaler, not the details of the skills involved in
producing that success.
Signals produced by skilled individuals must be costly to produce in order to
maintain their value as honest signals (Zahavi 1975; Zahavi and Zahavi 1997) and
prevent signalers from deceiving their audience about their skills. This reflects a
potential conflict of interest between the signalers and the recipients. Signalers
always benefit from receiving deference and other social perquisites from admirers,
regardless of their level of skill, whereas learners only benefit if they limit deference to skilled or knowledgeable tutors.
A formal model of this idea (Plourde 2008) supports the logic behind it and
demonstrates that when (1) the cost of making the signaling is higher for unskilled
than for skilled signalers, (2) the benefits gained by the signaler from deference
outweigh the cost of producing the signal, and (3) the benefits to learners (signal
responders) from increasing their skills outweigh the cost of deference, a signaling/
responding strategy can both invade a nonsignaling equilibrium, and can resist
invasion by a nonsignaling strategy. Although the model is an extreme simplification of the complexities that must be involved in prestige dynamics, it nonetheless
suggests that this kind of behavioral strategy and the attendant psychologies
supporting it provide a viable explanation for why prestige goods came into
existence.
This account is partially supported by data from ethnographic and historical
studies of traditional societies. These accounts suggest people who are considered
to be highly skilled within their community are preferentially copied, have higher
status, receive deference and privileges, and are excused from some social obligations (Henrich and Gil-White 2001, pp 180 187). However, in these accounts, it is
often unclear how competence and prestige were determined by community members, or whether the display of prestige goods influenced prestige. It is not known
whether any of the goods displayed actually reflected skills or knowledge that
deferential admirers were interested in learning. In contrast, the use of material
goods as signals of wealth and social class in contemporary stratified societies is
ubiquitous (Clark 1986, p 82; Johnson and Earle 2000). In many cases, these
displays do not provide evidence of expertise in specific domains that have particular importance for learners. This leaves us with the following puzzle: if prestige
goods came into existence in order to be used in signaling skills and expertise, how
is it that the most eye-catching examples of conspicuous displays in contemporary
settings signal wealth and social class, qualities which may have little to do with
specific skills or expertise? Answering this question requires us to reconsider why a
signaling strategy might have evolved.
6.4.3
Selection for Signaling in Prestige Competition
The time frame during which the psychological mechanisms underlying prestigebased exchange of knowledge for deference came into existence is unknown.
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However, it was likely to be part of the transition to fully modern Homo sapiens,
because prestige seems to be a universal feature of contemporary human societies,
and archeological evidence also attests to the increasing importance of cultural
knowledge during this transitional period. If signaling via prestige goods existed as
a strategy before this time, then the signal’s content could not have been to signify
wealth and social class, assuming that wealth and class distinctions did not yet exist.
This implies that signaling of wealth and social status probably tapped into phylogenetically older signaling psychologies and behaviors. This brings forward the
question of how and when the use of novel competitive strategies, including the
costly production of signals, might have been favored. Previously, I have suggested
two factors that could have changed the social context in which prestige dynamics
were operating in ways which may have favored an expertise-signaling strategy: (1)
increases in group size and (2) increases in the complexity and/or the number of
skills relevant to success (Plourde 2006, in press).
Increasing the size of groups could change the dynamics of prestige to favor a
signaling strategy because, all other things being equal, the extent of variation in
skill and expertise should increase with the number of individuals in the group.
From the perspective of learning individuals, in large groups, it should be more
difficult to identify the most skilled individual within the pool of potential models.
Thus, as group size increases, it would be more beneficial for skilled individuals to
advertise their expertise. Further, with more skilled individuals in the group, the
learning individual has more potential models from which to choose, thus increasing the competition between skilled individuals for followers.
Alternatively, if the complexity or number of skills and realms of expertise
increase, so should the difficulty associated with mastering all the skills necessary
for success, making it all the more important for followers to choose the most skilled
models. This should cause followers to invest more in deference to secure access to
the most skilled models. As the benefits derived from deference increase, it could be
worthwhile for skilled individuals to incur more costs in competition with other
potential models, and these costs may include the production of prestige goods.
It is worth noting at this point that these theories for the origins of prestige and
for the development of prestige signaling do not rely on the presence of a sociopolitical hierarchy, competition for offices of leadership, or the existence of social
inequality. However, the factors that promote increases in levels of competition for
prestige could very well result from the same kinds of environmental and social
factors that are often implicated in the emergence of social inequality: increases in
population size, increasing group circumscription, and technological innovation.
6.5
Prestige, Signaling and the Origins of Inequality
If the same overarching forces drove both competition for prestige and the emergence of socio-politico-economic differentiation, this might explain why evidence
for both of these often overlaps in the archeological record. An example that
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demonstrates this pattern comes from the Lake Titicaca Basin region in the SouthCentral Andean Highlands of South America, where the first unambiguous evidence of prestige goods (in fact, the earliest known beaten gold artifacts in the
Americas to date) beads made from nonlocally derived gold (Aldenderfer et al.
2008) appear in the archeological record before other kinds of indicators of social
ranking, including differences in house size, settlement sizes, or diet, intensified
agricultural production, or the creation of civic/ceremonial architecture (for a
review, see Stanish 2003). The relatively early appearance of prestige goods here
suggests that the production of such items predated the existence of heritable rank
and wealth differences, and could have been involved in their creation (Plourde
2006; Plourde and Stanish 2006). I hypothesize that prestige and competition for it,
including the use of material goods as signals of skill and expertise, often played
a critical role in “transegalitarian” societies (Owens and Hayden 1997; Hayden
2001) namely those in which the development of social and economic institutions
of inequality occurs primarily as a means by which leaders were chosen.
6.5.1
Prestige Competition and Selection for Leadership Roles
Ethnographic observations of how leaders are chosen in contemporary “egalitarian”
societies ones without formalized or permanent rank differences suggest that
prestige plays an important part in who is selected, or accepted, by the group to act
as a leader in collective activities, such as combat (Chacon 2004), intergroup
alliance, and trade (Johnson and Earle 2000). Leaders generally come from
among those who are recognized as having the most experience and/or skill relevant
to the task at hand: in other words, they are respected, and in all likelihood have
prestige in that particular domain of activity or knowledge. In such societies, a
leader’s authority is ephemeral and generally restricted in scope to the activity at
hand. But in groups experiencing increasing selection for the ability to act collectively, due to increasing circumscription and competition with neighboring groups
or some other combination of factors, the need for leadership would occur more
frequently and would be more important to group success, thus selecting for more
elaborate and permanent roles of authority.
Many ethnographic and historic examples support the idea that groups with the
ability to organize themselves in efforts for the common good, such as military
action and cooperative tasks for production, often do better than neighboring groups
(Richerson and Boyd 1999). For instance, if intergroup conflict over resources or
land increases in frequency and intensity, groups with an individual recognized to
lead and organize group defense and raiding might be more successful than groups
without such a leader. The same logic could apply to leading and organizing group
labor for public goods projects, such as the construction and maintenance of
irrigation systems, fish weirs, and the like. If efficient collective action becomes
more relevant to a group’s success, then the presence of a leader to coordinate such
activity may become more common over time. In this way, competition for
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personal prestige, and signaling skill via the display of prestige goods, could
become linked to competition for positions of authority.
6.5.2
Prestige Competition and Status Striving
Individuals who occupy positions of authority gain personal benefits and opportunities to augment the wellbeing of themselves and their families. For example,
Johnson and Earle (2000, p 126) observe that headmen and Big Men are often
polygynous, and thus likely have elevated reproductive success. Clark and Blake
(1994) coined the term “aggrandizer” to refer to ambitious men who are described
in ethnographic and historical accounts of tribal and chiefly societies. Aggrandizers
desire prestige and use material goods to gain prestige and attract followers, and are
thought to be largely responsible for the beginnings of social hierarchy.
Hayden’s (1998) model of the evolution of prestige technology posits that the
major changes in human societies that have led to the emergence and elaboration of
sociopolitical ranking result from the actions of aggrandizing individuals who
attempt to promote their personal self-interest through various means, including
the use of prestige goods. Hayden posits that “aggrandizing personalities” exist in
all human populations, but need only constitute a small percentage of a group in
order to provoke change in social structure. He defines a person with this personality as ambitious, socially, politically and economically aggressive, acquisitive, risktaking, as someone who manipulates other individuals in order to promote his or her
own self-interest, and who often acts selfishly rather than in the interest of the
community. He concludes that “. . .aggrandizers have, in effect, an inner motor, an
inner drive to increase their own standard of living and their own reproductive
success” (1998: 18 19).
The model of the function of prestige goods function that I have presented here
contributes to our understanding of the evolution of aggrandizing behaviors. An
evolutionary perspective suggests that all people will have an “inner motor” that
motivates them to improve their well-being and their own reproductive success, and
to enhance the welfare of their kin. I propose that the desire to attend to, admire,
own, and display prestige goods exists in all individuals, and is not limited to a
certain personality “type.” Instead, as an individual’s skill level changes over the
course of her lifetime, so too should her interest in attending to and obtaining and
displaying prestige goods. Which behavioral strategy a person uses at a given time
could be considered state-dependent, as the decision to obtain and display prestige
goods will depend on the level of skill and success currently possessed by the
individual, and will fluctuate over the course of the individual’s life as her competence changes or the availability of superior models changes.
The development of elevated social positions, and eventually of ranked social
classes, would have had considerable impact on the expression of these psychological dispositions. Both competition for prestige and the desire to possess and
display prestige goods will be augmented when the benefits derived from holding a
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leadership position increase. More importantly, once social ranking in wealth and
status classes is established, this would constitute a new kind of information that
would be beneficial to convey to potential mates, allies, and competitors. If prestige
goods are used as signals to relay status information, this could intensify the desire
to own and display prestige goods and influence leaders’ interactions with followers. For example, leaders might distribute prestige goods to their followers. This
would be an effective strategy because it would take advantage of the psychology
already in place that encourages ownership of prestige items as a means of
increasing prestige and deriving the associated benefits, and as a form of imitation
of the behaviors of successful individuals. Aggrandizing individuals use the display
and distribution of prestige items to group members to tap into these deep-seated
desires for their own benefit, and this constitutes one of the mechanisms by which
hierarchical power relations within the group develop. In fact, it would seem likely
that prestige competition could create a positive feedback system in which, as the
family and allies of would-be leaders contribute to the production of prestige goods,
the goods themselves would inevitably come to embody status. This sort of
Veblenian process has been modeled by Boone (1992, 1998, 2000); Boone and
Kessler 1999).
6.5.3
Signaling and Group Competition
Another significant transformation of human power relations is due to the increasing impact of interaction between politically defined and hierarchically organized
groups. The competitive interaction between communities that would favor the
existence of collective action and leadership, and the benefits to be gained from
occupying a leadership role for an individual and his/her kin and corporate group,
could lead to the formation of political factions within communities and hierarchical power relations between communities and their leaders. As politically and
economically stratified societies emerged through processes of conflict and alliance, the quality of group strength would become much more important than in
egalitarian societies. In terms of competition for prestige, this may also be viewed
to some degree as a result of leaders’ ability to control and direct the labor of others.
The amount of labor possessed by a group and the social power embodied in a
leader’s ability to direct that labor constitute an emergent quality that describes both
a leader and his group. As a consequence, the signal content of prestige goods was
further expanded to include this aspect of social power. Such signals would function
in competition between would-be leaders within a political group as well as
between groups.
This idea is not a new one; archeologists have often argued that prestige goods
act as signals of group and leader strength in models of the development of factional
and peer polity competition. Several scholars have drawn on evolutionary frameworks and methods to examine the costs and benefits of prestige goods and other
signals of competitive strength in terms of energy and ultimately, reproductive and
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cultural fitness. For instance, Neiman (1997) proposed that elaborate carved stone
monuments built by members of the Mayan ruling elite functioned as costly signals
of ability to win political contests (1997: 270). Success in such contests, he argues,
would depend on a variety of factors, including “physical size, physiological
condition, fighting skill, and psychological cleverness, which contribute in varying
proportions to an individual’s ability to build and maintain coalitions comprised of
kin and non-kin.” Individuals who win political contests increase their reproductive
success by gaining greater access to mates and resources. In this way, Neiman links
social power as a currency to the currency used in all other evolutionary analyzes of
animal behavior. This model is one of selection at the level of the individual but it
may also draw on kin selection to the extent that the performance of leaders in
political competition relies heavily on the actions of their immediate family and kin
networks (Neiman 1997, p 271). This logic can be extended to non-kin corporate
groups as well, and thus the problem could also be approached from a group
selection perspective.
Pletka (2001) adopts an explicitly group-selection approach in examining the
possible function of Neolithic and Bronze Age Danish earthwork monuments
(mounded graves and enclosures) as costly signals of a group’s ability to defend
its rights to land and resources. Earthen monuments, like carved stelae, are just one
of many possible ways to signal defensive capability, and I would argue that
Pletka’s model of honest signaling of group strength via the construction of earthen
monuments could be expanded and adapted to apply to all such signals. Like
Neiman and Pletka, Boone (2000) uses costly signaling theory to answer the
question of why conspicuous displays of wealth expenditure are universally associated with social inequality. His model addresses all material forms of display,
including elaborate monumental architecture, exotic and nonfunctional objects, and
also elaborate feasts and gift-giving performances (2000: 84 85).
All of these models share strong similarities with the theoretical literature on
contests in evolutionary biology. The signaling models developed by evolutionary
biologists demonstrate that signals between competitive rivals can evolve when
they convey information about unobservable qualities relevant to fighting. Honest
signals about fighting ability enable contestants to assess who is more likely to win
a fight, and avoid costly contests. However, the winners of such contests attain
dominance over their rival, it does not generate prestige. Thus, these models do not
provide much insight about how prestige goods first emerged or how such goods
were linked to leadership and ranking. I suggest that once leadership and social
ranking existed, the use of prestige goods was transformed through involvement in
competition between rivals, and took on an expanded signal content having to do
with coercion and dominance through their links with authority and social power.
The emergence of these two phenomena the formation of family and lineage
social rank differences and the increasing interaction between factions via their
leaders transformed the social landscape in which prestige goods were being used
in dramatic ways. Correspondingly, the content of the signal being made with
prestige goods was significantly expanded in these new social contexts. One result
was that self-promotion and aggrandizement using prestige goods, once rebuffed by
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social equals, became a tactic that was used by would-be leaders to advertise their
own social power and the success of their group, and to induce followers to
accepting their authority.
6.6
Conclusions
The focus of this chapter has been on the role of prestige in human systems of social
power. The recognition of the importance of prestige systems in human societies is
important for at least two reasons. First, prestige systems are a derived property of
human societies, and are not found in any other social species. Second, prestige is
arguably more important than dominance in determining interpersonal status relationships in human societies. Thus, an understanding of prestige systems will
further our understanding of social power and inequality in human societies.
Evolutionary models of prestige systems enhance anthropological and archeological theories of the evolution of social structures and institutions. For instance,
admiration of and desire for prestige goods are often cited as the reasons why the
display of prestige goods, and their distribution, were important in the emergence of
socio-political hierarchy, but the source of these feelings is also often left unexplained or underspecified. Evolutionary models of prestige systems can provide this
much needed explanation; here, I have hypothesized that admiration of and desire
for prestige goods emerged out of a more general desire for prestige as a means to
enhance status, specifically, because such goods acted as a means of advertising
skill and expertise. Changes in environmental and social contexts favoring the
development of leadership would make the advertisement of skills and expertise
pertinent to all group members, not just to the learning individuals, and I suggest
that it is because of this reason that prestige goods could play a critical role in the
emergence of the social structures and institutions of hierarchy and inequality. In
turn, this forged the link between the possession and display of prestige goods and
the occupation of positions of authority and wealth that not only predominates their
use in contemporary Western society but also appears to have played a powerful
part in the development of political hierarchy and social and economic inequality in
human societies around the world and through time.
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Part IV
Intergroup Relationships
Chapter 7
The End of the Republic
Laura Betzig
Abstract Virgil first read his Georgics, or “farm poem,” to the first Roman
emperor, Augustus, on his way back from the battle at Actium where Augustus
had put an end to the Roman republic, and established one-man rule. Virgil’s
natural history was bad: he thought that queen bees were kings, among other things.
But the point he was making was good. Communis natos, he wrote: in Rome, as in
Apis mellifera hives, sterile workers and soldiers would help raise the young of their
emperor, or queen. Like dominant members of eusocial species bees, ants, wasps,
gall thrips, termites, aphids, beetles, sponge-dwelling shrimp, and naked molerats Roman emperors had enormous reproductive success. They had sexual access
to hundreds or thousands of women, who may have borne hundreds or thousands
of children. And they got help defending their territories, and provisioning their
families, from millions of facultatively sterile workers and soldiers, and from
thousands of eunuchs who made up an obligately sterile caste. This example from
human history illustrates the unusual flexibility of human reproductive strategies.
7.1
Introduction
Over 2,000 years ago, in September of 31 BC, Gaius Octavius beat Marc Antony in a
naval battle at Actium off the coast of Greece. A year later, on his way back to
Rome, Octavian stopped to visit his friend Virgil in the south of Italy where he
recited his Georgics, or farm poem, for 4 days straight. Virgil’s natural history was
bad: he thought queen bees were kings, among other things. But the point he was
making was good. Equals would fight to the death, till just one was left on the nest.
L. Betzig
The Adaptationist Program, Ann Arbor, MI, USA
e mail: lbetzig@gmail.com
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 7, # Springer Verlag Berlin Heidelberg 2010
153
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L. Betzig
That, Virgil thought, was for the best. In his words: “Slay the weak rebel! bid the
usurper bleed! Slay, ‘ere he waste the hive.’ Defend the throne, and let the rightful
monarch reign alone.” He put it even better in the Aeneid he left on his deathbed:
“Spare the submissive, and war down the proud” (Virgil, Georgics, 4.89 91,
Aeneid, 6.852 853).
Virgil grew up on a farm in the north of Italy, the son of a man who greatly
increased his little property by buying up woodlands and raising bees; and he
devoted the fourth part of his Georgics to beekeeping, or apiculture (Suetonius,
Life of Virgil). After the honeybee fight to the death, Virgil knew something about
honeybee peace. As long as their king was alive, the hive was all “of one mind.”
Colony members would work for him; they’d fight for him; and they’d raise his
young. “Communis natos” is how Virgil put it: everybody helped the emperor breed
(Virgil, Georgics, 4.154 191, Whitfield 1956).
That made them truly social. Almost half a century ago, the term eusocial was
first used to describe insect societies, in which workers cooperatively care for a
monarch’s brood, as members of an obligately sterile caste (Batra 1966; Wilson
1971). Over the last few years, that definition has been expanded to include animals
in a variety of taxa from insects, including bees, ants, wasps, thrips, termites,
aphids and beetles, to a sponge-dwelling shrimp, to the East African naked molerat. Continuous definitions of eusociality include the wide range of species in which
some individuals help care for others’ young, and measure an index of reproductive
variance, or “skew” (Gadagkar 1994; Sherman et al. 1995; Lacey and Sherman
2005). Discrete definitions restrict eusociality to the small handful of species with
obligately sterile castes (Crespi and Yanega 1995; Costa and Fitzgerald 2005;
Crespi 2005). But in practice, these definitions overlap. In societies where some
individuals are obligately sterile, reproductive skew will usually be high; in societies without sterile castes, reproductive skew will usually be lower.
In either case, Virgil’s honeybees and Rome after Actium war exhibited some
striking parallels. Both Apis mellifera hives and the Roman Empire were provisioned and defended with help from obligately sterile workers, or eunuchs who
cared, directly or indirectly, for breeders’ young. And in both honeybee societies
and Imperial Rome, reproductive skew was at the high end of the continuum. In
Apis mellifera, as in most eusocial societies, direct reproduction is restricted to a
single queen, and other group members do not breed. In the Roman Empire, direct
reproduction was shared, but the emperors had sexual access to hundreds or
thousands of women, and thousands of obligately sterile eunuchs never bred.
That contributed to a reproductive division of labor, and moved Imperial Rome
down the eusocial continuum.
Virgil’s honeybees, Apis mellifera, live in colonies of tens of thousands of workers, but produce only about ten queens. The first of those queens to emerge searches
the others out and kills them, or is killed herself. Afterwards, the survivor’s tens of
thousands of sterile worker daughters clean cells, feed brood, store nectar, forage for
pollen, and defend the hive sacrificing their viscera, and their lives, with their barbed
stings. But the queen which grows up to twice their length, and lives up to 50 times
as long specializes as an egg-laying machine (Michener 1974; Seeley 1985).
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In a similar way, Roman emperors systematically had their competitors wiped
out; and their empires, like honeybee hives, were provisioned and protected with
help from thousands of eunuchs who made up a sterile caste. The Roman Empire
was defended by millions of celibate soldiers; it was supplied by millions of
celibate slaves; and it was administered by a praepositus sacri cubiculi, a eunuch
“set over the emperor’s sacred bedchamber,” and by other eunuchs “more in
number than flies around the flocks in spring” by the time the emperors left
Rome. But, like Apis mellifera queens, Roman emperors specialized as breeders.
They had sex with the freeborn women procured by family members and friends,
senators, and their praetorian guard; and they had sexual access to hundreds or
thousands of slaves.
7.2
Senators, Soldiers, Slaves and a Sterile Caste
Over the 357 years the empire remained in Rome, the distance between subject and
emperor increased. Some members of senatorial families who had administered the
res publica, or republic, were executed under the law of maiestas, or treason. And
others were punished for being promiscuous, under the “moral laws.” Millions of
soldiers were legally barred from marriage; millions of slave men had little access
to women. And by the time Roman emperors moved to Constantinople, thousands
of eunuchs worked in the civil service, fighting for and providing for the empire as
members of a sterile caste.
7.2.1
Senators
Augustus was the first to investigate a libel under the maiestas law in AD 12 being
“provoked” by the senator Cassius Severus, who’d made sarcastic remarks about the
burning of republican books (Tacitus, Annals, 1.72). Severus paid for his criticism
with an exile to Crete, ending his days on the rock of Seriphos. Others were punished
for treasonous acts. As early as 22 BC, Lucius Murena and Fannius Caepio, who’d
been suspected of a plot to assassinate Augustus, were “seized by state authority and
suffered by law what they’d worked to accomplish by violence” (Velleius, Compendium, 2.91). Even under benign emperors, hundreds of subjects were put to
death. Occasionally, whole lineages were wiped out (Bauman 1974, 1996; Fig. 7.1).
Overall, the number of dead was nontrivial though hard to pin down. Even
before he became an emperor, in December of 43 BC, Octavian put bounties on the
heads of 300 senators and 2,000 knights. Cicero was among the proscribed: hunted
down in a thicket on his way to the coast, his head and right hand were cut off
(Appian, Civil Wars, 4.5, Pliny, Natural History, 7.148). Augustus’ step-son,
Rome’s second emperor, Tiberius, had bodies up to 20 a day thrown on
the Stairs of Mourning leading down into the Forum (Suetonius, Tiberius, 61).
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Fig. 7.1 Victims of despotism in Marc Antony’s family. Antony had legitimate children by five legitimate wives; many of their descendants were eliminated
by Julio-Claudian emperors. See text
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And the third emperor, Augustus’ great-grandson Caligula, kept lists (The Dagger
and The Sword) of marked subjects, being credited with the remark: “I wish all
you Romans had just one neck” (Suetonius, Gaius, 50, Seneca, On Anger, 3.19).
Caligula’s uncle Claudius, picked by his soldiers to be Rome’s fourth emperor,
had up to 300 knights and 35 senators put to death along with lesser subjects “to
the number of the grains of sand and the specks of dust” (Seneca, Apocolocyntosis, 13 14, Suetonius, Claudius, 29). And the last man in Augustus’ dynasty, his
great-great-grandson Nero, was arguably the worst: Seneca (the Stoic philosopher,
his teacher), Lucan (the poet, and Seneca’s nephew), Petronius (the novelist, and
Nero’s “Arbiter of Taste”), and Thrasea Paetus (another Stoic philosopher) were
put to death; after a senatorial conspiracy in AD 65, the flow of blood “fatigued the
mind” though the number of casualties is unknown (Tacitus, Annals, 16.16).
Victims were accused along with their children, and their relatives were forbidden
to mourn (Suetonius, Tiberius, 61, Digest, 3.2.11.3). As a philosophical emperor
would later say, “what is no good for the hive is no good for the bee” (Marcus
Aurelius, Meditations, 6.54).
The survivors were asked to limit their direct reproduction. One day in 28 BC,
wearing a sword and steel corset under his tunic, with ten well-built bodyguards
around him, the first emperor persuaded 50 men to withdraw from the senate
voluntarily. Then, he “compelled” another 140 to follow their example. Eleven
years later, Augustus took another 200 senators off the lists; others were asked to
leave, in 11 BC and AD 4 (Syme 1939; Talbert 1984). Many had been politically
unfriendly; but others were censored for having too much sex. “On the strength of
their own knowledge of their families and their lives, he urged senators to become
their own judges.”
Augustus did the same to the equites, or knights who were often the richest
men in Rome. He “cross-examined” them on their personal affairs: “some, whose
lives proved to have been scandalous, were punished; others were only degraded”
(Suetonius, Augustus, 35, 39, Dio, History, 52.42.2).
Augustus’ contemporaries were appalled at censors’ powers. They invaded
“the privacy of our homes”; or, more explicitly, “throwing open every house and
extending the authority of the censors even to the bedchamber, they made that
office the overseer and guardian of everything that took place in their subjects”
houses’ (Pliny, Natural History, 29.8.18, Dionysius of Halicarnassus, Roman
Antiquities, 2.15). Other emperors conducted similar purges. Claudius brought
back the censorship in AD 47 48, revised the senate and struck knights off the
lists warning a notorious seducer of wives and virgins, “restrain your passions”
(Suetonius, Claudius, 16). Then, in AD 73 74, Vespasian, with his son Titus beside
him, reformed the senate and knights again, removing “undesirable members” and
“tightening discipline” (Suetonius, Vespasian, 8 9). Domitian made himself perpetual censor (censor perpetuus) in AD 84 85, and started another campaign for
“improving public morals”: men were sentenced for “unnatural” practices, and
women for not being chaste (Suetonius, Domitian, 8, Dio, History, 67.12.2).
On another day in 28 BC, the first emperor probably tried to pass his first set of
“moral laws” though he seems to have had them repealed. Propertius, the poet,
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L. Betzig
remembered how he had “rejoiced when that law was lifted”; and Livy, who wrote
histories, was not sure whether “corruption or its corrective” was worse (Propertius,
Poems, 2.7, Livy, History, pr.9). At any rate, in around 18 BC, Augustus tried again:
his lex Julia de maritandis ordinibus encouraged Roman bachelors to get married
and raise legitimate children; but his lex Julia de adulteriis had convicted adulterers
and adulteresses banished to remote islands with up to half of their property
confiscated by the state. Any man who “knowingly” made his house available for
adultery was exiled, as if he’d committed adultery himself; and any man who made
a profit out of adultery was punished, even if the adulterer was his spouse, “for it is
no small crime to have pimped for one’s wife” (Paulus, Sententiae, 2.26, Digest,
48.5.9, 30, Treggiari 1991).
7.2.2
Soldiers
Constraints on direct reproduction were not limited to the senatorial class: for
hundreds of years, soldiers’ marriages were banned. The law that soldiers could
not “legally have wives” may go back to Augustus: Ovid, the poet, found it odd that
the emperor’s legislation should encourage civilian breeders, in order to supply his
empire with unmarried soldiers; and Cassius Dio, the senator, dated the law against
soldiers’ marriages to Augustus’ reign (Ovid, Art of Love, i.113 114, Dio, History,
60.24.3).
The Roman Empire, like most empires, was built on conquest. Roman emperors
supported millions of soldiers, and many were killed in battle. Some of the
survivors had relationships with women, and fathered children. But for centuries,
they were legally unable to marry, and their children were considered bastards
(Wells 1999).
7.2.3
Slaves
There may have been 10 million slaves in a population of 60 million in the whole
Roman Empire in Augustus’ time, and none of those slaves had wives (Scheidel in
press). As Ulpian, a third-century jurist, summed up: “Conubium is the capacity to
marry a wife in Roman law”; and “there is no conubium with slaves” (Titului
Ulpiani, v.3 5, also Gaius, Institutes, i.57). Most slave men lived and worked on the
farms, or in the mines. In the mines, thousands “never saw the light of day” for
months; and they constantly risked being crushed to death. And on the farms,
thousands worked in chained gangs where they were driven by “whips rather
than words” in the fields, and slept in “an underground prison, as wholesome as
possible,” with light coming in through window slits high enough up that they
couldn’t be reached (Pliny, Natural History, 33.70, Columella, On Agriculture,
1.6.3, 1.8.5). Conditions were not conducive to family life.
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159
But a minority of slaves filled the imperial civil service: the Familia Caesaris.
They worked from the lowest level to the highest in menial positions, as footmen
(pedisequi), watchmen (custodes), name callers (nomenclatores), or post officers
(tabellarii); in middle management, as assistants (adiutores), archivists (commentarii), aides (vicarii), or accountants (tabularii); and in upper-level cabinet posts, as
secretaries of letters (ab epistulis), secretaries of documents (a studiis), secretaries
of petitions (a libellis), and secretaries of finance (a rationibus). The names of over
4,000 imperial slaves and freed slaves survive on Roman tombs (Weaver 1972). By
the end of Augustus’ dynasty, the emperor was so close to a slave that Rome
became a “slave to two emperors” at once; and by the end of the next dynasty,
“most emperors, though masters of their subjects, were the slaves of their freed
slaves” (Dio, History, 62.12.2, Pliny, Panegyric, 88). Some of those slaves had
relationships with women, and fathered illegitimate children who might, like their
fathers, have ended up in the Familia Caesaris (Rawson 1966; Weaver 1972).
7.2.4
A Sterile Caste
Early Roman emperors filled their bureaucracies with “outcasts” with foreigners
and the poor, or people who lacked important ancestors. Later emperors promoted
“dry trees” or people who lacked sons, or a sterile caste (Isaiah 56: 3 8, Betzig
2005, 2008).
By the end of three centuries , the imperial civil service was filled with eunuchs.
Even Maecenas who patronized Virgil, and worked as a regent for Augustus was
“attended in public” by a pair of eunuchi; and even Livia, who was Augustus’ last
wife, buried a rarius eunuchus, along with another half dozen cubicularii
bedchamber attendants, customarily castrated
in her family tomb (Seneca,
Moral Letters, 114, Dunlap 1924). Some cubicularii specialized as “workers,”
and others as “soldiers”: they advised emperors on foreign affairs; or were honored
at triumphs after foreign wars (Philo of Alexandria, Embassy to Gaius, 27.175,
Suetonius, Claudius, 28).
But there were more of them, and they were more important, as time went on. By
the end of the first century, there were “troops of eunuchs” at the imperial court; and
by the end of the second century, a hundred castrated Roman citizens waited on
an emperor’s wife (Suetonius, Titus, 7, Dio, History, 76.14). By the end of the third
century, the empire was administered by a praepositus sacri cubiculi the eunuch
“set over an emperor’s sacred bedchamber”; and by the fourth century, after Constantine moved the capital to Constantinople, the emperor was surrounded by a
thousand cooks, as many barbers and more butlers, a swarm of waiters, and
“eunuchs more in number than flies around the flocks in spring” (Malalas, Chronicle,
339, Libanius, Orations, 18.130). Castrensi managed the imperial bodyguard,
properties, movables, and treasury (Hopkins 1963; Jones 1964). Eventually, 8 out
of 18 administrative ranks were reserved for eunuchs, who consistently outranked
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L. Betzig
the “bearded” civil service (Constantine VII Porphyrogenitus, On Ceremonies,
2.52, Tougher 2002, 2008).
Remnants of the senatorial class disapproved. Eunuchs were all “extortionate or
despicable” lizards and toads: subservient to their masters, insolent to everybody else
(Ammianus Marcellinus, Roman History, 16.7.8, Basil of Caesarea, Letter 115).
7.3
Emperors
Emperors, on the other hand, bred. Roman emperors married just one, legitimate
wife at a time; but they had sex with as many women as they could afford (Betzig
1986, 1992a, b, 1993). Some of those women were freeborn Roman citizens (Syme
1960); but many of those women were slaves (Scheidel 2009). Friends, family
members, senators, and his praetorian guard brought freeborn wives and daughters
to the imperial bed. And on the side, Roman emperors had sexual access to
hundreds or thousands of slaves, whose daughters and sons vernae, or homeborn
slaves filled Latin law, literature, and the imperial civil service: the Familia
Caesaris.
7.3.1
Free Women
Even his friends admitted that Augustus was an adulterer. But they justified it: “he
attracted many women by his comeliness and high lineage.” Besides, he had sex
“for reasons of state”: he was trying to find out what his enemies were up to by
getting intimate with their daughters or wives who were stripped of their clothes,
“and inspected as though they were for sale” (Nicolaus of Damascus, Life of
Augustus, 5, Suetonius, Augustus, 69).
Other emperors were provided for by senators, bodyguards, family members,
and slaves. Tiberius got help from his slaves. Rummaging through the streets of
Rome for freeborn daughters and sons, they “rewarded compliance, overbore
reluctance with menaces, and if resisted by parents or relations kidnapped
their victims, and violated them on their own account” (Tacitus, Annals, 6.10).
Caligula abused his senate, asking legislators to dinner, and taking their wives to
bed, then coming back to announce in a loud voice “how they’d behaved in sexual
intercourse” (Seneca, On Constancy, 17.2). Claudius, who was sickly, depended on
his family like his mother and his grandmother, who put girls in his bed to satisfy
his “healthy” appetite for sex (Dio, History, 60.2.4 6). And Nero relied on his
praetorian guard. His praetorian prefect, Tigellinus, had a raft towed by gold and
ivory boats floated in the Campus Martius on Marcus Agrippa’s lake: brothels were
set up for high-ranking ladies (“the most beautiful and distinguished in the city”),
and whores (“naked prostitutes, indecently posturing and gesturing”) were lined up
on the quays (Suetonius, Nero, 27).
7
Human Reproductive Strategies
7.3.2
161
Slave Women
Mostly though, emperors had sex with their slaves (see Tables 7.1 7.3). There were
an estimated 6 million slaves in a population of 60 million in the whole Roman
Empire by the Augustus’ time, and most of them were owned by rich women and
men (Scheidel 2009). Great, late republican families saved places for 204, 634,
and 652 household slaves in their family tombs, and roughly a third had female
names (Treggiari 1975; Saller and Shaw 1984). Many were good looking or young:
ages of female slaves ranged from 4 to 35 at time of sale with a ripe median of
around 19; and slaves of “good appearance” cost more (Bradley 1978, Suetonius,
Caesar 47).
They were expected to be fertile. In legal sources, buyers were due a refund if a
slave menstruated twice a month, or never menstruated at all; if she regularly
produced “stillborn issue,” or was “so doctored that she cannot fulfill the function
Table 7.1 Sex ratios of slaves
on Roman tombs. After
Treggiari 1975 (Volusii,
Statilii, Liviae), Weaver 1972
(Familia Caesaris), and
Rawson 1986 (Alumnae,
Vernae)
Sample
Volusii family
Statilii family
Monumentum Liviae
Familia Caesaris
Alumnae
Vernae
Table 7.2 Ages of ancillae,
or female slaves, at time of
sale, as listed on Egyptian
papyri. After Bradley 1978
Range of ages
<5
5 9
10 14
15 19
20 24
25 29
30 35
Number (%) of Ancillae
1 (3.4%)
3 (10.3%)
5 (17.2%)
5 (17.2%)
7 (24.1%)
4 (13.8%)
4 (13.8%)
Table 7.3 Ages of vernae, or
homeborn slaves, on
inscriptions from Roman
tombs. Because vernae who
were commemorated on
tombs died young, the
implication is that the status
of those who survived
changed early to liberty,
or freed slaves. After
Rawson 1986
Range of ages
<5
5 9
10 14
15 19
20 24
25 29
30 100
Number (%) of Vernae
103 (32.0%)
100 (31.1%)
48 (14.9%)
34 (10.6%)
17 (5.3%)
9 (2.8%)
11 (3.3%)
Ratio of / = Slaves (% ?)
129:75 (63.2%)
421:213 (66.4%)
440:212 (67.5%)
3,325:291 (92.0%)
276:139 (66.5%)
381:183 (67.6%)
162
L. Betzig
of a woman” as it was “the highest and particular lot of woman to conceive and
conserve what she conceives.” On the other hand, a slave woman could be emancipated for giving birth to three children, or “if the first child she bears is male”; and it
was considered “a good reason for manumission where, for instance, anyone offers
for manumission before the council a natural son or daughter” (Digest, 1.5.15,
21.1.14 15, 35.5.10, Gaius, Institutes, i.19). Three out of four Roman epitaphs
belong to a freed slave, and vernae the “homeborn” children born on their
masters’ estates, to their masters’ slave women were most likely to be freed
(Taylor 1961; Hopkins 1978). Vernae were brought up in paedagogia along with
their masters’ legitimate children, attended by the same hairdressers (ornatrici),
anointers (unctores), teachers (praeceptori), and doctors (iatroliptae) (Rawson
1986; Bradley 1991). They often grew up to become knights, or sat in the senate;
and hundreds of Augusta vernae worked in the civil service (Tacitus, Annals, 13.27,
Weaver 1972).
7.3.3
Genius
Early in 44 BC, just months before the Ides of March, Julius Caesar was voted
“Father of his Country” by his senate, and the inscription pater patriae was
inscribed onto Roman coins (Suetonius, Caesar, 76, Dio, History, 44.4.4). Rumors
circulated about Caesar after the Ides of March, to the effect that members of the
senate had “actually ventured to suggest permitting him to have intercourse with as
many women as he pleased, because even at this time, though 50 years old, he still
had numerous mistresses.” Helvius Cinna, the people’s representative, or tribune,
was supposed to have drawn up a bill for the commons to pass while Caesar was out
of town, “legitimizing his marriage with any woman, or women, he pleased, ‘for the
procreation of children’” (Dio, History, 44.7.3, Suetonius, Caesar, 52). And within
days after Caesar was butchered in the senate, Cinna was supposed to have been
torn, limb from limb, by a crowd of angry men (Plutarch, Caesar, 68). But others
paid tribute to Caesar. They set up an altar in his honor, and raised a 6-m column of
Numidian marble in the Forum, with “To the Father of His Country” written on the
bottom (Appian, Civil Wars, 1.4, Suetonius, Caesar, 85). Cicero’s son-in-law,
Dolabella, had that column leveled, and made sure the masses that gathered to
make sacrifices were slaughtered. “Debauched and wicked free men” were thrown
down from the Tarpeian Rock; and “audacious and rascally slaves” were hung up
on crosses. As Cicero bragged to his friend Atticus, on May 1st: “Away with the
pillar! Contract for paving the site!” (Cicero, Ad Atticus, 14.15, Philippics, 1.5,
Gelzer 1968).
On a pair of bronze pillars outside his enormous, 90-m diameter, mausoleum in
Rome, Augustus inscribed his Res Gestae, or “History of His Reign.” The first
emperor was proud to be remembered for having served as a triumvir, a senator
7
Human Reproductive Strategies
163
Fig. 7.2 A coin issued under
emperor Maximinus II, a
contemporary of Constantine
the Great. The naked emperor
holds a cornucopia in his
left hand, and the head of
Serapis an Egyptian fertility
god in his right. The
inscription reads genio
augusti, in honor of the
emperor’s genius
and a censor, but he may have been proudest of the last accomplishment on
his list having been voted pater patriae, or “Father of his Country,” by his
subjects (Res Gestae, 35). Tiberius was offered that honor in the first year of his
reign, but in spite of “repeated popular pressure,” he turned it down (Tacitus,
Annals, 1.72).
Caligula, Claudius, and Nero, all capitulated in their first 12 months though
they all declined the epithet at first, in Nero’s case, “because of his youth.” In later
dynasties, pater patriae was a commonplace honor in literature, on coins, and in
architecture (Suetonius, Nero, 8, Gradel 2004).
In the months before March 15th, when the senate was voting excessive honors
for Julius Caesar, they decided that public prayers should be offered every year on
his behalf, and that people should “swear by” Caesar’s genius (Dio, History,
44.6.1). They had to do the same for Augustus. Romans had always made sacrifices
to the genius, or generative power, of the heads of their families, or gens; now they
would make the same sacrifices to their head of state (Fig. 7.2). When a family’s
household deities, or Lares, were put out to “feed from the dish,” the first emperor
was offered the same wine cup; and when a farmer came home from his field to food
and wine, he invoked Augustus “as a god,” along with his Lares, and offered the
emperor prayers (Ovid, Fasti, 2, February 22, Horace, Odes, 4.5). Tiberius vetoed
bills for the dedication of priests and temples to his divinity, and decided not to
allow subjects to swear by his generative power
though “if anybody after
swearing by it incurred the charge of perjury, he would not prosecute him”; but
Caligula had subjects boxed up in small cages or sawn in half, for “failing to swear
by his genius” (Suetonius, Tiberius, 26, Gaius, 27, Dio, History, 57.8.3, 58.2.8,
59.4.4). Bulls were sacrificed on public altars to the genius of Nero, and other
emperors, in order to ensure the harvests; and coins, covered with cornucopia, were
issued in honor of genio augusti, or the emperors’ genius (Fishwick 1987; Gradel
2004). But people were thrown to wild beasts in arenas, or consumed by fire, for
failing to “swear by the genius of Caesar,” for nearly 300 years (Acts of Polycarp, 9
and Acts of Perpetua, 6 by Musurillo 1972).
164
7.4
L. Betzig
Reproductive Skew
For more than 300 years after Augustus became the first emperor of the West, proud
Roman subjects were put in their place with respect to politics, and with respect to
sex. They were relegated to remote islands, or killed, for offending the majesty of
the emperor, under the law of maiestas. And they were thrown out of Rome, or
removed from the senatorial and equestrian orders, for being promiscuous, under
the censorship or lex Iulia de adulteriis. They were expected to worship the
emperors as divi Iulii, or gods; and they were turned into human torches, or thrown
to wild animals in the circus, for failing to make sacrifices to the emperor’s
generative power, or genius.
The Roman Empire, like an Apis mellifera colony, exhibited many parallels
with eusocial animals. There was a reproductive division of labor, with eunuchs
helping to care for the emperor’s freeborn and slave born children. And as a result,
reproductive skew was high. Like social insects from bees, to ants, to wasps
(Wilson 1971), to gall thrips (Crespi 1992), to termites (Thorne 1997), to aphids
(Aoki 1977), to beetles (Kent and Simpson 1992), like at least one crustacean
Synalpheus regalis, the sponge-dwelling shrimp (Duffy 1996), and like at least one
other social mammal Heterocephalus glaber, the naked mole-rat (Jarvis 1981),
Roman emperors specialized as breeders. They had sexual access to hundreds
or thousands of women, who may have borne hundreds or thousands of children.
And they got help defending their territories, and provisioning their families, from
the hundreds or thousands of workers and soldiers who made up an obligately
sterile caste.
For the more than 100,000 years of H. sapiens prehistory, reproductive skew was
low. In most cases, successful foragers raised roughly twice as many children as
average foragers; but most foragers managed to become parents (Hill and Hurtado
1996; Smith 2004). Like many insects, some birds and a handful of mammals
including primates, hunters, and gatherers have always been cooperative breeders:
adults, especially, closely related adults, have helped one other feed and protect
their broods. But with the exception of postmenopausal women, who have helped
raise their own grandchildren they have lacked an obligately sterile caste (Foster
and Ratnieks 2005; Hrdy 2005).
This changed around 10,000 years ago, with the origin of farming. By the time
the first historical records were being kept in the Ancient Near East, “beardless”
attendants were waiting on “bearded” kings who collected large numbers of
lukurs (or “king’s fallow,” or virgins) and nins (or “queens”), and left surviving
records of dozens of sons and daughters (Postgate 1994; Grayson 1995). In Old
Kingdom Egypt, the desert god Seth, whose testicles have become “impotent,”
helps administer the empire for pharaoh; and in the New Kingdom, the names of 49
sons generals, hereditary counts, chiefs of secrets, scribes survive from Rameseses II reign, along with another 111 unnamed sons (Fisher 2001; Allen 2005). In
the Sanskrit of India’s early imperial dynasties, the Maurya and Gupta, emperors
are waited on by “third genders”; and harem women are taught to hold an emperor’s
7
Human Reproductive Strategies
165
interest by speaking multilingually and talking to parrots, “even though he may
have thousands of other women” (Shamasastry 1951; Burton 1979). In China, in the
2nd millennium BC, there are huan guan court officers, customarily castrated on
Shang dynasty oracle bones; by the 2nd millennium AD, there were a record 100,000
Ming Dynasty eunuchs, and a Sui Dynasty emperor kept a record 100,000 women
(Tsai 1996; Ebrey 2003). Across Old World empires including Rome, eunuchs
oversaw administration and commanded armies. But emperors specialized as
breeding machines.
What accounts for the change? In any society, reproductive skew is expected to
increase: (1) as the genetic relatedness of group members goes up; (2) as the social
benefits of group membership go up; and (3) as ecological constraints on dispersal
increase (Vehrencamp 1983a, b; Emlen 1995).
In eusocial species, relatedness lowers the costs of helping (Hamilton 1964,
1972). And as expected, average relatedness in most skewed societies is high. In the
Hymenoptera, including Apis mellifera, haplodiploidy makes full sister bees, ants,
and wasps more closely related than mothers and daughters; so helping is favored
(Hughes et al. 2008). But in haplodiploid colonies where queens are inseminated
more than once, or in colonies with more than one queen, relatedness is not enough
to explain the existence of sterile castes (Keller 1993). And kinship is an inadequate
explanation for eusociality in diploid species from termites, to aphids, to beetles,
to shrimp, to naked mole-rats.
Social benefits including cooperative foraging, and cooperative defense can
also raise reproductive skew. Among other things, group members may benefit each
other as sentinels or fighters, groomers or hunters; and some may be willing to limit
direct reproduction, and to help others reproduce, as a result (Clutton-Brock 2006).
But in many social species, altruism is not voluntary, but enforced: breeders often
punish or evict nonbreeders who fail to help (Ratnieks and Wenseleers 2008).
Again, that has often been the case in human groups.
Strong evidence suggests that reproductive skew is often a response to ecological constraints. Ecological benefits including habitats safe from predation, and
with plenty of food may compensate nonbreeders for becoming nepotists (Emlen
1982, 1997). Eusocial species take advantage of resource patches across taxa from
insects, to crustaceans, to mammals (Jarvis et al. 1994; Duffy 1996; Thorne 1997).
Societies tend to colonize discrete nesting sites in sharply delineated habitats from
decaying logs, to sponge cavities on coral reefs, to 30 kg tubers dispersed in arid
ground. The fact that eusocial species “dominate the central, more stable areas of
habitats,” while solitary species “flourish in the peripheral, more ephemeral areas,”
(Wilson and Hölldobler 2005) may indicate that high skew is often an effect, rather
than a cause, of finding a good food source. For H. sapiens, that seems to have been
the case.
Most civilizations have probably risen up as an effect of ecological constraints.
Before sedentary societies spread with agriculture, reproductive skew in most
foraging societies was low. But around the Old World from Sumerian and later
civilizations on the Tigris and Euphrates, to the Egyptian civilization that lasted
for millennia on the Nile, to Harappan civilization on the Indus, to the Shang and
166
L. Betzig
later dynasties on the Yellow River and its tributaries, to Rome every ancient
empire began on “areas of circumscribed agricultural land” (Carneiro 1970,
1986). A small minority of men collected up to 100,000 women, and up to
100,000 eunuchs filled sterile castes.
Acknowledgments I thank my old friend, Bernie Crespi, for help with evolutionary biology; and
I thank my old friend, Walter Scheidel, for help with Roman history.
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Chapter 8
Intergroup Aggression in Primates and Humans:
The Case for a Unified Theory
Margaret C. Crofoot and Richard W. Wrangham
Abstract Human warfare and intergroup aggression among primates have traditionally been considered to be largely unrelated phenomena. Recently, however,
chimpanzee intergroup violence has been proposed to show evolutionary continuities with war among small-scale societies because both systems involve interactions
among temporary subgroups, deliberate attempts to hunt and maim, and demographically significant death rates. Here, we ask whether the functional similarities
between intergroup aggression among humans and chimpanzees can be extended to
troop-living primates. In most primates, patterns of intergroup aggression involve
brief encounters among stable troops, rare violence, and almost no killing.
Although they, therefore, show little behavioral resemblance to warfare, growing
evidence indicates that intergroup dominance is adaptively important in primates
because it predicts long-term fitness. This suggests that in all primates, including
humans, individuals use coalitions to maintain or expand access to resources by
dominating their neighbors. Thus, while the style of coalitionary aggression
depends on each species’ evolutionary ecology, we propose that the essential
functional reasons for intergroup competition are consistent across group-living
primates and humans: strength in numbers predicts long-term access to resources.
8.1
Introduction
Although societies can sometimes spend decades without practicing war, the
capacity for warfare is clearly a human universal. However, the question of why
humans readily engage in war is unresolved from an evolutionary perspective.
In this chapter, we review evidence suggesting that war between groups of
M.C. Crofoot (*) and R.W. Wrangham
Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA
email: mccrofoot@gmail.com, wranghamrichard@gmail.com
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 8, # Springer Verlag Berlin Heidelberg 2010
171
172
M.C. Crofoot and R.W. Wrangham
humans tends to serve the same essential functions as aggression between nonhuman primates. This might seem unsurprising given that there are obvious and
extensive behavioral similarities between human and nonhuman primate intergroup
aggression (van der Dennen 1995). Until recently, however, two major barriers
have inhibited the development of a unified theoretical explanation for these
two phenomena.
First, there has been considerable resistance among war scholars towards using
evolutionary theory to explain why war occurs. Thus, military organization has
been widely viewed as a socially constructed device that challenges rather than
conforms to evolutionary principles. For instance, Collins (2008) argues that war
systems are designed to overcome an instinctive tendency to avoid conflict.
Furthermore, because warfare is not archeologically visible until about 10,000
years ago, it is sometimes claimed to be a recent invention which, therefore,
requires explanation in terms of culture rather than biology (Ferguson 2000). A
similar argument notes that warfare is rare or unknown in some hunter-gatherers
and must therefore be unnatural (Fry 2006). The fact that aggression is oftentimes
not the main motivation of soldiers in battle also suggests an important discontinuity from intergroup aggression among animals (Hinde 1993). For these and many
other reasons, such as the complexity of human military and political organization
and the novelty of weapon technology, as well as the fear that an evolutionary
explanation of warfare will encourage more war (Sponsel 1996; Fry 2006), the
problem of war has often been considered to be social or cultural rather than
biological.
A second obstacle to conceptual unification has been the lack of a coherent
theory for intergroup aggression among primates. Aggression between primate
social groups is highly variable, whether in terms of the frequency and intensity
of encounters, the resources being contested, or the sex of the participants (Cheney
1987). This variation is observed not only between species, but also between
populations of the same species (e.g., Macaca fuscata: Saito et al. 1998; Sugiura
et al. 2000) and between seasons within the same population (e.g., Cercopithecus
sabaeus: Harrison 1983; Cercocebus galeritus: Kinnaird 1992). It has been difficult
to discern unifying patterns amid such variation, especially since studies of intergroup interactions (IGIs) tend to be opportunistic and have relatively small sample
sizes.
Other factors have also hampered the efforts to understand the broader significance of intergroup aggression in primates. First, a tendency to treat feeding and
mating competition between social groups as unrelated and, oftentimes, mutually
exclusive phenomena, has created an artificial division between species where
males compete over mates and species where females compete over food. Recent
studies have demonstrated that males can defend food resources either directly, or
as a by-product of their mate defense (Fashing 2001; Harris 2005, 2006a), highlighting the flaws of this dichotomy and indicating that closer attention must be paid
to the functional implications of intergroup aggression (Harris 2007). Second, the
role that intergroup resource competition plays in determining individual fitness
remains disputed. While Wrangham (1980) proposed that success in intergroup
8 Intergroup Aggression
173
competition provides reproductive advantages for individuals living in large social
groups, empirical data (Janson 1985), meta-analyses (Majolo et al. 2008), and
literature reviews (van Schaik 1983) have suggested. If, as van Schaik claims,
“intergroup feeding competition . . . [is not an] important determinant of an
individual’s fitness,” this calls into question the need for an adaptive theory for
intergroup aggression among primates.
Thus, traditionally neither students of war nor primatologists had much reason to
develop a common theory uniting human and primate intergroup aggression.
However, in recent decades, the discovery of human-like patterns of killing between neighboring communities of chimpanzees (Pan troglodytes) has provoked
evolutionary explanations of chimpanzee violence (Manson and Wrangham 1991;
Wrangham and Peterson 1996; Wrangham 1999; Wilson and Wrangham 2003;
Williams et al. 2004; Wilson et al. 2004; Watts et al. 2006; Boesch et al. 2008), and
has inspired parallel development of an evolutionary biology of human warfare
(van der Dennen 1995; Thayer 2004; Kelly 2005; Gat 2006; Roscoe 2007; Smith
2007). These efforts, which we review below, suggest that important elements of
intergroup violence among humans and chimpanzees can be explained by the
hypothesis that groups use aggression to achieve dominance over their neighbors.
According to this idea, intergroup dominance promotes fitness by a variety of
mechanisms, including access to more land and more females. We call this the
intergroup dominance hypothesis.
In this chapter, we consider whether the intergroup dominance hypothesis can
account for patterns of IGI among troop-living primates, chimpanzees, and humans.
8.2
Nonhuman Primates Living in Troops
Intergroup relationships have long been of interest to primatologists, as dominance
relationships are known to mediate competition for resources at the individual
level, and could therefore also do so at the group level (Huntingford and Turner
1987; Dunbar 1988). Yet, few studies have quantitatively investigated the relationships among neighboring primate social groups or explored how group-level
dominance influences access to resources. For example, of the 60 studies cited in
Cheney’s 1987 review of primate IGIs that included information about intergroup
dominance relationships, approximately half (25) concluded that such relationships
existed. However, most of these studies provided only verbal descriptions of the
relationships between social groups and only one-third (20/60) reported the number
of interactions on which their conclusions were based. In addition, the majority (28/35)
of studies that failed to find evidence for intergroup dominance were of species in
which groups defended home ranges as territories, and in which dominance relations are therefore hard to detect (Cheney and Seyfarth 1987). Single interactions
between territorial groups rarely result in noticeable boundary changes, but large
groups have been shown to have higher quality ranges (Cheney and Seyfarth 1987),
make more incursions into their neighbors’ ranges (Struhsaker 1967), and expand
174
M.C. Crofoot and R.W. Wrangham
their range at the expense of smaller neighboring groups (Cheney and Seyfarth
1987). Thus, it is not clear if these studies failed to detect intergroup dominance
relationships because such relationships did not exist or because the studies were
too short or too narrowly focused to adequately address the question.
To test the intergroup dominance hypothesis, three kinds of data are required.
First, numerous observations of encounters between neighboring, habituated primate groups are needed to determine if intergroup dominance relationships exist.
Investigating the relationships among several habituated social groups, rather than a
single habituated group and its unhabituated neighbors, is critical because the
presence of human observers may alter the behavior of unhabituated primates and
potentially decrease their competitive ability (Zinner et al. 2001). Large sample
sizes are essential because many factors can affect the outcome of intergroup
encounters; multivariate statistics may be needed to elucidate how these factors
interact to shape the relationships between neighboring groups (e.g., Kitchen et al.
2004a; Pride et al. 2006; Crofoot et al. 2008). Relatively few studies meet both
these criteria, and several that do have not yet published relevant analyses (e.g.,
Presbytis thomasi: Steenbeek 1999; Steenbeek and van Schaik 2001, Cercopithecus
mitis: Cords 2002, 2007). Nevertheless, studies meeting these criteria demonstrate
the presence of consistent intergroup relationships in a range of phylogenetically,
socially, and ecologically disparate species (see Table 8.1), suggesting that the
pattern may well be widespread among primates.
Second, data on how group dominance affects access to resources is required to
understand the functional implications of these relationships. Typically, high-ranking
groups are predicted to have priority of access to food resources and consume a
higher quality diet than their lower-ranking neighbors. However, few studies have
collected the detailed behavioral and ecological data required to demonstrate such
patterns (but see Table 8.1).
Finally, demographic data are needed to investigate whether the foraging advantages attained through intergroup dominance lead to increased fitness. Members of
high-ranking groups are expected to have higher reproductive rates, higher offspring survival rates, and/or lower mortality rates than their counterparts in lowranking groups.
In short, the combination of detailed behavioral, ecological, and demographic
data required to test the intergroup dominance hypothesis is found in only a small
number of primates. For this reason, we focus on three species where the data are
particularly complete.
8.2.1
Wedge-Capped Capuchins (Cebus olivaceus)
While studies of competition between primate social groups sometimes treat
numerical superiority as a sufficient proxy for group dominance (e.g., Koenig
2000; Cooper et al. 2004), the balance of power between opposing primate social
groups can be influenced by a range of additional factors, including the behavior,
Table 8.1 Intergroup interactions and dominance relationships in nonhuman primates: a selective review
Interactions
observed
Length of
study
IG
dominance?
Description of IG
relationship
Verreaux’s sifaka
(Propithecus
verreauxi)
5
19
3 months
Yes
6
White-faced
capuchins (Cebus
capucinus)
58
6 months
Yes
5
58
7 months
Seasonal
Savannah baboons (1) 4
(Papio
cynocephalus
ursinus)
10
12 months
No
Savannah baboons (2) 4
110
23 months
Yes
In 10 of the 11 interactions Subordinate groups slept
with a clear winner, the
in the core area of
group with more
their range
individuals (or, if
groups were equal in
size, with more males)
won
Large group size and
Groups traveled further,
proximity to home
faster, and stopped
range center increased
less frequently after
the likelihood of
losing interactions.
winning interactions
Small groups spent
more time feeding
and foraging and less
time socializing than
large groups
An IG dominance hierarchy Dominant groups were
able to monopolize
based on group size
existed in the birth
tourist feeding sites
season, but not in the
mating season
The two groups with
Most encounters did not
more males had
involve agonism and
larger, higher quality
mingling of troops was
home ranges, used
observed.
overlap areas more
Displacements
frequently, and had
occurred in two of ten
shorter day ranges
encounters
than their neighbors
No data available
Both location and rival
group identity
influenced the outcome
of intergroup
encounters, and groupdominance seemed to
depend on the relative
number of males
Tibetan macaques
(Macaca
thibetana)
Effects of IG dominance
on foraging success or
behavior
Effects of IG
dominance on
demographics
Reference
No data available
Benadi et al.
(2008)
No data available
Crofoot et al.
(2008),
Crofoot (2008)
No data available
Zhao (1997, 1999)
The two groups with
more males also
had higher birth
rates and a higher
proportion of
juveniles
Anderson (1981)
No data available
Kitchen et al.
(2004a)
175
Groups
studied
8 Intergroup Aggression
Species
(continued)
Length of
study
IG
dominance?
Description of IG
relationship
Effects of IG dominance
on foraging success or
behavior
Effects of IG
dominance on
demographics
Reference
6
Black and white
colobus (Colobus
guereza)
115
19 months
Yes
High ranking groups had
higher quality home
ranges than low
ranking groups
No data available
Harris (2006a,b)
Japanese macaques
(Macaca fuscata
(Yakushima))
7
151
14 years
intermittently
Yes
Large social groups
frequently displaced
their smaller
neighbors from food
trees
The ratio of infants to
adult females was
higher in large
groups, suggesting
that members of
large groups had
higher
reproductive rates
Suzuki et al.
(1998),
Takahata et al.
(1998),
Sugiura et al.
(2000)
Japanese macaques
(Macaca fuscata
(Kinkazan))
3
63
14 years
intermittently
?
Large social groups
rarely excluded their
smaller neighbors
from food resources
Group size was not
related to
reproductive rate
see Yakushima
169
9 years
Yes
Group dominance
relationships were
strong and linear and
depended on
characteristics of each
group’s male
Dominance relationships
were consistent and
stable over time.
Larger groups were
generally dominant to
smaller groups,
especially when the
difference in group size
was large
Groups rarely displaced one
another during
intergroup interactions
(IGIs) (7/63
interactions), but in
cases with clear
outcomes, larger social
groups tended to
dominate their smaller
neighbors (5/7
interactions)
Strong, stable dominance
relationships existed
among groups. Large
groups were dominant
to smaller groups, but
dominance may have
been more closely tied
to the number and
identity of adult males
than group size per se
High-ranking groups had
priority of access to
high quality areas
and consumed fruits
with higher sugar
contents. They spent
less time traveling
and foraging than
their lower ranking
neighbors
Both male and female Robinson (1988),
members of large
Srikosamatara
social groups had
(1987)
higher
reproductive rates
than their
counterparts in
small social groups
Groups
studied
12
Wedge-capped
capuchins (Cebus
olivaceus)
M.C. Crofoot and R.W. Wrangham
Interactions
observed
Species
176
Table 8.1 (continued)
Ring-tailed lemurs (2) 10
188
12 months
13 years
Territorial
Encounter location, rather
than group size
determined the
outcome of IGIs
No data available
Large social groups had Long-term data from
Pride et al. (2006),
higher quality home
Jolly et al.
this study site
ranges, and their
(2002), Pride
show that
members maintained
(2005a,b)
reproductive rate
higher food intake
decreases with
rates than members
group size, but that
of small groups
females in large
except during a
social groups may
period of atypically
experience lower
low food availability.
mortality
The costs of range
defense were lower
for individuals living
in large social groups
No data available
Females in
Takahata et al.
intermediate sized
(2008)
groups had higher
reproductive rates
than females in
either large or
small groups
8 Intergroup Aggression
Ring-tailed lemurs (1) 6
(Lemur catta)
177
178
M.C. Crofoot and R.W. Wrangham
temperament or size of the alpha male (e.g., Harris 2006b), the strength of relationships between group-mates (e.g., Starin 1991) and the location of the interaction
(e.g., Pride et al. 2006; Crofoot et al. 2008). For example, using 9 years of data on
interactions among 12 wedge-capped capuchin social groups, Robinson (1988)
demonstrated a linear dominance hierarchy among groups. Rather than depending
solely on group size, this hierarchy was ordered by the number and identity of the
adult and subadult males in each group (Robinson 1988). Groups with many males
tended to be high-ranking, but in some group dyads, the identity of the interacting
males also influenced intergroup relationships, such that a group with a smaller but
more potent male cohort outranked a group with a larger number of males.
Group dominance afforded several foraging benefits in this capuchin population.
High-ranking groups had greater access to fruit species that were clumped and
relatively uncommon, and were able to concentrate their foraging effort in areas
with high fruit tree density (Srikosamatara 1987). The fruit species consumed by
members of large groups also tended to have higher sugar content than the fruits
that made up the majority of small groups’ diets. Perhaps to compensate for the
costs of subordinacy, low-ranking groups spent more time moving and foraging
than high-ranking groups. They traveled further, particularly on days when they
encountered one of their neighbors (Srikosamatara 1987), presumably in an effort
to make up for decreased foraging efficiency. Such attempts, however, appeared to
be ineffective because females belonging to low-ranking groups had lower reproductive rates than their counterparts in high-ranking groups (Robinson 1988).
The demographic ramifications of the relationship between group size and
reproductive success in this capuchin population were striking. Because highranking groups grew faster than small groups, over time the percentage of the
population living in high-ranking groups is expected to increase. However, past a
certain size, resource competition within groups is expected to promote group
fissioning. The interaction between these opposing pressures structured population
growth in Robinson’s study population (1988). Low-ranking groups tended to be
small and to go extinct, while high-ranking groups grew and eventually “budded
off” new small groups. Resource competition between social groups also shaped
the genetic structure of this population, as both the female and male members of
high-ranking groups contributed disproportionately to population growth, and thus
to future generations (Robinson 1988; Valderrama Aramayo 2002).
8.2.2
Japanese Macaques (Macaca fuscata)
Relationships between neighboring social groups, and the effect that these relationships have on individual fitness, are expected to depend not only on the physical and
social characteristics of the species in question, but also on the distribution and
abundance of food and the density of conspecifics in their habitat (Horiuchi 2008).
The Japanese macaque populations on Yakushima and Kinkazan Islands illustrate
the strong effects that environmental variables can have on intergroup relationships.
8 Intergroup Aggression
179
These macaque populations have been studied intensively for almost three decades,
and numerous comparative studies of their social structure, behavior, ecology, and
demography have been undertaken (Yamagiwa 2008). These studies show that
intergroup relationships have much larger consequences in the high density
Yakushima population than in the low density Kinkazan population.
In Yakushima, relationships among social groups were determined by relative
group size (the difference in the number of adults belonging to each group), and
interactions between neighbors were found to influence both immediate foraging
opportunities and long-term resource access. Dominance relationships among
groups were consistent and stable over time (Saito et al. 1998). Larger groups
generally displaced smaller groups (74% of interactions: Sugiura et al. 2000)
especially when they had a large numeric advantage: when the larger group had
at least ten more members than their opponent, they won 94% of interactions
(Sugiura et al. 2000). This competitive advantage likely increased the short-term
foraging efficiency of females living in large social groups because the majority of
IGIs (100/151, i.e., 66.2%) ended with one group displacing the other (Sugiura et al.
2000), and 17% of interactions occurred when two groups simultaneously
approached a fruit or nut tree. Thus, members of small groups lost feeding opportunities as a direct consequence of encountering their neighbors. Intergroup dominance also appeared to have long-term consequences because, on average, larger
groups had higher reproductive rates, with the number of infants per female
increasing linearly with group size (Takahata et al. 1998). This effect emerged,
however, only during periods of resource scarcity; birth rates of large (highranking) groups were higher than those of their smaller (lower-ranking) neighbors
only during years with poor fruit production (Suzuki et al. 1998).
On the island of Kinkazan, approximately 1,300 km northeast of Yakushima, a
very different picture of Japanese macaque ecology and behavior emerges. In the
Kinkazan population, large group size did not confer competitive or reproductive
advantages. Groups encountered one another about one-third as often (0.012
encounters/hour on Kinkazan compared to 0.039 encounters/hour on Yakushima),
and fewer interactions were agonistic (11% vs. 49% in Kinkazan and Yakushima,
respectively) or involved one group displacing the other (11% on Kinkazan compared to 66% in Yakushima: Sugiura et al. 2000). Although bigger social groups
were dominant in five of the seven interactions with a clear winner (Sugiura et al.
2000), these rare displacements did not appear to affect the resource access of
smaller groups. As expected, therefore, larger groups did not have higher reproductive rates than their smaller neighbors. Group size was unrelated to reproductive
rate both in years with good fruit crops and in years with poor fruit crops (Suzuki
et al. 1998).
Why were intergroup relationships different in the Yakushima and Kinkazan
populations? In theory, Kinkazan population density could have been low relative
to food resources, thanks to hunting, predation, or disease. However, in groups of
all sizes on Kinkazan, birth rates were higher during years with large fruit crops
than in years with small fruit crops (Suzuki et al. 1998), indicating that the
population was food limited. Furthermore, the Kinkazan macaques had no
180
M.C. Crofoot and R.W. Wrangham
Table 8.2 Comparison of the Kinkazan and Yakushima population of Japanese macaques
Yakushima
Kinkazan
Intergroup interaction (IGI) ratea
0.039/h
0.012/h
70/151 encounters
7/63 encounters
Aggressive IGIsa
100/151 encounters
7/63 encounters
Displacementsa
Troop densityb
4.7/km2
0.2/km2
b
Average distance between home range centers
361 m
1,232 m
90 ha
221 ha
Home range sizeb
Average home range overlapb
58.70%
55.10%
1,802/ha
94/ha
Food tree densityb
70 m
151 m
Average distance between feeding boutsb
2.19 m/min
3.08 m/min
Average travel speedb
a
Sugiura et al. (2000)
b
Maruhashi et al. (1998)
predators (Takahata et al. 1998). The Kinkazan population, therefore, did not
appear to be living below carrying capacity.
Alternatively, the distribution and/or abundance of food resources in Kinkazan
might have made resource defense less economical than in Yakushima. If so,
feeding competition between groups is expected to be less intense (Wrangham
1980). Certainly, differences in resource distribution were clear (see Table 8.2).
Yakushima had a higher density of food patches than Kinkazan, i.e., 19 times more
food trees per hectare (Maruhashi et al. 1998), and a higher overall food abundance
(the basal area of food trees per hectare was 2.2 times greater than in Kinkazan). On
the other hand, the average size of food trees in Kinkazan (those providing fruits
and nuts) was larger (Maruhashi et al. 1998). These differences were correlated
with differences in foraging behavior. In Kinkazan, macaques had larger, more
evenly used home ranges, and traveled faster and further between feeding bouts,
indicating that they worked harder to meet their metabolic requirements (Maruhashi
et al. 1998). The more dispersed food sources and lower food abundance in
Kinkazan were thus associated with greater foraging effort, suggesting groups
competed primarily via scramble rather than contest competition. By contrast, the
higher resource density in Yakushima could have increased the profitability of
active resource defense, leading to a fitness advantage for individuals living in
larger and higher-ranking groups. Further research is thus needed to assess the
importance of differences in food distribution between Yakushima and Kinkazan,
and to distinguish effects of resource distribution and abundance from those due to
differences in population density and encounter rates (Horiuchi 2008).
8.2.3
Ring-Tailed Lemurs (Lemur catta)
Most, if not all, primate groups share some portion of their home range with their
neighbors. In the overlap zone, intergroup aggression can either occur over specific
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food patches or over space. Dominance in these interactions can be mediated by the
characteristics of the groups, as in wedge-capped capuchins where home ranges
overlapped completely and dominant social groups defeated their subordinate
neighbors throughout the entire area (Robinson 1988) (i.e., absolute dominance
sensu Kaufmann 1983). In other species, dominance can be context-dependent
because it depends on the location of the interaction (e.g., Lemur catta: Pride
et al. 2006, C. mitis: Cords 2002) (i.e., relative dominance sensu Kaufmann 1983).
Ring-tailed lemurs in Berenty provide the best-studied example of the latter system.
In this species a group’s fitness appears to depend on its ability to maintain the
ownership of a high-quality area (Pride et al. 2006).
In Berenty, groups of ring-tailed lemurs win interactions in their “typical” ranges
(their 85% minimum convex polygon home range), and tend to lose outside these
areas, regardless of the strength of the opposing group (Pride et al. 2006). Why large
social groups are not able to overpower their smaller neighbors is not understood.
This problem presents a challenge for the intergroup dominance hypothesis by
calling into question whether, in territorial species, groups are able to translate
competitive superiority into increased resource access or higher fitness. Although
large social groups do not seem to have a competitive advantage in any single IGI,
they might achieve high foraging success (and therefore high fitness) by using their
power advantage to defend territories of superior quality. The simplest way to test
this is by assessing the long-term effect of group size on reproductive rate.
Two studies have yielded conflicting data on this point. Jolly et al. (2002) found
that reproductive rate of lemurs at Berenty decreased with group size, thus indicating no benefits for larger groups. In contrast, Takahata et al. (2006) reported that
groups with an intermediate number of adult females had higher reproductive rates
than those with either few or many females. Since elevated within-group competition is expected in large groups and was demonstrated in their study, Takahata et al.
(2008) concluded that their data, based on ten groups over 13 years, supported the
hypothesis that large groups use social dominance over smaller groups to achieve
higher fitness.
The discrepancy between the results of Jolly et al. (2002) and Takahata et al.
(2006, 2008) has not been fully explained. Takahata et al. (2006) suggest that
differences in population density may be responsible, as their study groups were
in a high-density area of Berenty (542.3 individuals per km2), whereas Jolly’s study
included groups from a range of habitats with a broad range of densities (100 580
individuals per km2 in the scrub forest and near the tourist station, respectively).
However, this explanation is not fully supported because Jolly et al. (2002) found a
negative relationship between group size and reproductive rate even in the groups
near the tourist station where population density was highest.
Ring-tailed lemurs in Jolly et al.’s (2002) study thus show that large group size
does not necessarily confer a reproductive advantage. Nevertheless, two additional
lines of evidence from this population provide support for the intergroup dominance hypothesis. First, Pride et al. (2006) demonstrated that large groups defend
higher quality home ranges than small groups, and are able to do so at a lower cost
to individual members. Females in large social groups are able to share the burden
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of territorial defense with more group mates, and thus can maintain home ranges in
the most productive and stable areas at lower per capita cost of defense (Pride et al.
2006). Second, members of large social groups may have reduced mortality compared to members of smaller groups. Pride (2005b) demonstrated that glucocorticoid levels in females at Berenty predicted individual survival, and that females in
large social groups had lower cortisol levels than females in small groups (Pride
2005a). This pattern may be explained by the fact that individual participation in
intergroup contests declined with increasing group size (Pride et al. 2006). The
costs of territorial defense and intergroup resource competition thus seem to be
lower for females in large groups because they are shared among more individuals,
and this appears to have long term consequences for both survival and individual
fitness. In addition, competition for space is intense among ring-tailed lemurs, and
members of groups that lose control over their “typical” areas experience high
mortality (Koyama 1991; Hood and Jolly 1995; Jolly and Pride 1999; Koyama et al.
2002; Gould et al. 2003). For example, Jolly and Pride (1999) recorded a group
of ring-tailed lemurs expanding and fissioning over a 6-year period. In this case,
the group expanded as a result of increased resources coming from human food
(a tourist project). It reached 19 individuals, compared with group sizes of 3 12 in
11 other groups in the same area, and then fissioned. The two daughter groups were
both successful, one using aggression to extend its range at the expense of a
neighboring group and the other entirely taking over a neighboring range (Jolly
and Pride 1999). Thus, it is possible that even if smaller (and less dominant) groups
have higher reproductive rates, their long-term fitness is reduced by the mortality
risks associated with range loss and group extinction.
8.2.4
Troop-Living Primates: Discussion
Wedge-capped capuchins, Japanese macaques, and ring-tailed lemurs provide rare
examples of relatively complete studies of the long-term consequences of intergroup dominance in troop-living primates. In wedge-capped capuchins, the intergroup dominance hypothesis was clearly supported, because groups had predictable
dominance relationships that depended on fighting power, and members of higherranking groups had access to better resources and achieved higher reproductive
rates. In Japanese macaques, a high-density population experienced a similar
dynamic, whereas a low-density population did not. The effects of intergroup
dominance were more complicated in ring-tailed lemurs. Although it is not clear
whether members of large social groups have higher reproductive rates than
members of small groups, the data suggest that they experienced reduced mortality.
Similar evidence of the importance of long-term survival comes from toque
macaques (Macaca sinica, Dittus 1986). For 7 years, a group of 8 15 females
consistently dominated a neighboring group of 7 11 females in conflicts over
feeding sites, yet during this period, the reproductive rates of the two groups were
not significantly different. However, the dominant group then took over the range of
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its neighbor, and females in the subordinate group became members of the dominant group. Over the next 8 years, differences in reproduction and survival led to the
lineages of the dominant group having 20 females, compared to one descendant
from those in the subordinate group. This case suggests that over the long term, the
ability to control and defend a home range may be more important for fitness
maximization than short-term measures of reproductive rates. Where dominant
groups do not achieve high reproductive rates, they may alternatively have higher
rates of individual survival and/or superior physical condition.
Although data on the intergroup relations of troop-living primates remain too
sparse to provide a definitive test of the intergroup dominance hypothesis, intergroup dominance relationships have been shown to be important in an ecologically
and socially diverse set of species. These include both frugivores (Cebus olivaceus:
Robinson 1988) and folivores (Colobus guereza: Harris 2006a,b); territorial species
(L. catta: Jolly et al. 2002) and species with completely overlapping home ranges
(Cebus olivaceus: Robinson 1988); and in primates with one-male social systems
(C. guereza: Harris 2006a, b) and multi-male social systems (M. fuscata: Sugiura
et al. 2000). In each case, the results challenge van Schaik’s (1983) contention that
intergroup feeding competition is not important in determining an individual’s
fitness, and they indicate that when long-term data are available, the intergroup
dominance hypothesis is at least partly supported, i.e., that troop-living primates
can achieve long-term benefits from success in coalitionary aggression against
neighbors. Since hostile IGIs often have no obvious immediate effect in determining access to a particular food patch, these case studies suggest that the long-term
implications merit further research.
The behavioral implication of the intergroup dominance hypothesis is that
conflicts between groups are not necessarily over immediate access to resources,
but instead can represent fights over dominance status. Despite indications from
wedge-capped capuchins, Japanese macaques, and ring-tailed lemurs that the
benefits of winning such contests are high, escalated aggression rarely occurs
during intergroup conflicts in troop-living primates. One possible explanation is
that the costs of aggression are high. When the outcome of a conflict can be
predicted based either on previous interactions or on physical characteristics of
the participants, and when the cost of interacting is high, weaker opponents are
expected to withdraw rather than engage in a risky fight they are likely to lose
(Maynard Smith and Parker 1976). Neighboring primate social groups interact with
one another regularly and thus levels of intergroup aggression may be relatively low
because the outcome is a foregone conclusion. In addition, the social groups of
troop-living primates are, by definition, cohesive and thus intergroup aggression in
these species rarely involves the imbalances of power that are implicated in lethal
aggression of chimpanzees and human foragers (below). Observations of intergroup
killings in capuchin monkeys lend support to this hypothesis because they suggest
that troop-living primates will participate in escalated aggression if the costs are
sufficiently low (Gros-Louis et al. 2003). In this instance, the coalitionary nature of
the attacks meant that the aggressors could inflict serious wounds on their victim
without risking substantial injury themselves (Gros-Louis et al. 2003).
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An alternative explanation for the low intensity of intergroup aggression
observed in troop-living primates is that the collective action problem inherent in
group-level resource competition presents an obstacle to high individual investment
(Nunn and Lewis 2001; Nunn and Deaner 2004; Kitchen and Beehner 2007). Why
should any individual risk injury by participating in intergroup fights when the
benefits gained through such confrontations will be enjoyed by all group members,
including individuals that did not take part in securing them? Participation in
aggressive intergroup encounters is highly variable (Wilson et al. 2001; Wich
et al. 2002a, b; Kitchen 2004, 2006; Kitchen et al. 2004b), and why some individuals rush boldly towards an opposing group, risking injury, while others hang back
and watch the excitement from a safe distance remains poorly understood (Kitchen
and Beehner 2007). The fact that intergroup dominance relationships exist in a
number of troop-living species clearly suggests that primates are able to overcome
this collective action problem, but further study is required to demonstrate how this
is accomplished.
8.3
Chimpanzees
Chimpanzees form social communities that occupy a stable home range. Within
communities, individuals forage in parties (sub-groups) of variable size and composition, including sometimes being alone. Intercommunity interactions often
occur only at long distance, mostly through auditory contact. In three populations
(Taı̈, Mahale, Ngogo), they occurred at similar rates, 1 1.5 times per month.
Aggression is the principal form of interaction between communities. It occurs
mostly when parties meet by chance, but also when one party deliberately
approaches another, sometimes by stealth.
The principal actors are adult males and there are two main types of interaction.
Battles involve mostly bluff, including numerous calls and aggressive charges
made alone or jointly towards opponents. Physical contact is occasional and
generally mild, though it can lead to one individual being separated and attacked,
and in Taı̈, it includes herding and temporary forced consortships of females
(Boesch et al. 2008). Battles may continue in the same location for up to 45 min
(Wrangham pers. observ.), and normally end with one or both sides retreating.
Attacks, by contrast, involve a coalition of at least two and generally four or more
males violently attacking a member of the neighboring community. Attacks occur
both when parties meet by chance and when one party searches for potential victims
during boundary patrols or after detecting them at long distance. Attacks are much
less common than battles (Wilson and Wrangham 2003; Watts et al. 2006; Boesch
et al. 2008).
While chimpanzee intercommunity relations have not been studied in depth,
they appear to conform to the three components of the intergroup dominance
hypothesis. First, relationships between communities are generally predictable.
For example, in Gombe parties from the larger Kasekela community consistently
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defeated those from the smaller Kahama community (Goodall 1986). Nishida et al.
(1985) found a similar relationship at Mahale for M-group and K-group. However,
the outcome of specific interactions depends on the local context, such as the
relative numbers of males in each party, so parties from small communities can
sometimes win intergroup encounters (Wrangham 1999; Boesch et al. 2008).
Second, communities that win interactions improve their access to resources.
Thus, two dominant communities in Gombe and Mahale permanently extended
their territories at the expense of their neighbors (Nishida et al. 1985; Goodall 1986;
Williams et al. 2004). The M-group community in Mahale also exploited its
dominance seasonally by taking control of an area normally occupied by the
neighboring K-group, whenever the principal food-plant species in the shared
area came into fruit.
Third, success in intercommunity aggression had fitness pay-offs. In particular,
the dominant Gombe community experienced variation in territory size, which was
suspected to result from varying success in competition with neighboring communities. Larger territory size was associated with several indications of greater access
to resources, including higher individual body weights and larger parties, and
fitness gains are indicated by shorter interbirth intervals and higher infant survival
(Williams et al. 2004). Additionally, subordinate communities have twice been
observed to go extinct, apparently as a result of aggression from dominant neighbors (Kahama at Gombe, K-group at Mahale). While some individuals from these
subordinate communities survived the dissolution of their groups, almost all males
died and the females who were known to survive experienced high rates of infanticide (Nishida et al. 1985).
Intercommunity dominance accordingly appears to be beneficial for chimpanzees because it gives both sexes increased access to resources, while males can also
gain increased access to females. The question that links intergroup dominance in
chimpanzees to human warfare is why intergroup contests are so much more
aggressive among chimpanzees than among troop-living primates. In particular,
why do chimpanzees sometimes violently attack and kill members of neighboring
communities?
According to the imbalance-of-power hypothesis, the fission fusion social organization of chimpanzees facilitates lethal aggression against members of neighboring groups. Chimpanzees form temporary subgroups that vary in size, so parties
with several males sometimes encounter lone males or isolated mothers from
neighboring groups. When loners meet large parties, aggressive power is
distributed so asymmetrically that the dominant party can afford to express intense
violence while experiencing a very low risk of being hurt themselves. The proposed
advantage of damaging or killing an opponent is that by reducing the number of
coalitionary aggressors in the neighboring community, the attackers increase the
relative power of their community. As a result, they become more likely to win
future interactions, and therefore to achieve the fitness gains accruing from elevated
intercommunity dominance (Manson and Wrangham 1991; Wrangham 1999;
Williams et al. 2004; Wilson et al. 2004; Watts et al. 2006; Sherrow and Amsler
2007). The imbalance-of-power hypothesis predicts that the aggressors will be
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members of the philopatric sex, whether females (as in spotted hyenas, Crocuta
crocuta) or males (as in chimpanzees) (Wrangham 1999).
The proposal that an asymmetry of power tends to induce attack is supported by
data from Gombe, Mahale, Kibale and Taı̈ on the contexts of aggression. For
example, in 20 cases recorded by Watts et al. (2006) involving the Ngogo community in Kibale, attacks were conducted by at least three individuals on a victim that
was either alone when encountered, or was rapidly isolated from the rest of his/her
party. A victim who has members of his/her own community nearby has sometimes
been supported and rescued (Boesch et al. 2008). Thus, where power is more evenly
balanced, attacks are less likely or can be stopped. The importance of power
asymmetry is also indicated experimentally by playbacks showing that the probability of males approaching the location of a male stranger’s call, or the speed at
which they do so, is predictably increased by the number of males in the listening
party (Wilson et al. 2001). As expected, border zones tend to be avoided in general,
and males in small parties are particularly unlikely to visit them (Wilson et al. 2001;
Wrangham et al. 2007). In sum, the power asymmetries made possible by fission
fusion grouping make lethal violence cheap, provided that aggressors can assess the
relative fighting ability of parties correctly.
If escalated aggression is cheap and serves to increase the future dominance of
the aggressors’ community, it should be directed towards the most effective fighters
among the neighbors. Females are not active aggressors in intercommunity interactions in most sites. However, in Taı̈, females can take part, perhaps because
parties there are more stable than elsewhere, power asymmetries are reduced, and
intercommunity attacks are rare (Boesch et al. 2008). In other sites attacks are more
common and are indeed directed mostly at males. For instance, the probability of
attacks on strangers at Gombe was 100% for males (n ¼ 6 single males, 16 in
parties), <60% for females without sexual swellings (n ¼ 51) and <20% for
females with sexual swellings (n ¼ 23) (Williams et al. 2004). The sex difference
is particularly pronounced for lethal aggression. In a review of data from five
populations including 16 known and 16 suspected cases of adult deaths from
intercommunity aggression, Wrangham et al. (2006) found that 94% of the victims
were adult males (n ¼ 30 deaths). Intercommunity aggression also involves attacks
on infants. Unfortunately, observers can rarely detect the sex of infant victims, but
of eight cases where the sex of the victim was known, six were male (75%)
(Wrangham et al. 2006).
The imbalance-of-power hypothesis is thus supported by evidence that chimpanzees are sensitive to power imbalances, tend to reduce the number of males in
neighboring communities, and gain fitness advantages by doing so. Competing
hypotheses, to explain why chimpanzees make deliberate attacks on victims who
are outnumbered and over-powered, have mostly focused on the proximate stimuli
eliciting violence, and receive little support (Williams et al. 2004; Wilson et al.
2004). First, chimpanzees could, in theory, have a generalized tendency to attack
unfamiliar individuals. However, as we have seen, the likelihood of an attack
depends on context. Second, specific individuals might be particularly prone to
violence. However, although individual variation has been shown for predatory
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aggression by chimpanzees (Gilby et al. 2008) and for rank-related frequencies of
intracommunity aggression (Muller and Wrangham 2004), Wilson et al. (2001)
found equally strong responses to playbacks of strangers among all seven adult
males in their study. Third, attacks could be provoked by immediate competition
over resources. Relevant stimuli could include the presence of sexually active
females, the presence of preferred food patches, a season of ecological stress, or a
long-term shortage of land or females. None of these has yet been demonstrated to
be important, however (Wilson et al. 2004).
The imbalance-of-power hypothesis predicts that chimpanzees will rarely take
risks as aggressors in intercommunity interactions. Against this, Boesch et al.
(2008: 531) suggest that “chimpanzees can take large risks when potential benefits
are large or when failure to do so could inflict larger costs.” Aggressors have rarely
been wounded to date, but further data will enable these alternatives to be more
finely discriminated. In particular, evidence that aggressors expose themselves to
risk will suggest that competition over detectable resources is more important than
current data indicate. At present, the propensity for chimpanzees to violently attack
neighbors appears to be best explained by the intergroup dominance hypothesis,
including a tendency to use attacks to weaken the neighbors whenever possible.
Chimpanzees are the best-studied primate living in fission fusion communities,
but spider monkeys (Ateles spp.) have similar patterns of grouping and territoriality.
According to the imbalance-of-power hypothesis, therefore, spider monkeys should
show parallel forms of intergroup violence. No intergroup killing has yet been seen
in spider monkeys, but recent observations suggest that important elements of their
patterns of aggression are similar to those in chimpanzees. In particular, spider
monkeys show active defense of territories, larger parties tend to win interactions,
and small parties avoid the border zone (Aureli et al. 2006; Wallace 2008). Males
have been seen on intergroup raids making incursion into neighboring ranges and
attacking lone individuals (Aureli et al. 2006). Cooperative killing has been seen
within groups (Campbell 2006; Valero et al. 2006). On the basis of these observations, the imbalance-of-power hypothesis predicts that lethal attacks will eventually
be found also in spider monkeys.
The implication of the imbalance-of-power hypothesis for chimpanzees is that
selection has favored the propensity to attack male neighbors whenever the costs
are perceived to be sufficiently low. Roscoe (2007) presents an alternative idea. He
proposes that the reason why unprovoked attacks on strangers occur in chimpanzees
more than other nonhuman primates is that chimpanzees are exceptionally intelligent. As a result, he argues, the attackers are so skilled at assessing the long-term
benefits that they can evaluate the merits of a risky attack. The cognitive demands
implied by Roscoe’s proposal are high. According to Roscoe’s idea, a chimpanzee
is expected to perceive that a violent attack will lead to a reduction in the fighting
power of the neighboring group, and hence to an increased likelihood of the
aggressors’ community winning intercommunity interactions. The chimpanzees
should then be able to realize from this that they will obtain increased access to
resources. The cognitive challenges seem to us too great for this scenario to be
realistic, and we believe that a more parsimonious explanation is that, faced with an
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uncertain long-term pay-off, chimpanzees are motivated by a psychological reward
system that has been favored evolutionarily by the benefits that tend to accrue to
judicious killers.
8.4
Humans
War is sometimes defined as being a more exclusive activity than intergroup
aggression. For instance, Kelly (2000) and Fry (2006) defined warfare to exclude
feuding. Such a definition means that warfare is not considered to have occurred
among the Andamanese, for example, even though members of neighboring tribes
killed each other whenever they met vulnerable opponents (Fry 2006). Similarly,
Fry (2006) considered that among the Murngin, an Australian aborigine group,
there was no war even though they practiced six types of warfare according to
Warner (1958), their principal ethnographer. For example, “maringo” was defined
by Warner (1958: 166) as “Surprise attack by group, in revenge. Always woundings
or death.”
To avoid confusion and allow easy comparisons with primates, here we define
warfare inclusively to mean IGIs among humans, in which coalitions attempt to
aggressively dominate or kill members of other groups. Using this definition,
warfare is characteristic of most human societies. The few in which it has been
recently absent tend to be societies that were politically dominated by their neighbors (Fry 2006).
While cultural and socio-political diversity makes generalization difficult, two
broad styles of warfare can be recognized, below and above the military horizon
(Turney-High 1949). Below the military horizon, warfare is conducted anarchically
in the sense that individuals cannot be ordered to participate. Most interactions
involve asymmetric attacks, made either opportunistically or as a result of a
deliberate plan. In the cases of planned attacks, the typical motivation is revenge
for prior killings. Attacks can continue into a massacre if power is sufficiently
imbalanced. Males are the chief targets, but children and women can also be killed.
Battles involving deliberate confrontation of opposing sides are rare, though not
unknown. When battles occur, they tend to stop after a few deaths. This style of
warfare is characteristic of hunter-gatherers and small-scale farming societies (Gat
this volume). Hostility is often unrelenting between tribes with different languagegroups (“external war”). Within tribes, groups tend to oscillate between conditions
of war (“internal war”) and peace, often brought about by explicit peace-making
ceremonies (Wright 1942; Turney-High 1949; Keegan 1993).
Above the military horizon, warfare is practiced by armies, i.e., institutions in
which leaders devise plans and have the power to order soldiers into battle. While
asymmetric attacks remain common, battles are especially prominent in warfare
above the military horizon. Battles are rarely opportunistic and often require
the leaders of opposing forces to agree where and when to fight. The leaders’
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motivation for fighting includes complex political considerations, and tends to be
aimed at destroying or subjugating the opposing army (Keegan 1993; Collins
2008). The soldiers’ motivation for fighting varies widely. Individuals may fight
from a sense of duty; they may wish to fight out of patriotism or opportunities for
loot; or they may fear the consequences of not fighting, such as being killed by the
enemy, being killed by their leaders, or letting down their immediate comrades
(Keegan 1993).
Human warfare clearly conforms to the intergroup dominance hypothesis, because intergroup dominance relationships are routinely stable for years at a time,
and they predict access to resources such as valuable locations or trade routes.
Dominant groups also commonly flourish by expanding their territorial ranges or by
restricting the access to resources of individuals belonging to subordinate groups.
Gat (2006) gives numerous examples.
Warfare also appears broadly to fit the imbalance-of-power hypothesis. The fit is
particularly clear below the military horizon, though in humans, there are more
sources of power asymmetry than in chimpanzees. As in chimpanzees, power
asymmetry between opposing sides comes both from differences in party size and
from one side having the element of surprise. In addition, humans routinely use
night-time attacks (often initiating attacks just before dawn), and devise special
tactics and weapons to give themselves a military advantage. Practitioners of
internal war also use deceit (ensnaring familiars by guile) and treachery (betrayal
of a trust) to establish a power advantage (Turney-High 1949; Zegwaard 1959;
Wadley 2003; Gat 2006). The use of such tactics and the tendency to avoid battles
suggest that most killing below the military horizon occurs during asymmetric
interactions in which the killers experience low risk of being injured (e.g., Chagnon
1997). These generalizations suggest that the pattern of warfare among foragers and
small-scale farming tribes largely conforms to the imbalance-of-power hypothesis.
Above the military horizon, the calculus is more complex because of the
distinction between leaders (who are motivated to fight or direct others to fight)
and soldiers (who may be reluctant to fight). The existence of hierarchical relationships between leaders and soldiers means that leaders can take substantial risks,
deliberately allowing their armies to sustain large casualties. The lack of leaders in
chimpanzees or hunter-gatherers, therefore, contributes to explaining why they
rarely have lethal battles. Nevertheless, although a steep military hierarchy means
that warfare above the military horizon does not necessarily conform to the
imbalance-of-power hypothesis, we conjecture that within battles, and in numerous
engagements during prolonged wars, aggressive interactions tend to be conducted
according to the imbalance-of-power hypothesis. For example, military analyses
tend to find that most deaths occur not from direct confrontation, but as a result of
killing by the winning side, typically of soldiers who are helpless because they are
in retreat or have been captured (Collins 2008).
As for chimpanzees, coalitions of humans with a large power imbalance in their
favor could kill opponents either as a result of rational calculation or from emotional satisfaction (Roscoe 2007). Both factors seem likely to apply.
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Discussion
Our central question is whether intergroup aggression can be explained by the same
principles among troop-living primates, chimpanzees, and humans. Our review
suggests that in each case, the intergroup dominance hypothesis has substantial
explanatory power. Our findings differ from a number of recent reviews, which
have supported van Schaik’s claim that large group size does not provide functionally significant benefits in terms of resource competition in primates (Silk 2007;
Majolo et al. 2008). A critical component of our analysis, which may explain this
discrepancy, is that we focus on long-term rather than short-term reproductive
consequences of intergroup competition.
We note two ramifications. First, the intergroup dominance hypothesis suggests
that due to the social structuring of primate populations, individuals from dominant
social groups are expected to contribute disproportionately to future generations.
This indicates that source-sink dynamics will influence primate and human evolution with respect to intergroup aggression. Thus, in preferred habitats, groups are
expected to occur at high density and to act as genetic sources, exporting genes to
subpopulations in more marginal habitats. Groups within these successful subpopulations should compete aggressively, and success in competition will, therefore,
lead to high fitness for individuals that have evolved to fight well against neighboring groups. Dominant groups are thus expected to export genes which promote
success in intergroup aggression. This may contribute to explaining why aggressive
intergroup relations sometimes prevail in populations where intergroup aggression
provides no obvious benefits to dominant social groups, as discussed for Japanese
macaques.
Second, the evidence that dominant groups tend to have a fitness advantage in
nonhuman primates implies that many of the psychological mechanisms underlying
success in intergroup competition may be similar in humans and other primates.
Such mechanisms have hardly been studied. Wrangham (1999) suggested that for
chimpanzees, they might include the experience of a victory thrill, an enjoyment of
the chase, a tendency for easy dehumanization (or its equivalent for nonhuman
primates) and deindividuation, ready coalition formation, and sophisticated assessment of power differentials in the context of intergroup conflict. Depending on the
species (e.g., how important coalitions are within groups, or how often each sex
participates in aggression between groups), such mechanisms may be differentiated
by sex. The evidence that intergroup dominance is often critical in group-living
primates thus provokes a series of questions about the degree of similarity and
difference in the psychological mechanisms underlying coalitionary aggression
between humans and other species. (see Gat, this volume).
In sum, there are notable behavioral and functional similarities between human
warfare and intergroup aggression among nonhuman primates. They suggest that
coalitionary aggression in both systems is explicable by promoting intergroup
dominance and therefore tending to promote the aggressors’ fitness. There are also
important differences between human warfare and primate intergroup aggression,
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particularly above the military horizon where the interests of leaders and followers
are often in conflict and where lethal battles are a prominent feature. The conceptual
framework provided by the relatively simple case of nonhuman primates is, therefore, merely a starting-point for understanding the behavioral ecology and evolutionary psychology of warfare. Some basic outstanding evolutionary problems in
the study of warfare include a fuller accounting of individual costs and benefits
(such as the extent to which warriors are altruistic), understanding the nature and
importance of the emotional rewards experienced by fighters, and understanding
the role of social rewards conferred on warriors as a way to increase aggressive
motivations. Studies of the evolution of war are promising, but they are at a very
early stage.
Acknowledgments We are grateful to Robert Hinde, Joan Silk, and two anonymous reviewers for
comments.
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Part V
Foundations of Cooperation
Chapter 9
Why War? Motivations for Fighting in the
Human State of Nature
Azar Gat
Abstract The chapter addresses the causes of fighting among hunter-gatherers,
whose way of life represents 99.5% of the history of the genus Homo and about 90%
of that of Homo sapiens sapiens. Based on anthropological observations on the
behavior of extant and recently extinct hunter-gatherer societies, compared with
animal behavior, the chapter begins with somatic and reproductive causes. It
proceeds to demonstrate that other motives, such as dominance, revenge, the
security dilemma, and “pugnacity,” originally arose from the somatic and reproductive competition. Rather than being separate, all motives come together in an
integrated motivational complex, shaped by the logic of evolution and natural
selection.
9.1
Introduction
In contrast to long-held Rousseauite beliefs that reached their zenith in the 1960s
with the writings of Konrad Lorenz (1966) and Niko Tinbergen (1968), widespread
deadly violence within species including humans (Keeley 1996, Gat 1999, 2006;
LeBlanc and Register 2003) has been found to be the norm in nature. What are the
evolutionary rewards that can make this highly dangerous activity worthwhile?
This question relates to the age-old philosophical and psychological inquiry into the
nature of the basic human system of motivation. Numerous lists of basic needs and
desires have been put together over the centuries (Hobbes Leviathan, Chap. 6,
Maslow 1970; Burton 1990), but in the absence of an evolutionary perspective,
they have always had something arbitrary and trivial about them. They lacked
A. Gat
Department of Political Science, Tel Aviv University, Tel Aviv, Israel
e mail: Azargat@post.tau.ac.il
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 9, # Springer Verlag Berlin Heidelberg 2010
197
198
A. Gat
a unifying regulatory rationale that would suggest the reason why the various needs
and desires came into being, or the way in which they related to one another.
Arguing that the human motivational system as a whole should be approached from
the evolutionary perspective, I examine what can be meaningfully referred to as the
“human state of nature”, the 99.5% of the genus Homo’s evolutionary history in
which humans lived in small kin groups as hunter-gatherers. In this “state of
nature,” people’s behavior patterns are generally to be considered as having been
evolutionarily adaptive. They form the biological inheritance that we later carried
with us throughout history, when this inheritance constantly interacted with our
staggering cultural development.
Although I shall survey the reasons for warfare among hunter-gatherers one by
one, it is not my intention to provide yet another “list” of elements. Instead, I seek to
show how the various reasons come together in an integrated motivational complex,
shaped by the logic of evolution and natural selection for billions of years, including
the two million-year history of our genus, Homo, and the tens of thousands of years
of our species, H. sapiens sapiens. The aspects discussed include the pursuit of
subsistence resources and reproduction as ultimate causes, and behaviors relating to
dominance, revenge, the security dilemma, the supernatural, and playfulness, as
proximate and subordinate causes that arise from the first.
9.2
Subsistence Resources
Competition over resources is a prime cause of aggression and deadly violence
among humans, as in other animals. The reason for this is that food, water, and, to a
lesser degree, shelter against the elements are tremendous selection forces. As
Darwin, following Malthus, explained, living organisms, including humans, tend
to propagate rapidly. Their numbers are checked only by the limited resources of
their particular ecological habitats and by all sorts of competitors, such as conspecifics, animals of other species which have similar consumption patterns, predators, parasites, and pathogens.
When their environments suddenly expand, an unusual event in nature, demographic growth is dramatic. One of the best known examples is the rapid proliferation of Old World wildlife into new territories in the wake of the European age of
discovery. Humans propagated equally dramatically in similar circumstances. As
recently as several tens of thousands of years ago, the small groups that crossed over
from Asia into North America propagated into hundreds of thousands and millions
of people, filling up the Americas. Similarly, the small “founder groups” that
arrived in the Pacific islands during the last two millennia, in most cases no more
than a few dozens of people landing on each island, rapidly filled up their new
habitats, increasing in numbers to thousands and tens of thousands.
Such dramatic “breaking of the barriers” was rare, however. Contrary to the
Rousseauite imagination, humans, and animals, did not live in a state of primordial
plenty. Even in lush environments, plenty is a misleading notion, for it is relative,
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199
first, to the number of mouths that have to be fed. The more resource-rich a region
is, the more people it attracts from outside, and the greater the internal population
growth that takes place. As Malthus pointed out, a new equilibrium between
resource volume and population size would eventually be reached, recreating the
same tenuous ratio of subsistence that was the fate of most preindustrial societies
throughout history. The concept of “territoriality,” which became popular in the
1960s (Ardrey 1966; Lorenz 1966; Tinbergen 1968), should be more subtly defined
in this light. Among hunter-gatherers, territories varied dramatically in size
territorial behavior itself gained or lost in significance in direct relation to the
resources and resource competition. The same applies to high population density,
another popular explanation in the 1960s for violence. Except in the most extreme
cases, it is mainly in relation to resource scarcity that population density functions as
a trigger for fighting. Otherwise, Tokyo and the Netherlands would have been
among the most violent places on earth (Durham 1976; Dyson-Hudson and Smith
1978; Mueller 1983; Huntingford and Turner 1987; de Waal 1996).
Competition over resources existed in most hunter-gatherer cultures and sometimes escalated into conflict, mostly among, but occasionally also within, groups.
This competition was largely about nourishment, the basic and most critical somatic
activity of all living creatures, which often causes dramatic fluctuations in their
numbers. Resource competition, and conflict, is not, however, a given quantity but a
highly modulated variable. Resource competition and conflict change over time and
place in relation to the varying nature of the resources available and of human
population patterns in diverse ecological habitats. The basic question, then, is what
the main scarcities, stresses, and hence, objects of human competition, are in any
particular circumstances.
In extreme cases, such as the mid-Canadian arctic, where resources were
highly diffused and human population density was very low, resource competition and conflict barely existed. In arid and semi-arid environments, like those
of Central Australia, where human population density was also very low, water
holes were often the main cause of resource competition and conflict. They were
critical in times of drought, when whole groups of Aborigines are recorded to
have perished. For this reason, however, there was a tendency to control them,
also violently, even when stress was less pressing (Meggitt 1965b, p 42). In wellwatered environments, where there was no water shortage and hence, no water
competition, food often became the chief cause of resource competition and
conflict, especially in times of stress, and also in expectation of and preparation
for stress (Ember and Ember 1992: 242 262; also Hamilton 1975: 146). As
Lournados (1997: 33) writes with respect to Aboriginal Australia: “In southwestern
Victoria, competition between groups involved a wide range of natural resources,
including territory, and is recorded by many early European observers throughout
Victoria.” Lournados’s next sentence shows that his “competition” also includes
“combat.”
The nature of the food in question varied with the environment. Still, it was
predominantly meat of all sorts that was hotly contested among hunter-gatherers.
This fact, which is simply a consequence of nutritional value, is discernible
200
A. Gat
throughout nature. Herbivores rarely fight over food, for the nutritious value of
grass is too low for effective monopolization. Fruit, roots, seeds, and some plants
that are considerably more nutritious than grass are often the object of competition
and fighting, both among animals and humans. Meat, however, represents the most
concentrated nutritional value in nature and is the object of the most intense
resource competition: hence, the inherent state of competition and conflict found
between Stone Age human hunters.
Let us understand more closely the evolutionary calculus that can make the
highly dangerous activity of fighting over resources worthwhile. In our affluent
societies, it might be difficult to comprehend how precarious people’s subsistence
in premodern societies was (and still is). The specter of hunger and starvation was
ever-present. Affecting both mortality and reproduction, they constantly trimmed
down population numbers. Thus, struggle over resources was very often evolutionarily cost-effective. The benefits of fighting also had to be matched against possible
alternatives (other than starvation). One of them was to move elsewhere. This, of
course, often happened, especially if one’s enemy was much stronger, but this
strategy had clear limitations. By and large, there were no “empty spaces” for
people to move to. In the first place, the quality of space is not uniform, and the best,
most productive habitats were normally already taken. One could be forced out to
less hospitable environments, which may also be already populated by other less
fortunate people. Indeed, finding empty niches required exploration, which again
might involve violent encounters with other human groups. Furthermore, a move
meant leaving a habitat with whose resources and dangers the group’s members
were intimately familiar, and traveling into uncharted environments. Such a change
could involve heavy costs. Moreover, giving in to pressure from outside might
establish a pattern of victimization. Encouraged by its success, the alien group
might repeat and even increase its pressure. A strategy of conflict concerns not only
the object presently in dispute but also the whole pattern of future relations.
Standing for one’s own might, in fact, mean lessening the occurrence of conflict
in the future. No less, and perhaps more, than actual fighting, conflict is about
deterrence.
Having discussed the possible benefits and alternatives of fighting, deterrence
brings us to the costs side. Conflict becomes an evolutionarily more attractive
strategy for those who resort the it to lower their risk of incurring serious bodily
harm and death. Consequently, displays of strength and threats of aggressive
behavior are the most widely used weapons in conflict, both among animals and
humans. Furthermore, when humans, and animals, do resort to deadly violence, they
mostly do so under conditions in which the odds are greatly tilted in their favor
(Crofoot and Wrangham, this volume). Among animals, it is mostly the defenseless
young, chicks and eggs that fall victim to deadly violence, whereas adult animals
are very cautious of fighting to the finish with their peers for fear of self-injury.
Among hunter-gatherer and other prestate societies, it is not the open-pitched battle
but the raid and ambush that constitute the principal and, by far, the most lethal
form of warfare. Asymmetrical fighting is the norm in nature, including the human
state of nature (Gat 1999).
9 Why War?
9.3
201
Reproduction
The struggle for reproduction is largely about access to sexual partners. There is a
fundamental asymmetry between males and females in this respect, which runs
through most of nature. At any point in time, a female can be fertilized only once.
Consequently, evolutionarily speaking, she must take care to make the best of it. It
is quality rather than quantity that she seeks. She must select the male who looks
the best equipped for survival and reproduction, so that he will impart his genes,
and his qualities, to the offspring. In those species, like the human, where the male
also contributes to the raising of the offspring, his skills as a provider and his
loyalty are other crucial considerations. In contrast to the female, there is theoretically almost no limit to the number of offspring a male can produce. He can
fertilize an indefinite number of females, thus multiplying his own genes in the
next generations. The main brake on male sexual success is competition from
other males.
All this, of course, is only an abstract, around which sexual strategies in nature
are highly diverse (Symons 1979; Daly and Wilson 1983; Ridley 1994; Buss and
Malamuth 1996). Some species are highly polygynous; yet access to females can be
more evenly spread, all the way down to pair-bonding. However, although pairbonding reduces, it by no means terminates, male competition. In pair-bonding
systems, the quality of the female partner also gains significance. If the male is
restricted to one partner, it becomes highly important for him as well to choose the
partner with the best reproductive qualities he can get: young, healthy, and optimally built for bearing offspring; that is, in sexual parlance, the most attractive
female.
The need to take care of very slowly maturing offspring, which required
sustained investment by both parents, turned humans towards pair-bonding. However, competition over the best female partners remains. Furthermore, humans, and
men in particular, are not strictly monogamous. In the first place, males tend to have
more than one wife when they can. Only a minority can, however. Although in most
known human societies polygyny was legitimate, only a select few well-to-do men
were able to support, and thus have, the extra wives and children. Second, in
addition to official or unofficial wives, men tend to search for extramarital sexual
liaisons with other women, married or unmarried.
How does all this affect human violent conflict and fighting? The evidence
across the range of hunter-gatherer peoples tells the same story. Within the tribal
groupings, women-related quarrels and violence were rife, often constituting the
principal category of violence, and occasionally escalating to blood feuds and
homicide. Incidents were caused by competition among suitors, by women’s
abduction and forced sex, by broken promises of marriage, and by jealous husbands
suspicious of their wives’ fidelity. Between groups, the picture was not very
different and was equally uniform (but see Chapais, this volume). Warfare regularly
involved the stealing of women, who were then subjected to multiple rape, or taken
for marriage, or both.
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So, hunter-gatherer fighting commonly involved the stealing and raping of
women, but was this the cause or a side effect of hunter-gatherer fighting? In recent
anthropological literature, this question has been posed by R.B. Ferguson in respect
to Yanomamo warfare. Ferguson (1995: 355 358), who holds that warfare is
caused by material reasons alone, has disputed Napoleon Chagnon’s claim that
the Yanomamo fought primarily for women. Chagnon (1977: 123, 146), for his
part, dismissed the materialist position, enlisting the testimony of Yanomamo men
who had told him amused: “Even though we like meat, we like women a whole lot
more!” However, even Chagnon wavered on occasions on whether Yanomamo
warfare was really about women.
The Yanomamo are hunters and horticulturalists rather than pure hunter-gatherers.
However, the fundamental question in dispute is relevant to pure hunter-gatherers as well. As argued here, this is a pointless question that has repeatedly led
anthropologists to a dead end. It artificially takes out and isolates one element
from the wholeness of the human motivational complex that may lead to
warfare, losing sight of the overall rationale that underpins these elements. It
is as if one were to ask what people are really after when they go to the
supermarket: meat, bread, or milk (Ferguson 2000; Gat 2000). Both somatic and
reproductive elements are present in humans; moreover, both these elements are
intimately interconnected, for people must feed, find shelter, and protect themselves in order to reproduce successfully. Conflict over resources was at least
partly conflict over the ability to acquire and support women and children, and
to demonstrate that ability in advance, in order to rank worthy of the extra
wives. Brian Hayden (1994) has advanced an anthropological model whereby
simple resources in resource-rich societies are accumulated and converted to
luxury items in an intensified competition for status, prestige, and power (see
Plourde and Henrich, this volume). He could add women to the list of converted
goods. Thus, competition over women can lead to warfare indirectly as well as
directly. As with mass and energy in Einstein’s equations, resources, reproduction, and, as we shall see, status, are interconnected and interchangeable in the
evolution-shaped complex that motivates people. Motives are mixed, interacting,
and widely refracted. Nonetheless, it is the purpose of this chapter to show that
this seemingly immense complexity and inexhaustible diversity can be traced
back to a central core, shaped by the evolutionary rationale.
Wealth, status, matrimonial success, and power were interconnected among the
“big men” of northern Australia (Hart and Pilling 1964: 18, 50). The same pattern
applied to the “big men” (umialik) of the Eskimo hunter-gatherers of the Alaskan
coast: “If he [an umialik] had more than one wife, his ties of blood and marriage
were greater than those of others, and he could depend on many persons for support.
Furthermore, by being an umialik he was a person whose opinions the others
respected” (Oswalt 1967, p 178; also Burch 1974, p 6). A positive feedback
mechanism was in operation. Chagnon (1979) has shown one way this mechanism
worked with the Yanomamo, and Keen (1988: 290) has independently detected the
same pattern among the Australian hunter-gatherers. The largest clans in a tribe,
those comprising more siblings and cousins, acted on the principle of kin solidarity
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vis-à-vis the rest of the tribe. They moved on to increase their advantage by
controlling the leadership positions, resources, and marriage opportunities at the
expense of the others. As a result, large clans tended to dominate a tribe, politically
and demographically, over time. The notion that there is a self- and mutually
reinforcing tendency which works in favor of the rich, mighty, and successful,
facilitating their access to the “good things of life,” goes back a long way.
Polygyny was a significant factor in many hunter-gatherer societies. Australia
constitutes our best laboratory. Its size, near complete isolation, and ecological
diversity make it far superior to other, more recently studied and more publicized
cases that are mostly confined to arid environments. Polygyny was legitimate
among all the Aborigines tribes of Australia and highly desired by the men.
However, comparative studies among the tribes show that men with only one
wife comprised the largest category among married men, often the majority. Men
with two wives comprised the second largest category. The percentage of men with
three or more wives fell sharply, to around 10 15% of all married men, with the
figures declining with every extra wife (Meggitt 1965a; Long 1970). To how many
wives could the most successful men aspire? There was a significant environmental
variation here. In the arid Central Desert, four, five, or six wives were the top. Five
or six was also the top figure mentioned by Buckley for the Aborigines living in the
region of Fort Philip (Melbourne) in the south-east in the early nineteenth century.
However, in the more rich and productive parts of Arnhem Land and nearby islands
in the north, a few men could have as many as 10 12 wives, and in some places, in
the most extreme cases, even double that number. There was a direct correlation
between resource density, resource accumulation and monopolization, social ranking, and polygyny (Berndt and Berndt 1964: 172; Hart and Pilling 1964, pp 17 18,
50; Meggitt 1965b, pp 78, 80 81; Morgan 1980, p 58; Keen 1982; Lournados 1988,
p 151 152).
Data from other hunter-gatherer societies reveal a similar picture. Resource
scarcity reduced social differentiation, including in marriage, but did not eliminate
it. The leaders of the Aka Pygmies were found to be more than twice as polygynous
as ordinary people, and to father more children (Betzig et al. 1991, p 410). Among
the !Kung of the arid Kalahari Desert, polygyny was limited, but 5% of married
men still had two wives (Daly and Wilson 1988: 285). Women-related feuds were
the main cause of homicide among them. In the extremely harsh conditions of the
mid-Canadian arctic, where resources were scarce and diffused, fighting over
resources barely existed. Because of the resource scarcity, marriages among the
native Eskimo were also predominantly monogamous. One study registered only
three polygynies out of 61 marriages. Still, wife-stealing was a widespread, probably the main, cause of homicide and “blood feuds” among the Eskimos (Betzig
et al. 1991). “A stranger in the camp, particularly if he was traveling with his wife,
could become easy prey to the local people. He might be killed by any camp fellow
in need of a woman” (Daly and Wilson 1988, p 222; citing Balikci 1970, p 182).
Among the Eskimos of the more densely populated Alaskan Coast, abduction of
women was a principal cause of warfare. Polygyny, too, was more common among
them, although restricted to the few (Oswalt 1967, pp 178, 180, 182, 185, 187, 204;
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A. Gat
Burch and Correll 1972, p 33; Dickemann 1979, p 363; Symons 1979, p 152;
Nelson 1983, p 292, 327 329; Irwin 1990, pp 201 202). Strong Ingalik (“big men”)
often had a second wife, and “there was a fellow who had five wives at one time and
seven at another. This man was a great fighter and had obtained his women by
raiding” (Betzig et al. 1991, p 410).
The resource-rich environment of the Northwest Coast accentuated resource
competition and social ranking. Conflict over resources was therefore intense.
However, resource competition was not disassociated from reproduction, but constituted, in fact, an integral whole with it. Women are not even mentioned in
R. B. Ferguson’s elaborate materialist study of Northwest Coast Indian Warfare
(1984). Nonetheless, they were there. Most natives of the Northwest Coast were
monogamous. However, the rich, strong, and powerful were mostly polygynous.
The number of wives varied from tribe to tribe, but “a number” or “several” is
normally quoted, and up to 20 wives are mentioned in one case. The household of
such successful men is repeatedly described as having been very substantial and
impressive indeed. Furthermore, as is universally the case, the mainly female slaves
taken in the raids and working for their captors also shared their masters’ bed
(Drucker 1951, p 301, 1965, p 54; Krause 1970, p 154; Rosman and Rubel 1971,
pp 16 17, 32, 110; Donald 1997, p 73).
Naturally, the increase in the number of a man’s wives generally correlated with
his reproduction rate (number of children). Statistics for hunter-gatherers, beyond
those already cited, are scarce, and most of the following derives from simple
horticulturalists who may have had more impressive reproductive skews. Among
the Xavante horticulturalists of Brazil, for example, 16 of the 37 adult males in one
village (74 out of 184 according to a larger survey) had more than one wife. The
chief had five, more than any other man. He fathered 23 surviving offspring who
constituted 25% of the surviving offspring in that generation. Shinbone, a most
successful man among the Yanomamo of the Orinoco basin, had 43 children. His
brothers were also highly successful, so Shinbone’s father had 14 children, 143
grandchildren, 335 great grandchildren, and 401 great-great grandchildren, at the
time of the research (Chagnon 1979; Symons 1979, p 143; Daly and Wilson 1983,
pp 88 89, 332 333). Again, women are such a prominent motive for competition
and conflict because reproductive opportunities are a very strong selection force
indeed.
To be sure, this does not mean that people always want to maximize the number
of their children. Although there is some human desire for children per se and
a great attachment to them follows once they are born, it is mainly the desire for
sex Malthus’s “passion” which functions in nature as the powerful biological
proximate mechanism for maximizing reproduction. As humans, and other living
creatures, normally engage in sex throughout their fertile lives, they have a vast
reproductive potential, which, before the introduction of effective contraception,
mainly depended on resource availability for its realization.
Polygyny (and female infanticide) created a scarcity of women and increased
men’s competition for, and conflict over, them (Divale and Harris 1976).
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In conjunction with the other motives surveyed here, this was a major reason for the
high violent mortality rate among hunter-gatherers. Among Aboriginal Australian
tribes, about 30% of the Murngin adult males are estimated to have died violently,
and similar findings have been recorded for the Tiwi. The Plains Indians showed a
deficit of 50% for the adult males in the Blackfoot tribe in 1805 and 33% deficit in
1858 (but by the nineteenth century, they already possessed guns and horses), while
during the reservation period the sex ratio rapidly approached 50 50. Among the
Eskimos of the central Canadian arctic, where group warfare was practically
nonexistent, the rate of violent deaths, in the so-called “blood feuds” and “homicide,” was estimated at one per 1,000 persons per year, ten times the 1990 US rate
which is the highest in the developed world. Among the !Kung of the Kalahari
Desert, dubbed the “harmless people,” there were 22 cases of homicide in the
period of study, 1963 1969; 19 of the victims were males, as were all of the 25
killers. This amounts to a rate of 0.29 person per 1,000 per year, and had been 0.42
before the coming of state authority, 3 4 times higher than the 1990 US rate
(Gat 2006, pp 129 132, for references to this and the following paragraphs).
The data for prestate agriculturalists is basically the same. Among the Yanomamo,
about 15% of the adults died as a result of inter and intragroup violence: 24% of
the males and 7% of the females. The Waorani (Auca) of the Ecuadorian Amazon
hold the registered world record: more than 60% of adult deaths were caused by
feuding and warfare. Among the many peoples in Highland New Guinea, violent
mortality estimates are very similar: among the Dani, 28.5% of the men and 2.4% of
the women; among the Enga, 34.8% of the adult males; among the Goilala, whose
total population was barely over 150, there were 29 (predominantly men) killed
during a period of 35 years; among the Lowland Gebusi, 35.2% of the adult males
and 29.3% of the adult females. Archeology unearths similar findings. In the
Neolithic site of Madisonville, Ohio, 22% of the adult male skulls had wounds
and 8% were fractured.
Another consequence of sexual deprivation in young adult males is their marked
restlessness, risk-taking behavior, and belligerency. Young adult males are genetically inclined to greater risk-taking, for their matrimonial status-quo is highly
unsatisfactory. They still have to conquer their place in life. Thus, they have always
been the most natural recruits for violent action and war. Male murder rates peak in
both London and Detroit although 40 times higher in the latter at the age of 25
(Daly and Wilson 1983, pp 92 97, 297 301; Jones 1993, p 92).
The interconnected competition over resources and reproduction is the root
cause of conflict and fighting in humans, as in all other animal species. Other
causes and expressions of fighting in nature, and the motivational and emotional
mechanisms associated with them, are derivative of, and subordinate to, these
primary causes, and originally evolved this way in humans as well. This, of course,
does not make them any less “real” but only explains their function in the evolutionshaped motivational complex, and, thus, how they came into being. It is to these
“second-level” causes and motivational mechanisms, directly linked to the first, that
we now turn.
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Dominance: Rank, Power, Status, Prestige
Among social animals, possessing higher rank in the group promises one a greater
share in the communal resources, such as hunting spoils, and better reproductive
opportunities. While there is considerable diversity among species, rank is hotly
contested for that reason. It is the strong, fierce, and among our sophisticated
cousins, the chimpanzees also the “politically” astute, that win status by the actual
and implied use of force. Rivalry for rank and domination in nature is, then, a
proximate means in the competition over resources and reproduction (Watts, this
volume).
In determining one’s status, image and perception have always been as important as tangible reality. Thus, both overt and subtler displays of worth are a constant
human activity. It is limited only by the desire to avoid the provocation of a
negative social response, because other people as well jealously guard their honor
in the social competition for esteem. In traditional societies in particular, people
were predisposed to go to great lengths in defense of their honor. The slightest
offense could provoke violence. Where no strong centralized authority existed,
one’s honor was a social commodity of vital significance, affecting both somatic
and reproductive prospects.
Does this mean that what people who strive for leadership or esteem “really”
want is sexual opportunity or resources? Not necessarily. Wanting is subjective, and
mentally it can be genuinely disassociated from ultimate evolutionary aims. For
instance, people widely desire love and sex for their own sake rather than for the
resulting offspring, whom they often positively, and even desperately, do not want.
In the same way, the pursuit of rank and esteem in humans, as with animals, was
closely associated with better somatic and reproductive prospects, and evolved as a
proximate means for achieving them, even though the evolutionary aim often
lacked conscious expression. Again, to remove all too prevalent misunderstandings
regarding the evolutionary rationale, the argument, of course, is not that these
behavior patterns are a matter of conscious decision and complex calculation
conducted by flies, mice, lions, or even humans. It is simply that those who failed
to behave adaptively became decreasingly represented in the next generations, and
their maladaptive genes, responsible for their maladaptive behavior, were consequently selected against. The most complex structural engineering and behavior
patterns have thus evolved in, and program, even the simplest organisms, including
those lacking any consciousness (Dawkins [1976], 1989, pp 96, 291 292).
As with competition over women, competition over rank and esteem could lead
to violent conflict indirectly as well as directly. For instance, even in the simplest
societies people desired ornamental, ostentatious, and prestige goods. Although
these goods are sometimes lumped together with subsistence goods, their social
function and significance are entirely different. Body and clothes ornamentation are
designed to enhance physically desirable features that function everywhere in
nature as cues for health, vigor, youth, and fertility (Darwin [1871], 1962, pp
467 468, Low 1979, pp 462 487, Diamond 1992: Chap. 9). For example, articial
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coloring is used to enhance eye, lip, hair, and skin color; natural and by extension,
added symmetrical, orderly, and refined features signal good genes, good nourishment, and high-quality physical design; tall and magnificent headgear enhances
one’s size. It is precisely on these products of the “illusions industry” cosmetics,
fashion, and jewellery that people everywhere spend so much money. Furthermore, where some ornaments are scarce and therefore precious, the very fact that
one is able to afford them indicates wealth and success: hence, the source of what
economist Thorstein Veblen, referring to early twentieth century American society,
called “conspicuous consumption.” In Stone Age societies as well, luxury goods,
as well as the ostentatious consumption of ordinary ones, became in themselves
objects of desire as symbols of social status. For this reason, people may fight
for them.
Indeed, plenty and scarcity are relative not only to the number of mouths to be
fed but also to the potentially ever-expanding and insatiable range of human needs
and desires. Human competition increases with abundance
as well as with
deficiency taking more complex forms and expressions, widening social gaps,
and enhancing stratification. While the consumption capacity of simple, subsistence, products is inherently limited, that of more refined, lucrative ones is practically open-ended. One can simply move up the market.
9.5
Revenge: Retaliation to Eliminate and Deter
Revenge is one of the major causes of fighting cited in anthropological accounts of
prestate societies. Violence was activated to avenge injuries to honor, property,
women, and kin. If life was taken, revenge reached its peak, often leading to a
vicious circle of death and counter-death.
How is this most prevalent, risky, and often bloody behavior pattern to be
explained? From the evolutionary perspective, revenge is retaliation that is intended
either to destroy an enemy or to foster deterrence against him, as well as against
other potential rivals. This, of course, applies to nonphysical and nonviolent, as well
as to physical and violent action. If one does not pay back on an injury, one may
signal weakness and expose oneself to further injuries not only from the original
offender but also from others. A process of victimization might be created. I suspect
that experts would be able tell us that a similar behavioral pattern occurs, if only
rudimentarily, within other social species (Aureli et al. 1992). All the same, humans
have far longer memories than do animals, and, thus, revenge the social settling of
accounts with those who offended them assumes a wholly new level with them.
Of course, depending on one’s overall assessment of the stakes and relative balance
of power and if the challenger is much stronger than oneself, it is equally common
for one to accept in silence an injury and the consequences of reduced status. This
rationale applies wherever there is no higher authority that can be relied upon for
protection, that is, in the so-called anarchic systems. In modern societies, it thus
applies to the wide spheres of social relations in which the state or other
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authoritative bodies do not intervene. In prestate societies, however, it applied far
more widely to the basic protection of life and property.
But are people not driven to revenge by blind rage rather than by calculation? I
raise this typical question only in order to reiterate the point which is all too often
misunderstood with respect to the evolutionary rationale. Basic emotions evolved,
and are tuned the way they are, in response to very long periods of adaptive
selective pressures (Fessler and Gervais, this volume). They are proximate mechanisms in the service of somatic and reproductive purposes. To work, they do not need
to be conscious, and the vast majority of them indeed are not in humans let alone
in animals. Thus, the instinctive desire to strike back is a basic emotional response
which evolved precisely because those who struck back of course, within the
limits mentioned above were generally more successful in protecting their own.
Indeed, this rationale is remarkably supported by the famous computerized game
that found tit-for-tat the most effective strategy a player can adopt (Axelrod 1984).
Tit-for-tat poses a problem. One’s offender cannot always be eliminated. Furthermore, the offender has kin who will avenge him, and it is even more difficult to
eliminate them as well. In many cases, tit-for-tat becomes a negative loop of
retaliation and counter-retaliation from which it is very hard to exit. One original
offense may produce a pattern of prolonged hostility. Thus, retaliation might
produce escalation rather than annihilation or deterrence. In such cases, fighting
seems to feed on, and perpetuate, itself, bearing a wholly disproportional relation to
its “original” cause. People become locked into conflict against their wishes and
best interests. It is this factor that has always given warfare an irrational appearance
that seems to defy a purely utilitarian explanation.
How can this puzzle be explained? In the first place, it must again be stressed that
both the original offense and the act of retaliation arise from a fundamental state of
interhuman competition that carries the potential of conflict, and is consequently
fraught with suspicion and insecurity. Without this basic state of somatic and
reproductive competition and potential conflict, retaliation as a behavior pattern
would not have evolved. Indeed, sometimes revenge is merely a pretext for conflict
emanating from more fundamental reasons. However, while explaining the root
cause of retaliation, this does not in itself account for retaliation’s escalation into
what often seems to be a self-defeating cycle. A prisoner’s dilemma-like situation is
responsible for the emergence of such cycles. In the absence of an authority that can
enforce mutually beneficial cooperation on people, or at least minimize their
damages, the cycle of retaliation is often their only rational option, though, exposing them to very heavy costs, is not their best option.
Like any game, the prisoner’s dilemma is predicated on its assumptions. It has
proven so fruitful because it has been found that many situations in real life exhibit
elements of the dilemma. Indeed, the prisoner’s dilemma is of great relevance when
explaining the war complex as a whole and not only that of revenge and retribution.
Still, it ought to be emphasized that not all violent conflicts or acts of revenge fall
under the special terms of the prisoner’s dilemma. In the context of a fundamental
resource scarcity, if one is able to eliminate, decisively weaken, or subdue the
enemy, and consequently reap most of the benefits, then this strategy is better for
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one’s interests than a compromise. It is only when such a decisive result cannot be
achieved that conditions similar to those specified by the prisoner’s dilemma come
into play.
9.6
Power and the Security Dilemma
Revenge or retaliation is an active reaction to an injury, emanating from a competitive and, hence, potentially conflictual basic state of relations. However, as Hobbes
saw (Leviathan, Chap. 13), the basic condition of competition and potential conflict, which gives rise to endemic suspicion and insecurity, invites not only reactive
but also preemptive response, which further magnifies mutual suspicion and insecurity. It must be stressed that the source of the potential conflict here is again of a
“second level.” It does not necessarily arise directly from an actual conflict over the
somatic and reproductive resources themselves, but from the fear, suspicion, and
insecurity that the potential of those “first-level” causes for conflict creates. Potential conflict can thus breed conflict. When the “other” must be regarded as a
potential enemy, his very existence poses a threat, for he might suddenly attack
one day. For this reason, one must take precautions and increase one’s strength as
much as possible. The other side faces a similar security problem and takes similar
precautions.
Things do not stop with precautionary and defensive measures, because such
measures often inherently possess some offensive potential, indirectly or directly.
Indirectly, a defended home base may have the effect of freeing one for offensive
action with less fear of a counter-strike it reduces mutual deterrence. Directly, a
defensive alliance, for example, may be transformed into an offensive one. Thus,
the measures that one takes to increase one’s security in an insecure world often
decrease another’s security and vice versa.
What are the consequences of this so-called “security dilemma”? (Herz 1950;
Jervis 1978). In the first place, it tends to escalate arms races. Arms races between
competitors take place throughout nature. Through natural selection, they produce
faster cheetahs and gazelles; deer with longer antlers to fight one another; more
devious parasites and viruses and more protected “hosts.” Many of these arms races
involve very heavy costs to the organisms, which would not have been necessary if
it were not for the competition. This, for example, is the reason why trees have
trunks. Trees incur the enormous cost involved in growing trunks only because of
their intense struggle to outgrow other trees in order to get sunlight. As with
humans, competition is most intense in environments of plenty, where more
competitors can play and more resources be accumulated. This is why trees grow
highest in the dense forests of the water-rich tropical and temperate climates.
Arms races often have paradoxical results. The continuous and escalating effort
to surpass one’s rival may prove successful, in which case the rival is destroyed or
severely weakened, and the victor reaps the benefits. However, in many cases,
every step on one side is matched by a counter-step on the other. Consequently,
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A. Gat
even though each side invests increasing resources in the conflict, neither gains an
advantage. This is called, after one of Alice’s puzzles in Lewis Carroll’s Through
the Looking-Glass, the “Red Queen effect”: both sides run faster and faster only to
find themselves remaining in the same place. Arms races may, thus, become a
prisoner’s dilemma. If the sides gave up the hope of outpacing each other and
winning the contest, they could at least save themselves the heavy costs incurred,
which anyway cancel each other out. However, they are often unable to stop the
race, because of suspicion, faulty communication, and inability to verify what
exactly the other side is doing.
Thus arms races are, in general, the natural outcome of competition. The special
feature of arm races created by the security dilemma is that their basic motivation
on both sides is defensive. Again, one way to stop the spiral is to find a means to
reduce mutual suspicion. Marriage ties used to be a standard measure for achieving
this aim in all premodern societies (Chapais, this volume). Fostering familiarity and
demonstrating good will through mutual friendly visits and ceremonial feasts were
other prominent universal measures. For all that, suspicion and insecurity are
difficult to overcome for the reasons already mentioned. Furthermore, even ostensibly friendly overtures sometimes turned out to be treacherous. However, there is
another way to reduce the insecurity. Although both sides in the security dilemma
may be motivated by defensive concerns, they may choose to actively preempt their
opponents; that is, take not only defensive precautions but attack in order to
eliminate or severely weaken the other side. Indeed, this option in itself makes
the other side even more insecure, making the security dilemma more acute.
Warfare can thus become a self-fulfilling prophecy. Since full security is difficult
to achieve, history demonstrates that constant warfare can be waged, conquest
carried afar, and power accumulated, all truly motivated by security concerns,
“for defense.” Of course, in reality motives are often mixed, with the security
motive coexisting with a quest for gain.
The basic condition of interhuman competition and potential conflict thus
creates “second-level” causes for warfare, arising from the first, such as the cycle
of revenge and the security dilemma. This does not mean that actual competition
over somatic and reproductive resources has to exist on every particular occasion
for the security dilemma to flare up. Still, it is the prospect of such competition that
stands behind the mutual insecurity, and the stronger the competition and potential
conflict, the more intense the security dilemma will grow.
9.7
World-View and the Supernatural
But what about the world of culture that after all is our most distinctive mark as
humans? Do not people kill and get killed for ideas and ideals? From the Stone Age
on, the spiritual life of human communities has been imbued with supernatural
beliefs, sacred cults and rituals, and the practice of magic. Here, the difference
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between humans and other animals is the most marked, even if rudimentary culture
forms already manifest themselves in primates and hominids. It should be noted,
however, that the capacity for culture itself evolved as a biological adaptation, and
its various forms undergo evolutionary selection, both biological and cultural.
The evolutionary status of religion is beyond our scope here. Like warfare,
religion is a complex phenomenon which is probably the result of several different
interacting factors. Some scholars believe religion to be detrimental for survival
and hold that it emerged as a “bug,” “parasite,” or “virus” on H. sapiens sapiens’
advanced intellectual “software” (Dawkins [1976], 1989, pp 189 201, 329 331,
2006; Bowker 1995; Boyer 2001). By contrast, functionalist theorists, from Emile
Durkheim on, have argued that religion’s main role was in fostering social cohesion, inter alia in war (Durkheim 1965; Ridley 1996: 189 193; Wilson 2002;
Hayden 2003). In evolutionary terms, this means that in those groups in which
common ritual and cult ceremonies were more intensive, social cooperation became
more habitual and more strongly legitimized, which probably translated into an
advantage in warfare.
But how did the hunter-gatherers’ supernatural beliefs and practices affect the
reasons for conflict and fighting? I argue that on the whole they added to,
sometimes accentuating, the reasons we have already discussed. The all-familiar
glory of the gods, let alone missionary quests, never appear as reasons for huntergatherers’ warfare. These will appear later in human cultural evolution. The
supernatural reason for fighting among hunter-gatherers most cited by anthropologists is fear and accusations of sorcery. It should be noted, however, that these
did not appear randomly, but were directed against people whom the victim of the
alleged sorcery felt had reasons to want to harm him. This, of course, does not
necessarily mean that they really did. It certainly does not mean that these people
actually did harm the victim by witchcraft. What it does mean is that competition,
potential conflict, animosity, and suspicion were conducive to fears and accusations of sorcery. To further clarify the point, it is not that these “imagined” fears
and accusations did not add to the occurrence of deadly violence beyond the “real”
or potentially “real” causes that underlie them. They certainly did. But, to a greater
degree than with the security dilemma, the paranoia here reflects the running amok
of real, or potentially real, fears and insecurity, thus further exacerbating and
escalating the war complex.
Supernatural elements sometimes came into play in connection with motives for
warfare other than fear and insecurity. For instance, trespassing was often regarded
in hunter-gatherer societies as an offense against a group’s sanctified territory. In
other cases, an act of sacrilege against the clan’s totem was regarded as an insult to
the clan itself. In both these instances, the supernatural element functioned as a
sanctified symbol of less imagined goods: resources and honor. The totem was thus
like an emblem or flag. Of course, in some cases, supernatural reasons were evoked
as mere pretexts for other motives. However, even when they were not, the
supernatural elements added an extra dimension to existing motives, taken from
the realm of the spiritual and sanctified.
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9.8
A. Gat
Playfulness, Adventurism, Ecstasy
For all that we have said about the evolution-shaped aims of warfare, do not people
sometimes fight for no particular purpose, just for the fun of it, as a game, an outlet,
arising from sheer pugnacity?
Playing and sports have often been regarded indeed, defined as purposeless,
expressive, pure fun activity. What is its evolutionary logic? After all, it is an
activity that consumes a great deal of energy for no apparent gain. In reality,
though, its purpose is physical exercise and behavioral training for the tasks of
life, such as hunting, escaping predators and natural dangers, fighting, nurturing,
and social cooperation in all these. For this reason, in all mammalian species
(distinctive for their learning ability and playing activity), it is the young who
exhibit the most active and enthusiastic play behavior, compared with the more
mature and experienced (Fagan 1981; Smith 1984; Huntingford and Turner 1987:
198 200). Since adaptive behaviors are normally encouraged by emotional gratifications, play and sport are generally enjoyable.
So, games and sports serve, among other things, as preparation for fighting. In
this light, fighting may even be perpetrated in rare cases as playful training for more
serious fighting. However, is fighting sometimes not perpetrated only for evoking
the sort of emotional gratifications associated with play or sport behavior? Do
emotional gratifications sometimes not take on an end of their own in perpetrating
fighting? I claim that they do, but as an extension rather than a negation of the
evolutionary logic.
In the first place, it should be borne in mind that even wholly playful or
“expressive” fighting behavior developed within a general evolutionary context in
which conflict was normal and fighting a distinct possibility and, therefore, a deeply
rooted behavior pattern. In this respect, wholly “purposeless” violence is a “misplaced” or “misactivated” expression of a “normal,” evolution-shaped behavior.
We shall return to this in a moment. Second, as with respect to accusations of
sorcery, it should be noted that even seemingly purposeless violence is not purely
random. It is much more often directed against aliens or competitors than against
perceived friends. Thus again, it is often an extension of, or over-reaction to, a state
of competition and potential conflict.
Still, allowing that some “purposeless,” “expressive” violence does exist, at least
marginally, what does it mean to describe such behavior as “misplaced,” or “misactivated”? Surely, the intention is not to pass any sort of value judgment. Rather,
the terms describe behavior which, while having an evolutionary root, is expressed
out of its evolutionarily “designed” context, and thus is typically also maladaptive.
But if so, how does it survive? In reality, maladaptive traits are constantly selected
against. For this reason, their prevalence remains marginal. Still, they do exist. It is
not only that natural selection is perpetual because of mutations, the unique gene
recombination that occurs with every new individual, and changing environmental
conditions; the main reason is that no mechanism, whether purposefully designed
by humans or blindly by natural selection, is ever perfect, 100% efficient, or fully
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Why War?
213
tuned. Like any other design, the products of natural selection, for all their marvels,
vary greatly in their level of sophistication, have limitations, flaws, and “bugs,” can
only operate in a proximate manner, and are, thus, far from optimal. The only
requirement they are bound to meet is that they are good enough to survive in a
given environment and facing given competitive challenges. The emotional
mechanisms controlling violence have all the above limitations. Thus, they can
be triggered or “misactivated” into “purposeless,” “expressive,” “spontaneous,” or
“misdirected” violence. However, like overeating or sleeplessness to give more
familiar examples such behavior should be understood as a range of deviation
from an evolutionarily shaped norm.
Ecstatic behavior is another case in point. Ecstasy is a feeling of elation and
transcendence produced by an increasing flow of hormones such as adrenaline,
serotonin, and dopamine. It reduces body sensitivity to pain and fatigue, raises its
energy to a high pitch, and lowers normal inhibitions. In nature, ecstatic behavior
can be produced during extreme bodily exertion, often associated with struggle and
fighting. However, humans very early on found ways to arouse it artificially for the
feel-good effect itself, for instance, through rhythmic dance or by the use of
narcotic substances. In some cases, narcotic substances were consumed before
fighting and in preparation for it; a few shots of alcohol before an assault was
ordinary practice in most armies until not very long ago. However, in other cases,
the ecstatic condition itself can breed violence; again, drunkenness greatly contributes to the occurrence of violence in many societies. Furthermore, in some
cases, the sequence is reversed, with fighting entered into in order to produce
ecstatic sensations. For example, in addition to “ordinary” reasons, such as
money, females, social esteem, and so forth, this motivation plays a prominent
role often in conjunction with alcohol consumption in perpetrating “purposeless” youth gangs’ violence. Again, what we have here is a mostly maladaptive
outgrowth and deviation from an evolution-shaped behavioral pattern.
9.9
Cooperation in Fighting
Fighting in the human state of nature is carried out at the individual and group
levels. Cooperation in fighting takes place among family, clan, and tribe (regional
group) members. In principle, there are strong advantages to cooperation. In
warfare, for example, there is a strong advantage to group size (Crofoot and
Wrangham, this volume). However, the problem with cooperation throughout
nature is that one has a clear incentive to “free ride” reap the benefits of
cooperation while avoiding one’s share in the costs. Three or four different
mechanisms overlap to secure a measure of cooperation in hunter-gatherer groups.
First, as the theory of inclusive fitness predicts, people risk their lives in support
of close kin, with whom they share more genes. Family members tend to support
one another in disputes and clashes with members of other families. In interclan
rivalry, clans which are intermarried are likely to support one another against other
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A. Gat
clans. Companions for raids, the most common form of fighting among huntergatherers, come mainly from one’s family and clan. The members of regional
groups and confederation of regional groups, numbering in the hundreds and
more, are not as closely related as family and clan members, and yet (weaker)
cooperation among them takes place, particularly in conflict against alien regional
groups. In part, the same logic that, in J.B.S. Haldane’s famous formulation, makes
it evolutionarily beneficial to sacrifice one’s life in order to save more than two
siblings or eight cousins, and take risks at even lower ratios, holds true for 32
second cousins, 128 third cousins, or 512 fourth cousins. This, in fact, is pretty
much what a regional group is. Moreover, although not every member of the
regional group is a close kin of all the others, the regional group is a dense network
of close kinship through marriage ties (Chapais, this volume). Marriage links crisscross the regional group, making families and clans ready to take risks in support of
one another. Since most marriages take place within the regional group, there is a
wide gap between the “us” of the tribe and outsiders (Hamilton 1975, p 144;
Silverman 1987, p 113; Bowles 2006). Hunter-gatherers only felt safe to go
where they had kin.
Secondly, social cooperation can be sustained in groups that are intimate enough
to allow mutual surveillance and social accounting. If detected, a “free rider” faces
the danger of being excluded, “ostracized,” from the system of cooperation, which
is on the whole beneficial to him. People not only keep a very watchful eye for
“cheaters” and “defectors,” but in comparison with other animal species, they also
have very long memories. They would help other people on the assumption that
they would get similar help in return, but are likely to cease cooperating if the
expected return fails to arrive. This is the basis for the so-called “reciprocal
altruism” in human relations, which explains most of human seeming altruism
towards non-kin (Trivers 1971; Alexander 1987; Frank 1988; Ridley 1996). The
regional group is small enough to have dense kinship networks, as well as for all its
members to know one another, to be in contact with them, and to hold them to
account.
Thirdly, apart from biology, humans have culture, and are differentiated by their
cultures. This is a human universal that set humans far apart from other animals. As
culture, particularly among hunter-gatherers, was local and thus closely correlated
with kinship, cultural identity became a strong predictor of kinship (Irwin 1987,
p 131 156). Moreover, culture sharing is also crucial for human social cooperation.
Cooperation is dramatically more effective when cultural codes, above all language, are shared (Silk and Boyd, this volume). Like genes, culture changes over
time, only much faster. In Australia, for example, where the time depth of the
Aboriginal population measures in tens of thousands of years, lingual diversity
among the hundreds of regional groups or “dialect tribes” was great. There were
more than 200 different languages and even more dialects (Lournados 1997, p 38).
The tribal groupings, differing from their neighbors in their language and
customs, were thus the most effective frameworks of social cooperation for their
members. Outside them, people would find themselves in a great disadvantage.
Therefore, shared culture in a world of cultural diversity further increases the stake
9 Why War?
215
of a regional group’s members in their group’s survival. This factor may not have
been sufficiently recognized in the literature. The regional group is bound together
by mutually reinforcing and overlapping ties of kinship, social cooperation, and
cultural distinctiveness. Hence, the phenomenon of “ethnocentrism,” a human
universal that started at the level of the hunter-gatherer regional group and would
be expanded onto larger ethnic groupings later in history.
Fourthly, there is the contentious issue of group selection. Modern evolutionary
theory centers on individual or gene survival, with cooperation explained by the
principles of “kin selection” and “reciprocal altruism.” However, according to an
older view, first raised as a possibility by Darwin and now affecting a comeback,
biological selection takes place not only at the individual or gene level but also
among groups. A group which is biologically endowed with greater solidarity and
with individual willingness to sacrifice for the group would defeat less cohesive
groups. In rejection of this view, it used to be claimed that genes for self-sacrifice on
behalf of the group would have the effect of annihilating those who possessed them
much faster than aiding them through improved group survival, and that “cheaters”
would proliferate. However, a modulated multilevel selection, working through the
individual, family, and larger group levels, is supported by mathematical modeling
(Hamilton 1975; Levitt 1980; Wilson and Sober 1994, 1998; Hamilton 1996;
Wilson and Wilson 2007).
As can be inferred from Bowles (2006), one should guard against a sharp
empirical distinction between kin selection and group selection. For in reality,
throughout the vast majority of human evolutionary history, groups were anyhow
small kin-groups. The extended family group of a few dozens, the basic human
group, consisted of close kin. Even the regional group of a few hundreds consisted
of medium-range kin cris-crossed by marriage ties. Truly large societies of non- (or
remote-) kin emerged only very recently, with agriculture and civilization.
9.10
Conclusion: Fighting in the Evolutionary State of Nature
The hunter-gatherer way of life covers 99.5% of the history of the genus Homo, and
more than 90% of the history of the species H. sapiens. Agriculture and the state
are recent cultural inventions, starting in the most pioneering groups of our species
only some 10,000 and 5,000 years ago, respectively, and having little effect on the
human genome. Thus, to speak in a meaningful manner about human nature is to
address human adaptations to the human natural habitats, which are responsible for
the human biological inheritance.
Conflict and fighting in the human state of nature, as in the state of nature in
general, were fundamentally caused by competition. While violence is evoked and
suppressed by powerful emotional stimuli, it is not a primary, “irresistible” drive; it
is a highly tuned, both innate and optional, evolution-shaped tactic, turned on and
off in response to changes in the calculus of survival and reproduction. It can be
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A. Gat
activated by competition over scarce resources, as scarcity and competition are the
norm in nature because of the tendency of organisms to propagate rapidly when
resources are abundant. Deadly violence is also regularly activated in competition
over women, directly as well as indirectly, when men compete over resources in
order to be able to afford more women and children.
From these primary somatic and reproductive aims, other, proximate and derivative, “second-level,” aims arise. The social arbiters within the group can use their
position to reap somatic and reproductive advantages and hence the competition
for and conflict over esteem, prestige, power, and leadership, as proximate
goods. There are highly complex interactions at work here, which are, however,
underpinned by a simple evolutionary rationale. An offense or injury will often
prompt retaliation, lest it persists and turns into a pattern of victimization. Tit-fortat may end in victory or a compromise, but it may also escalate, developing into a
self-perpetuating cycle of strikes and counter-strikes, with the antagonists locked in
conflict in a sort of prisoner’s dilemma situation.
Similarly, in a state of potential conflict, security precautions are called for,
which may take on defensive but also offensive or preemptive character. The
security dilemma variant of the prisoner’s dilemma breeds arms races that may
produce an advantage to one side but often merely produces a “Red Queen” effect,
by which both sides escalate their resource investment only to find themselves in
the same position vis-á-vis one another. Organisms can cooperate, compete, or fight
to maximize their survival and reproduction. Sometimes, fighting is the most
promising choice for at least one of the sides. At other times, however, fighting,
while being their rational choice, is not their best one.
Competition and conflict are, thus, “real” in the sense that they arise from
genuine scarcities among evolution-shaped self-propagating organisms and can
end in vital gains for one and losses for the other. At the same time, they are
often also “inflated,” self-perpetuated, and mutually damaging, because of the logic
imposed on the antagonists by the conflict itself in an anarchic, unregulated
environment. In a way, this justifies both of the prevalent polarized attitudes to
war: the one that sees it as a serious business for serious aims and the other that is
shocked by its absurdity.
Finally, a few comments on the evolutionary perspective that underpins this
study. As our grand scientific theory for understanding nature, evolutionary theory
does not compete with scholarly constructs such as psychoanalytic theories in
explaining motivation; rather, evolutionary theory may encompass some of their
main insights within a comprehensive interpretative framework. For instance,
Freud, Jung, and Adler were divided over the elementary drive which each posited
as the underlying regulating principle for understanding human behavior. These
were respectively: sex; creativeness and the quest for meaning; and the craving for
superiority. All these drives, in fact, come together and interact within the framework of evolutionary theory, which also explains their otherwise mysterious origin.
Evolutionary theory explains how long-cited motives for fighting like William
Graham Sumner’s (1968: 212) hunger, love, vanity, and fear of superior powers
came into being and how they hang together and interconnect.
9
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217
Some readers may wonder why evolutionary theory should be presented here as
different from and superior to other scholarly approaches. Indeed, it is because
evolutionary theory is nature’s immanent principle rather than an artificial analytical construct. It is the only nontranscendent mechanism for explaining life’s
complex design. This mechanism is blind natural selection in which at every
stage those who are endowed with the most suitable qualities for surviving and
reproducing survive. There is no reason for their survival other than that they
proved successful in the struggle for survival. “Success” is not defined by any
transcendent measurement but by the immanent logic of the evolutionary process.
This brings us to another widespread cause of resistance to “sociobiology,” the
belief that it upholds biological determinism in a subject which is distinctively
determined by human culture. For once humans developed agriculture, they set in
motion a continuous chain of developments that have taken us far away from our
evolutionary natural way of life. Original, evolution-shaped, innate human wants,
desires, and proximate behavioral and emotional mechanisms now express themselves in radically altered, “artificial” conditions. In the process, they have been
greatly modified, assuming novel and diverse appearances. At the same time,
however, cultural evolution has not operated on a “clean slate,” nor has it been
capable of producing simply “anything.” Its multifarious and diverse forms have
been built on a clearly recognizable deep core of innate human propensities. It has
been working on a human physiological and psychological “landscape” deeply
grooved by long-evolved inborn predispositions. Cultural takeoff took place much
too recently to affect the human genome in any significant way (except for some
well known aspects such as genes for lactose absorption, disease resistance, and a
few other cases of strong selection) (Cavalli-Sforza and Feldman 1981; Lumsden
and Wilson 1981; Boyd and Richerson 1985; Durham 1991; Richerson and Boyd
2005). Genetically, we are virtually the same people as our Stone Age forefathers
and are endowed with the same predispositions. With cultural evolution, all bets are
not off they are merely hedged.
Unfortunately, space is too limited for a discussion of how the motives for
human fighting and fighting itself have endured, and how they have been affected,
by cultural evolution, through history. Interested readers are referred to my book
(Gat 2006, Chaps. 12 and 17).
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Chapter 10
From Grooming to Giving Blood: The Origins
of Human Altruism
Joan B. Silk and Robert Boyd
Abstract Cooperation plays an important role in the lives of most primates,
including ourselves. However, the magnitude and scope of cooperation varies
considerably across taxa: callitrichids pool efforts to rear a pairs’ offspring, male
langurs jointly challenge resident males for access to groups of females, female
baboons groom one another equitably, and male chimpanzees exchange support for
mating opportunities. All of these forms of cooperation have analogs in human
societies, but humans cooperate in more diverse contexts, with a wider range of
partners, and at larger scales than other primates. The evolutionary foundations of
cooperation in nonhuman primates kinship, reciprocity, and mutualism also
generate cooperation in human societies, but cooperation in human societies may
also be supported by group-level processes that do not exist in other primate
species. The human capacities for culture may have created novel evolutionary
forces that altered the selective benefits derived from cooperation.
10.1
Introduction
Humans are exceptionally altruistic creatures. We honor promises, make donations
to charity, vote in elections, recycle bottles, give blood, participate in political
protests, punish cheaters, and go to war. We are moved by prosocial sentiments,
such as empathy and compassion, that influence our responses to others in need, and
moral emotions, such as a concern for fairness, that shape our judgments about what
we should do in particular situations. Although other animals can be altruistic, our
species is unusual because our altruistic impulses extend to people who lie outside
the circle of close kin and beyond networks of reciprocating partners. This suggests
J.B. Silk (*) and R. Boyd
Department of Anthropology, University of California, Los Angeles, CA, USA
e mail: jsilk@anthro.ucla.edu, rboyd@anthro.ucla.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 10, # Springer Verlag Berlin Heidelberg 2010
223
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J.B. Silk and R. Boyd
that the evolutionary processes that are thought to underlie altruistic cooperation in
other animals, kin selection, contingent reciprocity, and biological market processes,
may not be sufficient to account for the patterning of altruism in humans.
How were humans transformed from smart, sociable, and cooperative apes into
other-regarding altruists? In this chapter, we attempt to provide some answers to
this question. We begin with a discussion of the evolutionary processes that
underlie altruism in nature, and then review the distribution, scope, and limits of
altruism in nonhuman primates. We also review a growing body of data that
provides insight about the nature of preferences that underlie cooperation in
nonhuman primates. This information provides the context for understanding the
similarities and differences among humans and other primates, and for considering
the kinds of selective processes that may have played a role in the evolution of
altruism in human societies. Then, we discuss the evolutionary processes that may
have favored the evolution of altruistic, other-regarding creatures like ourselves.
10.2
The Evolution of Altruism
Evolutionary biologists define altruism as any behavior that reduces the genetic
fitness of the actor and increases the genetic fitness of the recipient. There is
considerably less consensus among evolutionary biologists about the definition of
cooperation. Sometimes, cooperation is used as a synonym for altruism, and
sometimes it is used to encompass any type of coordinated mutually beneficial
behavior. Here, we will adopt the narrower definition of cooperation as a synonym
for altruism, and we will use the terms cooperation and altruism interchangeably.
Natural selection is not expected to favor indiscriminate altruism because
altruists always bear the costs of the altruistic behaviors that they perform on behalf
of others, so the average fitness of an allele that increases the likelihood of
performing altruistic behaviors will be lower than the average fitness of the nonaltruistic allele. In order for altruism to evolve, there must be some cue that causes
altruists to direct benefits selectively to other altruists. In nature, three types of cues
seem important: recent common descent, proximity in viscous populations, and
previous behavior.
Selection can favor altruism toward close relatives because recent common
descent provides a reliable cue of genetic similarity. Individuals that are descended
from the same ancestors have some probability of inheriting copies of the same
genes. Thus, individuals who carry genes that are associated with altruistic behavior
are more likely to have relatives who carry copies of the same genes than individuals drawn at random from the population. If individuals can identify their
relatives and preferentially behave altruistically toward them, they will tend to
confer benefits on individuals who also carry copies of the the genes that lead to
altruistic behavior. Selection can also favor indiscriminate altruism toward other
individuals if limited dispersal in viscous populations causes neighbors to be more
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closely related than chance would dictate even though they do not share a recent
common ancestor.
Both these processes are often lumped together under the heading of kin
selection (Hamilton 1964). What has come to be known as Hamilton’s rule predicts
that altruism will be favored when br > c. The quantities b and c represent the
benefits and costs associated with the altruistic act. The quantity r measures how
much the possession of a particular gene in one individual predicts the presence of
the same gene in a second individual. Kin selection relies on the existence of a cue
that allows individuals to direct altruism toward kin, and natural selection has
produced a variety of mechanisms for kin recognition (rodents: Holmes and
Mateo 2007; primates: Widdig 2007; social insects: Gamboa 2004; amphibians:
Blaustein and Waldman 1992; birds: Komdeur and Hatchwell 1999).
The same basic logic underlies the theory of reciprocal altruism that was first
introduced by Trivers (1971) and later formalized by Axelrod and Hamilton (1981).
Reciprocal altruism is a form of contingent reciprocity in which the past behavior of
other group members provides a cue about whether they carry alleles that lead to
altruistic behavior. When individuals interact repeatedly, contingent altruistic strategies can arise. In the first interaction, an individual who carries the gene that leads
to altruistic behavior provides help, and continues to help only if its partner
reciprocates. If individuals follow this tit-for-tat rule, then contingent altruists
will channel help toward other altruists after the first interaction. These kinds of
contingent strategies can be sustained as long as (1 1/t) b>c, where b is the benefit
derived from the other’s helpful act, c is the cost of the helpful act, and t is the
expected number of interactions between the two. It is not possible to satisfy this
inequality when t ¼ 1, so multiple interactions are required for contingent reciprocity to be favored. It is also easier to satisfy the inequality when the ratio of
benefits to costs is high.
Although the logic underlying kin selection is fundamentally similar to the logic
underlying contingent reciprocity, the outcome can be quite different because there
are multiple equilibria. When the conditions (1 1/t) b>c are fulfilled, contingent
reciprocity can persist as an evolutionarily stable strategy (ESS), but unconditional
defection is also an ESS, as are a variety of other strategies. In a world of
unconditional defectors, contingent reciprocators will not prosper because they
will invariably encounter partners who do not cooperate. In order for mutually
beneficial strategies like tit-for-tat to evolve, there must be some factor that shifts
the balance in their favor and makes it more likely that the population will move
toward a cooperative equilibrium.
The solution to this problem was provided by Axelrod and Hamilton (1981).
When pairs of related individuals interact, the odds of encountering another individual with the same rare strategy are substantially increased. If the benefits obtained
over time by a cooperating pair are sufficiently greater than the benefits obtained
by two unconditional defectors, then these rare cooperators can compensate for
the fact that they do poorly when they are paired with unconditional defectors.
Quite small amounts of relatedness may allow reciprocating strategies to invade a
population composed of unconditional defectors. Axelrod and Hamilton termed this
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the “ratchet effect,” because small amounts of relatedness can ramp up cooperation.
Although the ratchet effect is powerful, it is also quite restrictive: it only works
when groups are small (Boyd and Richerson 1988).
Below, we briefly review empirical evidence which suggests that kin selection
and contingent reciprocity have shaped the distribution of altruistic behavior in
nonhuman primate groups.
10.3
Kin Biases in Behavior
Primatologists have produced a rich body of information about maternal kin biases
in behavior over the last 30 years (reviewed in Silk 2002, 2005; Chapais and
Berman 2007). Perhaps the best way to summarize this extensive literature is to
say that female primates behave as though they understand the logic of Hamilton’s
rule. In nearly every species in which females live in groups with their relatives,
there are pronounced nepotistic biases among females in the distribution of altruistic behaviors, such as grooming, coalitionary support, and food sharing. Moreover,
the most costly forms of altruism, including reproductive suppression and defense
against higher ranking opponents, are limited to very close kin.
The most extreme form of nepotism occurs in cooperatively breeding groups of
marmosets and tamarins. Marmoset and tamarins, members of the subfamily Callitrichinae, live in small territorial groups (French 1997; Tardif 1997). Cooperatively
breeding callitrichid groups typically contain only one breeding pair, who are assisted
by several nonbreeding adults, who may be same-sexed siblings of the breeding pair,
and mature offspring from previous litters (Dietz 2004; French 1997; Tardif 1997).
Breeding females typically give birth to fraternal twins and can produce two litters
per year (in contrast, most other primates give birth to singletons at considerably
longer intervals). After females give birth, nonbreeding group members provide
extensive help carrying and provisioning infants. In golden lion tamarins, the species
for which we have the most complete data in the wild, coresident adult males are
generally close kin, but only one male sires offspring (Dietz 2004).
Nepotism is also a pronounced feature of behavior in the well-studied Cercopithecine societies, which include baboons, macaques, and vervets. In these species,
mothers support their immature daughters when they are involved in conflicts with
members of lower-ranking families, and daughters acquire rank positions just
below their mothers (Silk 2002, 2005). Females form matrilineal dominance hierarchies in which all members of one matriline rank above or below all members of
other matrilines. This process has long-lasting impacts on females because matrilineal dominance hierarchies are remarkably stable over time. High-ranking
females have priority of access to resources, including food and water, and generally reproduce more successfully than lower-ranking females.
Male philopatry characterizes a much smaller set of primate species, including
chimpanzees, bonobos, spider monkeys, muriquis, and wooly spider monkeys
(Pusey and Packer 1987). Unlike males in most other primate species, males in
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these species form close ties with one another. For example, chimpanzee males
spend a considerable amount of time in parties with other males, and males groom,
hunt, share meat, aid, and patrol the borders of their territories with one another
(Mitani 2005; Muller and Mitani 2005). In chimpanzee communities, males tend to
form close relationships with their maternal brothers when they are available
(Nishida 1979; Goodall 1986; Langergraber et al. 2007), but many males do not
have brothers in their groups and kinship does not seem to be a necessary ingredient
of close relationships among male chimpanzees (Langergraber et al. 2007).
Until recently, analyzes of nepotistic biases in favor of paternal kin were
complicated by uncertainties about paternity (at least on the part of observers),
which made it impossible to identify the paternal kin. The development of molecular genetic techniques for assessing paternity and noninvasive methods for obtaining genetic material from free-ranging animals now allow primatologists to identify
the sires and to study the effects of paternal kinship on the distribution of altruistic
behavior in primate groups (reviewed by Widdig 2007).
In baboon and rhesus macaque groups, females are more likely to associate with
and groom paternal half sisters than unrelated females (Widdig et al. 2001, 2002;
Smith et al. 2003; Silk et al. 2006a). In general, females show considerably stronger
preferences for the maternal half sisters than for their paternal half sisters. This may
reflect some degree of uncertainty about paternal relatedness or differences in
the value of relationships with maternal and paternal sisters. If social bonds
reinforce alliances, then maternal sisters may be more valuable allies than paternal
sisters. Widdig et al. (2006) found that female rhesus macaques do not selectively
support their paternal half sisters in agonistic encounters, but they do avoid intervening against them. Paternal kinship does not seem to play an important role in
the distribution of altruistic behavior among adult male chimpanzees (Langergraber
et al. 2007), young chimpanzees (Lehmann et al. 2006), juvenile mandrills
(Charpentier et al. 2007), or white-faced capuchins (Perry et al. 2008).
10.4
Cooperation Among Reciprocating Partners
Until recently, most analyzes of the patterning of cooperation among unrelated
individuals were based on the assumption that these interactions were the product of
some form of contingent reciprocity. However, some researchers have argued that
primates lack the cognitive ability to keep track of interactions with multiple
partners across time (de Waal 2000; Barrett and Henzi 2002, 2005) and have
cognitive biases, such as a preference for immediate rewards, that constrain the
evolution of contingent reciprocity (Stevens and Hauser 2004; Stevens et al. 2005).
Skepticism about the plausibility of contingent reciprocity as a strategic option for
primates has led to interest in alternative processes, particularly the biological
market model (Noë 2005, 2006). In biological markets, transactions are influenced
by economic forces, such as supply and demand; trading partners act as buyers and
sellers, negotiating prices for commodities based on their value to each party and
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the availability of alternative trading partners. Because buyers and sellers both
obtain immediate benefits from their exchanges, there is no need to develop longterm relationships with partners.
The theoretical foundations of these approaches are quite different, but it is
difficult to disentangle these processes empirically. For example, positive correlations between the amount of grooming given and received might be the product of
contingent reciprocity or the outcome of trade in a biological market. Similarly, the
absence of correlations in the amount of support given and received might mean
that contingent reciprocity is not operating, that support is being exchanged for
some other commodity (and there is some mechanism for enforcing trades), or that
the frequency of interactions provides a poor index of the benefits accrued or costs
incurred. Below, we summarize the pattern of interactions among unrelated individuals, although it is not always clear whether these patterns of exchange are the
product of contingent reciprocity or market forces.
10.4.1 Patterns of Exchange and Interchange
Monkeys and apes generally spend the most time grooming those from whom they
receive the most grooming, although the magnitude of the correlations in grooming
given and received varies considerably (Schino and Aureli 2008). In addition, in
some species, grooming within dyads is associated with the distribution of other
commodities, such as access to infants (Henzi and Barrett 2002) and food (de Waal
1997), and with the distribution of some types of services, including support
(Schino 2001; Watts 2002; Schino and Aureli 2008), participation in border patrols
(Watts and Mitani 2001), and mating opportunities (Duffy et al. 2007).
Several recent studies indicate that grooming is more evenly balanced across
multiple bouts than within single bouts (Frank and Silk 2009a; Gomes et al. 2009;
Schino et al. 2009). For example, grooming among pairs of adult female baboons
was significantly more evenly balanced over an 18-month study period than within
single grooming bouts (Frank and Silk 2009a). These results strongly suggest that
monkeys and apes are able to keep track of the distribution of grooming given and
received over substantial periods of time.
Several lines of evidence suggest that cooperation may be limited to cooperating
partners. First, female baboons and male chimpanzees form the strongest and most
enduring social relationships with those that groom them most equitably, and this
holds for both related and unrelated females (Silk et al. 2006b; Mitani 2009). We do
not know whether equitable grooming relationships are more likely to be maintained across time, or whether relationships among close associates become more
equitable over time. Either way, female baboons and male chimpanzees selectively
maintain relationships with those that groom them most equitably.
Several factors seem to influence the distribution of cooperation within dyads.
Grooming is often directed up the dominance hierarchy in macaque groups, as
low-ranking females groom their partners more than they are groomed in return
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(Schino 2001). This imbalance may exist because low-ranking females trade
grooming for support as Seyfarth (1977) originally suggested. It may also exist
because low-ranking females use grooming to appease high-ranking partners and
reduce the risk of harassment from them. Barrett and her colleagues have shown
that such trades become more imbalanced when the risk of aggression from higherranking females increases (Barrett et al. 1999). In some cases, females may use
grooming to obtain access to other kinds of resources. For reasons that are not
entirely clear, females are highly motivated to handle other females’ infants, and
often use grooming as a means to this end (Maestripieri 1994; Henzi and Barrett
2002). In some groups, females spend more time grooming mothers of newborns
when few other infants are present than when many infants are available (Henzi and
Barrett 2002; Manson 1999), but in others, the supply of infants does not seem to
influence the grooming behavior (Frank and Silk 2009b). Similarly, female baboons
groom higher-ranking mothers more than lower-ranking mothers in some groups
(Henzi and Barrett 2002), but not in others (Frank and Silk 2009a).
Several naturalistic experiments have been designed to detect contingencies in
cooperative behavior. Wild vervet monkeys were more attentive to the taperecorded distress calls of unrelated group members if they had been groomed
recently by the caller than if they had not been groomed recently by the same
monkey (Seyfarth and Cheney 1984). In contrast, grooming among closely related
monkeys did not influence the likelihood of responding to distress calls. Similarly,
when disputes over food were instigated by researchers, long-tailed macaques were
more likely to intervene on behalf of monkeys who had recently groomed them than
monkeys who had not groomed them (Hemelrijk 1994). Grooming also enhances
feeding tolerance among chimpanzees (de Waal 1997). In this experiment, chimpanzees were provisioned with leafy branches and all food transfers were recorded.
In addition, grooming behavior before the provisioning event was monitored. The
chimpanzees were more tolerant to individuals that had previously groomed them
than they were to other individuals, and the effects of previous grooming were most
pronounced for pairs that did not frequently groom at other times.
More formal laboratory experiments that were designed to assess how individuals respond to the helpful or unhelpful behavior of their partners have generated
mixed results (see Silk 2007a for a review). In some cases, researchers have
detected biases that favor partners who have provided help in previous trials (e.g.,
Cronin and Snowdon 2008; de Waal and Berger 2000; Hauser et al. 2003), while in
others little or no evidence of contingency has been detected (Brosnan et al. 2009;
Melis et al. 2008).
10.5
Limits of Altruism in Primate Groups
In nonhuman primate groups, cooperative interactions involve relatively small
numbers of familiar individuals, often close relatives or reciprocating partners.
Grooming involves pairs of individuals, coalitionary aggression may involve
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several individuals, and sizable numbers of individuals may participate in intergroup encounters.
Responses to strangers and members of neighboring groups generally range
from passive avoidance to active hostility (see Crofoot and Wrangham this
volume). Members of different groups rarely groom, and there are no reports of
groups forming coalitions against other groups. In contrast, even human huntergatherer societies can orchestrate cooperative activities involving hundreds, sometimes thousands, of individuals. In market economies, goods and services are traded
among strangers.
In human societies, people who violate social norms, break rules, or commit
crimes are often punished by other group members. Punishment of this sort is
altruistic because the individuals who impose sanctions on transgressors incur costs,
while the benefits flow to the entire community. Hauser and Marler (1993a, b)
provided the first evidence for altruistic punishment in primate groups. Rhesus
macaques give distinctive calls when they find desirable foods. Hauser and Marler
hid piles of coconut, a rhesus treat, in the monkeys’ home range, and monitored
what happened when these caches were discovered. In some cases, the monkeys
who discovered the piles of coconut called, and in other cases, they remained silent.
Female macaques were more likely to be attacked by other monkeys, particularly
young males, if they remained silent after finding food than if they gave food calls.
The authors hypothesized that females were being punished for attempting to
conceal the location of these prized foods. This could constitute a form of altruistic
punishment if the screams of the victim alert other group members to the site of the
food, giving many animals an opportunity to profit from the aggressor’s actions, or
if harassment reduces the likelihood that the victims will remain silent after finding
food in the future.
Subsequent work on food calling in white-faced capuchins (Gros-Louis 2004)
provides an alternate interpretation for aggression in this context. Capuchins who
called after finding food were less likely to be approached by others than monkeys
that remained silent. In addition, individuals who gave food calls when they were
approached by higher-ranking animals were less likely to receive aggression than
monkeys who did not call. Gros-Louis (2004) suggests that food calls may function
to establish the ownership of resources and signal the owners’ willingness to defend
them. This would explain why monkeys are especially likely to call when they are
approached by high-ranking monkeys, who might challenge them for possession of
food items.
10.6
Motives Underlying Altruism in NHPs
It is easy to perceive similarities between the altruistic behaviors we observe in
primate groups and some forms of altruistic interactions in humans. A chimpanzee
who is being groomed looks very much like someone getting a good massage
deeply relaxed and contented. When a juvenile baboon nestles in the lap of a male
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who is defending him from harassment, he seems to be relieved and reassured.
Despite these parallels, it is not clear whether the motives and sentiments that
underlie altruism are the same in humans and other species. In humans, altruism
seems to be motivated at least in part by an understanding of others’ thoughts
and desires, concern for the welfare of others, and a preference for outcomes that
benefit others (Fehr and Fischbacher 2003; Henrich et al. 2006). We may also be
motivated by a concern for reputation that makes us want others to think that we are
generous, fair, or charitable (Haley and Fessler 2005, Fessler and Gervais this
volume). Below, we discuss the evidence for empathy and other regarding preferences in nonhuman primates. For a discussion of the evidence for fairness, see
Lakshminarayan and Santos, this volume.
10.6.1 Empathy
Until recently, discussions of empathy in other primates were based on anecdotal
reports of one individual helping another or reacting to another animal’s distress
(Silk 2007b). Although these events are intriguing, anecdotal data present several
problems. First, interpretations of singular events are based on subjective impressions of observers, and are very difficult to verify. Second, observers may be more
likely to notice and remember incidents that suggest that primates are empathetic
than they are to take note when they seem oblivious or indifferent. Third, observers
may be more likely to offer more anthropomorphic interpretations of the behavior
of some animals, such as apes or dogs, than others.
This has led researchers to try to devise more systematic ways to assess the
empathic responses of other primates. One study capitalized on the fact that
macaques and baboons display elevated rates of self-directed behaviors, such as
scratching, when they are under stress. Rates of self-directed behaviors rise sharply
after monkeys are threatened or harassed by other group members (Aureli and van
Schaik 1991; Castles and Whiten 1998). If monkeys experience empathy, then
mothers would be expected to experience distress when their infants are distressed.
However, Japanese macaque females showed no obvious signs of stress when their
infants were harassed (Schino et al. 2004). Moreover, mothers did not approach
their infants or offer comfort after their infants were victimized.
These results are consistent with the results from a study of mothers’ reactions
when their infants were exposed to danger (Cheney and Seyfarth 1990). In these
experiments, mothers had learned that a dangerous or frightening object, such as a
model of a snake, was concealed in a box in their enclosure. Mothers made no effort
to stop their infants or warn them of danger when their infants, who were ignorant
of danger, approached the box. Maternal indifference in these situations strongly
suggests that monkeys may not have the capacity for empathy.
Female macaques may not respond to their infants’ distress or protect them from
potential dangers because they do not have a well-developed understanding of
others’ knowledge, feelings, and desires (Tomasello and Call 1997). Apes seem
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to have a more complete understanding of others’ minds, and stronger claims are
made about their capacity for empathy (Preston and de Waal 2002; de Waal 2003).
The capacity for empathy might underlie chimpanzees’ responses to the victims
of aggression. Third-party affiliation after conflicts has been documented in a
number of chimpanzee populations (reviewed in Silk 2007a). De Waal and van
Roosmalen (1979) hypothesized that these interactions are designed to alleviate the
distress of the victims of aggression, and applied the label “consolation.” This
functional interpretation implies that actors are motivated by empathy for the
victim (Palagi et al. 2006; Fraser et al. 2008), but studies designed to evaluate the
function and effectiveness of third-party affiliation after conflicts have produced
conflicting results. Palagi et al. (2006) found that chimpanzees did not selectively
console kin or close associates, as might be expected if consolation is based on
empathy. More recently, Fraser et al. (2008) have found that chimpanzees are
significantly more likely to offer consolation to valued social partners than to
other group members. Consolation effectively reduced self-directed behavior in
one group of chimpanzees (Fraser et al. 2008), but not in another (Koski and Sterck
2007). The inconsistency among these results makes it difficult to draw strong
conclusions about the function of consolation.
10.6.2 Other Regarding Preferences
The difficulties associated with identifying the sentiments that underlie altruistic
behavior have led researchers to borrow techniques developed by behavioral
economists to investigate the nature of social preferences in primates. In these
experimental studies, subjects are faced with decisions about how to allocate
resources to themselves and others. The choices that they make in these situations
provide insight about their social preferences.
For example, in one set of experiments, chimpanzees were presented with the
opportunity to deliver food rewards to themselves and/or other individuals. To
implement their choices, the animals manipulated simple mechanical apparatuses
that were baited with food. One of the options provided identical food rewards to
the actor and to the occupant of the other enclosure (the 1/1 option); and the other
option provided a food reward only to the actor (the 1/0 option). Individuals might
prefer the 1/1 option because they have prepotent biases toward larger numbers of
rewards (regardless of the distribution), so a control condition was included in
which there was no potential recipient present. If individuals are concerned about
the welfare of others, they are expected to prefer the 1/1 option over the 1/0 option,
and this preference is expected to be stronger when another individual is present
than when the actor is alone. Chimpanzees at three different sites, using four
different apparatus, were as just as likely to choose the 1/1 option when another
chimpanzee was present as when they were alone (Silk et al. 2005; Jensen et al.
2006; Vonk et al. 2008). It is possible that actors did not choose the 1/1 option more
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often because they were unaware that their partners preferred this outcome
(Warneken et al. 2007). However, Vonk et al. (2008) found that potential recipients
made begging gestures before the actor had made a choice in some trials, and
clearly directed their begging gestures toward the side of the apparatus from which
they would obtain rewards. Begging did not increase the likelihood that the actor
would deliver rewards.
These experiments with chimpanzees have been followed with similar experiments in other species. In each case, actors were offered choices with different
payoff structures. Capuchins demonstrate prosocial preferences in two different
experimental paradigms (de Waal et al. 2008; Lakshminarayanan and Santos 2008),
while results for two cooperatively breeding species are mixed. Cooperatively
breeding common marmosets, Callithrix jacchus, were presented with an opportunity to deliver food to another individual, but received no reward themselves
(0/0 vs. 0/1). The marmosets were significantly more likely to choose the 0/1 option
when another marmoset was present than when they were alone (Burkart et al.
2007), satisfying the criterion for prosocial preferences. However, different results
were obtained for another cooperatively breeding callitrichid, the cotton-top tamarin, Saguinus oedipus (Cronin et al. 2009). In this case, cotton-top tamarins did not
distinguish between partner-present and partner-absent trials for two different
payoff distributions (1/1 vs. 1/0; 0/1 vs. 0/0).
In another experimental study, Cronin and Snowdon (2008) evaluated tamarins’ willingness to deliver rewards to their mates when (a) both parties got
rewards, (b) both parties were rewarded on alternate days, (c) both parties shared
access to a monopolizable reward, and (d) one partner could deliver rewards to
its partner in repeated trials. The tamarins were most likely to solve the task, and
deliver rewards to their partners, when both got rewards (100% trials), and they
were least likely to solve the task when they did not receive rewards themselves
(46%). Although helping was not extinguished completely when actors did not
obtain rewards, latency to solve the task increased and there was more variability
across pairs.
Chimpanzees seem to be more inclined to provide instrumental help to humans
and other group members than to deliver food rewards to them. In two sets of
experiments, chimpanzees retrieved objects that human experimenters could not
reach (Warneken and Tomasello 2006; Warneken et al. 2007), and their performance was not influenced by the availability of food rewards (Warneken et al.
2007). In another experiment, chimpanzees were given an opportunity to provide
help to other group members. In this experiment, the door to an adjacent room was
fastened by a chain. The actor could remove a peg and release the door, but could
not enter the room. The potential beneficiary could not reach the peg, but could
enter the room. During experimental trials, a bowl of food (visible only to the
potential beneficiary) was placed in this room. In control trials, food was placed in
another room that was inaccessible to both chimpanzees. During all trials in which
food was placed in the accessible room, potential beneficiaries oriented toward the
door of the room, while they oriented toward the door to the other room in only half
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of the control trials. Moreover, actors were significantly more likely to release the
door to the accessible room in experimental trials than in control trials. Thus, actors
helped potential beneficiaries get into the locked room, and were sensitive to cues
about their desires.
These experiments indicate that chimpanzees understand others’ needs and
desires, but do not provide unambiguous information about the motivations that
underlie their behavior. In these situations, chimpanzees could be motivated by
other regarding preferences or expectations of delayed reciprocity.
It is not easy to explain why these experiments lead to prosocial behavior in
marmosets and capuchins, but not in tamarins or chimpanzees, and why chimpanzees display prosocial preferences when they are given an opportunity to provide
instrumental help to others at some cost to themselves, but not when they are given
an opportunity to provide food to others at no cost to themselves. Having a large
brain and sophisticated knowledge of others’ thoughts and desires is apparently
neither necessary nor sufficient for the development of other regarding preferences.
Burkart et al. (2007) suggested that cooperative breeding in callitrichids and
humans might generate prosocial preferences, but this explanation does not fit the
prosocial results for capuchins or the absence of prosocial preferences in tamarins.
Warneken and his colleagues (Warneken and Tomasello 2006; Warneken et al.
2007) have speculated that chimpanzees may not display prosocial preferences in
these experiments because they perceive food as a limited resource, and are not
predisposed to provide food to others. This might explain why capuchins, which are
remarkably tolerant of scrounging, display prosocial preferences. But it does not
explain why prosocial responses are seen in marmosets, but not in tamarins, as food
transfers play an important role in both taxa (Brown et al. 2004). It is possible that
chimpanzees view all interactions involving food as zero-sum games because food
supplies are limited in nature (Warneken et al. 2007), and have selfish preferences
about food. If such preferences biased their behavior in these experiments, they
would be expected to show consistent preferences for the 1/0 option; instead they
choose the 1/1 and 1/0 option with equal frequencies.
10.7
The Origins of Other Regarding Preferences in Humans
When the human lineage diverged from the great ape lineage 5 7 million years ago,
our ancestors were probably something like modern chimpanzees smart, sociable,
and cooperative. They would have been helpful to group members in some situations, and hostile to strangers. They might have exchanged goods, services, and
favors with reciprocating partners, but were not unconditionally altruistic and may
have had limited capacities for empathy and sense of fairness. To understand the
origins and evolution of group-level cooperation and generalized other regarding
preferences in humans, we need to consider two related questions. First, why did
selection favor the evolution of the group-level cooperation and other regarding
preferences in ancestral human populations, but not in other closely related species
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of primates? What evolutionary processes sustain these other regarding preferences
in human groups?
To answer this question, it is useful to consider how the social organization and
subsistence strategies of human foraging societies differ from those of other
primates. Like the cooperatively breeding callitrichids, humans often get considerable help from others in rearing their offspring (Hrdy 2005; Burkart et al. 2007).
Humans make more use of resources that are difficult to obtain and complicate to
process than other primates do (Kaplan et al. 2000, 2003). We also rely more
heavily on social learning to acquire the knowledge, skills, and information we need
to make a living (see Whiten this volume). Finally, warfare has played an important
role in the history of human societies (Gat this volume), but lethal intergroup
conflict is absent in nonhuman primate groups, with the exception of chimpanzees
(Crofoot and Wrangham this volume). All of these factors have been implicated in
the evolution of other regarding preferences of humans. Here, we focus on cooperative breeding, complex foraging, and cultural evolution. For a more complete
discussion of the role of warfare in the evolution of human societies see Gat (this
volume) and Crofoot and Wrangham (this volume).
10.7.1 Cooperative Breeding
Hrdy (2005) hypothesizes that the high costs of producing and supporting slowgrowing human children favored the development of extensive allomaternal care
networks, which included fathers, grandmothers, and older siblings. In societies
with high levels of infant mortality, alloparental care was an integral element of
females’ reproductive strategies. Hrdy considers humans to be cooperative breeders
because multiple individuals contribute to children’s care. This definition conflates
taxa in which there is only one breeding pair who are assisted by nonbreeding
helpers (e.g., wild dogs, meerkats) with taxa in which there are multiple breeding
females who share some maternal tasks and may be assisted by other group
members (e.g., lions, banded mongoose). We reserve the term cooperative breeders
for the former, and use the term communal breeders for the latter. By this definition,
callitrichids are cooperative breeders and humans are communal breeders.
Hrdy suggests that the ability to engage caretakers and elicit investment would
be advantageous for infants, and this would favor the evolution of cognitive
capacities that allow young children to assess the intentions and predict the
responses of others. Over the course of our evolutionary history, selection elaborated these capacities to produce empathy and a well-developed theory of mind.
According to this argument, delayed maturation and cooperative/communal breeding coevolved, and both of these developments preceded the marked expansion of
brain size in humans and the origin of other regarding preferences.
Burkart et al. (2007) have also emphasized the link between cooperative breeding
and other regarding preferences in humans and marmosets. They suggest that
“. . .unsolicited prosociality, which arose in the context of provisioning, carrying,
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and sharing was then generalized toward the sharing of information and psychological states.” When unsolicited prosociality was added to the the ape-like brain of our
ancestors, it precipitated a “cascade of further developments” including language,
teaching, and the development of other regarding preferences and group-level
cooperation.
Generalized prosocial preferences in small groups of closely related individuals
could evolve through kin selection. Callitrichid groups are typically small and
group members are closely related to the infants that they care for. Although
there is some evidence that altruistic responses are not limited to kin and not all
group members are equally altruistic (Burkart et al. 2007), it is possible that there
are few opportunities for prosocial behavior toward nonrelatives in the wild. Thus,
group-level cooperation evolved through kin selection.
It is more difficult to invoke the same argument for the evolution of other
regarding preferences in humans. Contemporary human foragers live in larger
and more complex groups than cooperatively breeding monkeys do. Allomaternal
care in such groups is typically nepotistic, and grandmothers are the most common
caregivers for children (reviewed by Hrdy 2005). Male provisioning and direct care
of infants may be a form of parenting effort or mating effort (Anderson et al. 1999a,
b; Marlowe 1999a,b); in both cases, mens’ contributions to childcare linked to their
own fitness benefits. Selective pressures favoring allomaternal care and communal
breeding in human groups have not produced indiscriminant altruism toward
children, and it seems difficult to link communal breeding directly to the emergence
of other regarding preferences in human groups.
The similarity in the responses of marmosets and humans in the prosocial task
may arise because group-level cooperation is favored in both taxa, not because both
species are cooperative/communal breeders. Kin selection may favor group-level
cooperation in marmosets while other forces may generate group-level cooperation
in humans. Below we consider two possible mechanisms underlying group-level
cooperation in human societies.
10.7.2 Complex Foraging
The cooperative breeding hypothesis does not explain why humans mature more
slowly in relation to their body size than other primates and why human infants
require so much more care than the infants of other primates. Kaplan and his
colleagues suggest that humans mature slowly because it takes a long time to
acquire the knowledge and skills that human food foragers need to make a living
(Kaplan et al. 2000, 2003). Human foragers rely heavily on complex foraging skills,
including hunting and extractive foraging, and exploit a much wider range of
resources using a larger repertoire of tools and techniques than other primates do.
Comparative data indicate that extracted and hunted foods account for about 5% of
chimpanzee diets, while these types of food account for about 90% of the diet of
human hunter-gatherers. Kaplan and his colleagues emphasize the fact that humans
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Origins of Human Altruism
237
specialize on resources, such as meat, roots, and nuts, that are rare and patchy, but
provide rich sources of nutrients.
The reliance on complex foraging techniques may have favored economic
interdependence within families and groups. If foraging skills are difficult to
master, it makes sense to develop foraging specializations. Such diversification,
by sex, age, or ability, will pay off if specialists share the products of their foraging
efforts. Sexual division of labor is a universal feature of human foraging societies,
as men mainly hunt and women mainly gather. Sharing may also buffer the
economic risks associated with hunting (Winterhalder 1986). On some days, hunters return with carcasses large enough to feed many people, but on other days, they
come back empty-handed. Sharing provides one way to insure against such risks.
Sharing networks that extend beyond family or household, buffer risk even further
(Gurven 2004).
According to this argument, the importance of complex foraging techniques in
human subsistence strategies may have favored delayed development, extended
periods of parental provisioning, division of labor within families, and the formation of extended sharing networks. In hunter-gatherer groups, resource exchanges
are influenced by the dynamics of contingent reciprocity, as well as by the norms of
fairness (Gurven 2004, 2006). To explain the origins of prosocial preferences,
however, we need to take the argument one step further, and consider the role of
cultural evolution.
10.7.3 Cultural Evolution
Complex foraging strategies may be linked to the evolution of social learning and
the capacity for culture. Early Pleistocene hominins occupied a considerably wider
range of habitats than any contemporary apes do today. The knowledge and
subsistence technology required for complex foraging varies greatly from one
habitat to another, and it would have become more and more difficult for individuals to acquire this information on their own. Social learning allows human
populations to gradually accumulate useful knowledge as individuals learn from
others, make modest improvements, and pass this accumulated knowledge on. This
kind of cumulative cultural change can give rise to complex habitat-specific
adaptations much more rapidly than genetic evolution can (Boyd and Richerson
1985, 1996, McElreath this volume). Although we may have underestimated the
social learning capacities of chimpanzees and other primates in the past (Whiten
this volume), there is no doubt that humans rely on social learning to a much greater
extent than any other primates do.
The cultural transmission of information may have been especially important for
our ancestors during the Middle and Upper Pleistocene. During this period, world
temperatures fluctuated widely. At some points, average world temperatures
changed as much as 10 C in 1,000 years (Richerson et al. 2001). In this kind of
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J.B. Silk and R. Boyd
world, the ability to make rapid adjustments to changing conditions would have
provided strong selection for the evolution of cultural capacities.
Social learning may have enabled humans to adapt to changing conditions, but it
also had the potential to generate considerable cultural variation among groups.
Social interactions commonly give rise to multiple adaptive equilibria. (a nonbiological analog of this process would be conventions about which side of the road to
drive on: it is equally efficient to drive on the right or the left, but essential that
everyone follows the same rule.) Systems of reciprocity, reputation management,
and punishment can stabilize a vast range of behaviors including ones that lead to
large-scale cooperation (Axelrod 1986; Boyd and Richerson 1992; Nowak and
Sigmund 1998; Henrich and Boyd 1998; Panchanathan and Boyd 2004). Adaptive
processes, including both individual learning and the tendency to imitate successful
individuals which generates conformist biases, will cause local populations to
evolve toward different equilibria. This tendency will be counteracted by the flow
of ideas between groups, just as genetic variation among groups is counteracted by
migration. However, if individuals adopt the ideas and traditions of their new
groups, then cultural variation among groups will be maintained. Cultural adaptation can proceed much more quickly than genetic adaptation, so it is likely that as
cultural adaptation became more and more important, the amount of variation in
behavior and social organization among human groups also increased (Richerson
and Boyd 2005).
Increased variation between groups could have had important effects on the
cultural evolution of group-beneficial traits. To understand the why, it is helpful to
adopt the formulation derived by Price (1970) which partitions genetic evolutionary
change into two components:
Dp /
VG bG þ Vw bw
|{z}
|{z}
between groups
within groups
The effect of selection on the change in frequency of a gene, Dp, is proportional to
the amount of genetic variation between groups (VG) and the amount of genetic
variation within groups (VW). Behaviors are beneficial to the group when the
behavior increases group fitness, or bG > 0. If it is costly to the individual,
bW < 0. When behaviors are beneficial to the group and costly to individuals (as
is the case for altruistic traits), then the outcome will depend on the relative
magnitude of the variance within and between groups. When groups are large,
selection is weak, and rates of migration are not too low, VW will greatly exceed VG
(Rogers 1990), and group selection cannot overcome opposing individual selection.
These conditions often hold in nature, and group selection is generally not thought
to be an important force.
Group selection can play a more important role when there are multiple stable
equilibria. To see how this works, consider a situation in which there are two
alleles, A and B, and homozygotes have higher fitness than heterozygotes (WAA >
WBB > WAB). This means that when A is common, B will not be able to invade
10
Origins of Human Altruism
239
because WAB < WAA; when B is common, A will not be able to invade because
WAB < WBB. In this situation, there is a paradoxical result: when B is common,
individual selection will not favor the A allele, even though it confers a fitness
advantage. However, group selection can lead to the spread of the B allele when
there are multiple stable equilibria. Imagine that a large population is divided into a
number of separate groups: A is common in some groups, and B is common in other
groups. There is little variance within groups, so VW will be low. Selection favors A
in some populations and B in other populations, so the average value of bW across
groups will also be small. This means that the within-group component of the Price
equation will approach zero, and selection within groups will have little impact on
the frequency of A and B. If the A allele has higher average fitness, then group
selection can favor the spread of the A allele. The A allele may spread if carriers of
the A allele are more successful in forming new groups, or if groups with lower
frequencies of the A allele are more likely to become extinct and are replaced by
individuals carrying the A allele (Boyd and Richerson 2002). Thus, when there are
multiple stable equilibria and selection is strong compared to migration, selection
will preserve the variation among groups and favor the evolution of traits that
increase group fitness.
Although the Price equation was formulated to describe the effects of selection
on genetic traits, the same basic logic applies to cultural variants. When there are
multiple stable equilibria, processes that reduce the amount of cultural variation
within groups will reduce the within-group component of variation and strengthen
the forces of selection among groups. This will favor the evolution of groupbeneficial cultural traits that increase the competitive ability of groups. Competition between groups will favor the spread and elaboration of cooperative cultural
norms that makes groups larger, more productive, and more successful in conflicts
with neighboring groups. Cooperative cultural norms may be maintained by
concerns about reputation, desire to maintain reciprocity, or fear of costly sanctions. This, in turn, may have favored the development of new prosocial emotions,
such as compassion, guilt, and shame. Individuals who lack these new social
emotions would have violated the prevailing norms more often, and as a result
they may have been punished, denigrated, denied access to community resources,
or ostracized. Cooperation and group identification in intergroup conflict may
have set up an arms race that drove social evolution to progressively greater
extremes of in-group cooperation. Eventually, human populations came to resemble the hunter-gathering societies of the ethnographic record. These societies are
egalitarian and political power is diffuse. People readily punish others for transgressions of social norms, even when their own personal interests are not directly
at stake.
These new motivations did not replace the motivational biases that we inherited
from our primate ancestors. We still care strongly about our own welfare, are biased
in favor of our relatives, and place special trust in reciprocating partners. However,
we are also moved by broader loyalties to clan, tribe, class, caste, and nation. In
some cases, these loyalties conflict: we wince when we pay our taxes, although we
realize that our taxes support schools, hospitals, and other worthy institutions.
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J.B. Silk and R. Boyd
Conclusions
In the last 5 million years or so, a smart, sociable, and cooperative forest ape was
transformed into a slow growing, highly adaptable, technologically sophisticated,
other-regarding altruist. Although much of the ancient ape remains within us, we
differ from other apes in our exceptionally slow life history, our reliance on social
learning and cultural adaptations, and in our development of group beneficial social
norms and social preferences. Our understanding of this transformation is based on
work from many academic disciplines. Primatologists have accumulated a wealth
of information about the distribution of altruistic behavior in primate groups, which
provides a broad comparative framework for understanding the roots of cooperation. Evolutionary theorists have developed a rich body of theory about the evolution of altruism and cultural evolution, which has enabled us to understand the
dynamics of these processes in a more rigorous way. Finally, developmental
psychologists, behavioral economists, human behavioral ecologists, and evolutionary psychologists have contributed a diverse set of methods and extensive empirical
evidence about motivations, preferences, and behavior of contemporary people.
This body of work has helped us to define the continuities in the behavior of humans
and other primates, and to illuminate the gaps. It is clear that our understanding of
the origins of human altruism is incomplete, but we have begun to get some traction
on the problem.
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Watts DP, Mitani JC (2001) Boundary patrols and intergroup encounters in wild chimpanzees.
Behaviour 138:299 327
Widdig A (2007) Paternal kin discrimination: the evidence and likely mechanisms. Biol Rev
82:319 334
Widdig A, Nürnberg P, Krawczak M, Streich WJ, Bercovitch FB (2001) Paternal relatedness and
age proximity regulate social relationships among adult female rhesus macaques. Proc Natl
Acad Sci USA 98:13769 13773
Widdig A, Nürnberg P, Krawczak M, Streich WJ, Bercovitch FB (2002) Affiliation and aggression
among adult female rhesus macaques: a genetic analysis of paternal cohorts. Behaviour
139:371 391
Widdig A, Streich WJ, Nürnberg P, Croucher PJP, Bercovitch FB, Krawczak M (2006) Paternal
kin bias in the agonistic interventions of adult female rhesus macaques (Macaca mulatta).
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Winterhalder B (1986) Diet choice, risk, and food sharing in a stochastic environment. J Anthropol
Archaeol 5:369 392
Chapter 11
Evolved Irrationality? Equity and the Origins
of Human Economic Behavior
Venkat Lakshminarayanan and Laurie R. Santos
Abstract While the economic approach to human decision-making characterizes
our choices in terms of how we maximize utility, in many cases, such an approach
fails to predict the decisions we actually make. Specifically, preference-biases such
as loss-aversion and reference-dependence demonstrate that decision-makers make
relative (rather than absolute) comparisons in judging the quality of their rewards.
In the case of inequity aversion, decision-makers take into account not just their
own earnings, but also the quality of another individual’s reward. In this chapter, we
discuss an evolutionary approach to uncovering the origins of these irrational
economic strategies. To do this, we review recent experiments showing that
nonhuman primates possess a variety of economic tendencies previously thought
to be unique to humans. The existence of economic decision-making biases outside
of our species implies that ancestral organisms may have possessed these same
tendencies suggesting that humans’ biased economic decisions may actually have
been adaptive in evolutionarily ancient environments, even though they might be
characterized as irrational in contemporary economic settings.
11.1
Homo economicus: Model Subject or Tall Tale?
One of the major challenges facing humans from all cultural backgrounds is how to
make good decisions. In the human species, decision-making seems remarkably
complicated judging the potential benefit of even simple choices, like walking to
the theater to see a movie, can be incredibly tough. Is there a better movie playing
V. Lakshminarayanan (*) and L.R. Santos
Department of Psychology, Yale University, New Haven, CT, USA
e mail: venkat.lakshminarayanan@yale.edu, laurie.santos@yale.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 11, # Springer Verlag Berlin Heidelberg 2010
245
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V. Lakshminarayanan and L.R. Santos
somewhere else? Am I being charged a fair price for the ticket? Is the person in the
next theater enjoying himself more than I am? And how does seeing the movie
compare to something else I could be doing with my time, like re-reading my
favorite novel? The answers to each of these questions will, undoubtedly, affect the
value of my choice to go to the movies. In addition, even posing these kinds of
questions enlists a vast array of sophisticated mental operations: taking others’
perspectives, making comparisons to other similar situations, imagining the consequences of other hypothetical actions, and so on. Making even very simple
decisions is therefore a tough cognitive task, not just for our own species but for
other nonhuman creatures as well.
The question of how decision-makers should go about making their decisions
is one that has plagued a number of academic fields for some time. One of the
most heralded answers to this tough question comes from the field of classical
economics. The economic approach thinks of the rational decision-maker as
someone who maximizes his utility in any decision-making scenario. Economists
begin by assuming that a decision-maker has certain stable preferences, and that
he makes decisions that optimally satisfy these preferences. If a decision-maker
did indeed make decisions based on factors that were consistent across many
contexts and possessed preferences that were not heavily influenced by the way he
processed information, then one would conveniently be able to interpret his
behavior according to this utility-maximizing approach. A perfectly rational
subject such as this a Homo economicus would therefore be expected to
choose to go to the movies whenever this action would deliver more utility than
the other options that Homo economicus would have to sacrifice in order to attend
the film.
Notably, several important economic phenomena such as pricing behavior
in markets are, in fact, neatly consistent with the Homo economicus utilitymaximizing model.1 Despite the potential advantages of a traditional economic
approach in such cases, a similarly notable body of empirical research suggests
that humans do not always behave like aspiring Homo economici. Many phenomena that are common in human decision-making for example, giving anonymously to charity, helping a stranger cross the street, or switching your
preferences based on context do not necessarily make sense from the perspective
of perfect utility maximization (see reviews in Kahneman et al. 1982; Camerer
1998). Indeed, when considering real human behavior, using Homo economicus as
a normative standard leads to strong and often unrealistic assumptions about how
1
It’s not just humans that sometimes obey the maxims of utility maximization. Indeed, researchers
have previously identified economically rational feeding strategies in a variety of organisms (for
reviews, see Glimcher 2003; Krebs and Davies 1993). To take one famous example, great tits
behave as though they are optimizing their foraging utility when they make decisions about
remaining in (or leaving) a feeding patch (Cowie 1977).
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people ought to act: for example, the assumption that decision-makers always
selfishly maximize their own wealth level (which would presumably conflict with
anonymously giving money away), or the assumption that preferences are stable
regardless of how information is presented. Studies conducted by social psychologists, anthropologists, and economists suggest that, instead of behaving like
Homo economicus, decision-makers’ choices are constrained in predictable
ways: we sacrifice self-interest to invest in fairness norms (see Kahneman et al.
1986) and punish wrongdoers (Henrich et al. 2004), and will change our decisions
systematically based on the wording of different problems (Kahneman and
Tversky, 2000). In short, humans do not actually behave like classical economists
would predict, even when their own self-interest is at stake.
These seemingly “irrational” human tendencies can be considered puzzling not
just from a classical economic perspective, but also from an evolutionary one.
Presumably, the decision-making behavior of modern humans has been shaped
over time by natural selection: the economic strategies observed today are thus
likely to have been shaped over generations of competition for scarce resources.
Because rational decision-making behavior plausibly increases an organism’s
chance of survival, one would assume that our human ancestors might indeed
have figured out ways to optimally maximize their expected payoffs, which would
thus lead to maximizing the ultimate evolutionary currency survival and reproductive success. In this way, our deviation from the optimal normative model of
expected utility maximization poses not just an economic dilemma, but also an
evolutionary one.
How then did humans develop such irrational strategies for making decisions?
Some researchers have attempted to explain the origins of human irrationality
through an adaptive (or, following Tinbergen’s terminology, functional) approach
that examines whether purportedly irrational strategies actually can work to increase survival and reproductive success (e.g., Gigerenzer et al. 1999; Gigerenzer
and Selten 2001). Here, we approach the problem of human irrationality from a
slightly different level of analysis. Specifically, we take a phylogenetic approach to
these seemingly irrational behaviors, one that focuses on whether these tendencies
exist not just in human decision-making but also in the decision-making of our
close primate relatives. We argue that people violate the standard assumptions of
rational choice because their economic decision-making reflects a set of mental
processes that are actually evolutionarily quite ancient, even though they conflict
with certain norms of economic rationality. In this sense, economic irrationalities
may have originated in ancestrally related species, and may therefore emerge
independently of human-specific cultural experiences.
In this chapter, we examine the evolutionary origins of several key features of
our “irrational” economic behavior. We first review each of these phenomena in
human economic behavior and then investigate the evolutionary origins of these
phenomena by exploring whether similar features exist in the decision-making of
closely related primates. We then examine what each of these phenomena means
for the evolutionary origins of human economic behavior more generally.
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V. Lakshminarayanan and L.R. Santos
The Irrationality of Human Preferences
One of the standard assumptions made by rational accounts of human decisionmaking is the idea that decision-makers should have preferences that are stable over
time and consistent across contexts. Indeed, much of the field of economics rests on
assumption that preferences are stable enough to be modeled and used to make
formal predictions. Unfortunately, despite the usefulness of assumptions
concerning the stability of our preferences, there is a growing body of empirical
work demonstrating that humans do not obey these classic assumptions. Much of
the early empirical evidence for these preference violations came from the groundbreaking work of Daniel Kahneman and Amos Tversky (e.g., Kahneman and
Tversky 1979; Tversky and Kahneman 1981, 1986). Beginning in the 1970s,
these researchers developed elegant empirical demonstrations of situations in
which decision-makers reliably failed to make rational choices.
One of Kahneman and Tversky’s most famous early observations was that
people’s choices seem to vary based on the context in which they are made. Such
context-specificity in choice seems to violate a central assumption of rational
choice, the invariance assumption, namely, that people should treat any presentation of the same information identically. Rather than always choosing the option
with the highest expected payoff, people seem to make different decisions when
problems are described in different ways. Take, for example, the two scenarios first
presented by Kahneman and Tversky (1979):
Scenario 1: Gains
You have been given $1,000. You are now asked to choose between:
a. 50% chance to receive another $1,000 and 50% chance to receive nothing [16%
of subjects choose this option]
b. Receiving $500 with certainty [84% of subjects choose this option]
Scenario 2: Losses
You have been given $2,000. You are now asked to choose between:
a. 50% chance to lose $1,000 and 50% chance to lose nothing [69% of subjects
choose this option]
b. Losing $500 with certainty [31% of subjects choose this option]
Note first that the two scenarios are simply alternate formulations of the same final
outcomes: each scenario involves a choice between either a risky final payoff of
either $2,000 or $1,000 [A] or a sure final payoff of $1,500. People may vary in how
much they’d like to gamble by taking choice A over choice B, but they should not
have a different preference for risk-taking in Scenario 1 and Scenario 2, since the
two scenarios actually present participants with the same two choices. But as shown
above, people did show different preferences across the two scenarios. When the
final payoffs were represented or “framed” in terms of a gain, as in Scenario 1, the
majority of people chose to go for the safe option. In contrast, when the payoffs
were framed as losses, most people chose the riskier option. Kahneman and
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Tversky labeled this phenomena the “reflection effect”: when given a safe and risky
choice with the same average payoff, people paradoxically choose the risky gamble
when the options are framed to emphasize losses, but not when the same choices are
presented as gains These examples demonstrate the puzzling result that when
subjects are given alternate formulations of the same set of choices, their preferences flip from being risk-averse to being risk-seeking.
The reflection effect nicely illustrates two central features of human economic
choice that violate the assumptions of a Homo economicus model. The first feature
is that human economic choice is based on relative rather than absolute payoffs.
Kahneman and Tversky described this phenomenon in terms of what they called a
“reference point” bias; people seem to evaluate different options in regards to
a particular (usually arbitrary) reference point (e.g., one’s current asset position
in a particular experimental gamble, etc.). The second feature concerns the fact that
people treat options differently depending on whether their choices lead to positive
(gains) or negative (losses) outcomes relative to their reference point. As the
scenarios above demonstrate, people tended to be risk-averse when dealing with
outcomes that are gains relative to their reference point they chose sure smaller
gains over larger riskier gains but became risk-seeking when dealing with losses
they preferred a risky chance not to have any loss over a sure small loss. Kahneman
and Tversky famously described this phenomenon in terms loss aversion people
work harder to avoid losses than they do to seek out equally sized gains.2 As
Kahneman and Tversky (1979) observed in the above scenario, the disutility that
decision-makers experience from losses tends to be greater than the utility they
experience with identically sized gains. Such loss aversion makes relative changes
in the negative direction far more salient than negative changes in the positive
direction (see Kahneman and Tversky 2000 for review).
These two features of human economic choice reference dependence and loss
aversion explain many real-world irrationalities in human economic decisionmaking. For example, investors are reluctant to sell real estate (Genesove and
Mayer 2001) or stocks (Odean 1998) for less than their buying price, even when
doing so would be profitable. Making choices based on reaching predetermined
reference-levels also leads to poor business decisions in the real world. The
behavioral economist Colin Camerer famously observed that reference dependence
and loss aversion lead New York cab drivers to work less hard than they should
when presented with more lucrative conditions (such as on rainy days when the
supply of potential clients is higher) in part because they tend to work until they hit
2
Kahneman and Tversky (1979, 1981) developed a formal descriptive account of choice behavior
known as Prospect Theory to explain reference dependence and loss aversion. Prospect Theory
posits that people represent choices in terms of their value. Values are described in relative terms
and are measured as losses or gains relative to a specified (yet often arbitrary) reference point.
Because of loss aversion, there is a kink in the value function at the reference point, such that
receiving a gain of a certain size (e.g., gaining $100) is associated with an increase in value that is
far smaller (i.e., about half as large) than the decrease in value associated with an identically sized
loss (e.g., losing $100).
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a certain reference payoff level; this leads cab drivers to mistakenly work longer
days when the supply of clients is low and shorter days when the supply of clients is
higher. Finally, reference dependence and loss aversion are thought to be responsible for an irrational phenomenon known as the endowment effect, a bias in which
people overvalue objects they own. The endowment effect can lead to real-world
problems in bartering and bilateral trade (see Thaler 1980). In one famous example,
Kahneman et al. (1990) gave a group of participants one of two identically priced
items a coffee mug or a box of pens. They then allowed subjects to barter with
each other for the alternative item. Few objects seem to trade hands in part because
owners tended to demand a larger price to sell or trade their object than non-owners
were willing to pay to buy it. Kahneman and colleagues interpreted this phenomenon in terms of loss aversion: owners think of bartering in terms of losing their
owned object. Losses affect well-being more severely than equally sized gains,
which can lead an owner to systematically over-estimate how much money he
should demand when selling his possession. In fact, Kahneman and colleagues
observed that owners sometimes demand nearly twice as much as buyers are willing
to pay in order to give up an owned item.
In summary, humans decision-making appears to exhibit two phenomena that
violate the tenets of rational utility-maximization and lead to irrationalities in
economic markets. First, and most critically, human think about choices in relative
terms; rather than focusing on our absolute payoffs, we instead make decisions with
reference to arbitrary information that is typically irrelevant to the problem at hand.
Second, we are more sensitive to losses relative to reference points than we are to
equally sized gains. Such loss aversion can lead us to think about problems
differently when they are framed in negative versus positive terms.
11.3
Irrational Equity-Seeking and the Emergence
of Human Fairness Norms
The framing effects discussed above are not the only situations in which people
exhibit loss aversion and relative preference switches across contexts. Another
situation in which people exhibit strikingly irrational tendencies to forgo earnings
is when thinking about their own payoffs relative to those of others. Behavioral
economists have long observed that one of the most salient reference points for our
own decisions is “what others are getting.” Indeed, researchers have observed that
people are often willing to give up much in terms of their absolute happiness and
wealth in order to be better off than others. In one striking example, Solnick and
Hemenway (1998) gave people the choice between earning $50,000 while others are
earning $25,000 or earning $100,000 while others are earning $250,000. Surprisingly, almost half of the respondents were willing to take the first choice, despite the fact
that this choice would cut one’s actual wealth level and purchasing-power in half.
Human decision-makers’ sensitivity to the relative payoffs of others is central
to another way that human economies violate self-interested utility-maximization:
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we care a bit more than a Homo economicus should about equity. For example,
Kahneman et al. (1986) identified a number of economic scenarios in which
people’s interpretation of what should be done depends not on a decision’s profitability, but instead on a decision’s perceived equitability. Consider the following
scenario:
A small company employs several workers and has been paying them average wages. There
is severe unemployment in the area and the company could easily replace its current
employees with good workers at a lower wage. The company has been making money.
The owners reduce the current workers’ wages by 5%.
Kahneman and colleagues observed that 77% of people found the company’s action
in this case to be unacceptable. Interestingly, people’s perceptions seem to change
depending on the payoff received by the company; when the researchers changed
the above scenario to read that the company “has been losing money,” only 32% of
people thought the company’s wage cuts were unacceptable. In this way, people
seem to use fairness constraints over economic constraints when judging the
acceptability of economic decisions; as in the above, people judge a wage-cut for
the employees as more acceptable when the company is also experiencing a loss
than in cases when it is not.
Our human concern for equitable outcomes seems to lead to a number of realworld violations of selfish utility-maximization. First, people often give more than
is minimally required in experimental giving or trust games. One such scenario
involves a well-known economic game known as the dictator game (e.g., Camerer
2003; Henrich et al. 2004). In this game, two anonymous individuals play a oneshot game in which one individual, the dictator, must decide how to split a cash
offer (e.g., $10). Because the game is anonymous and played only once, there is
little incentive from a utility maximization perspective for the dictator to give any
of the cash allotment to the second player. Nevertheless, participants do not
selfishly keep all of the money. Indeed, most people give nonzero offers with
many people splitting the allotment 50 50.
A second set of cases in which people exhibit a preference for fair behaviors
concerns altruistic punishment instances in which people are willing to sacrifice
individual gain in order to punish wrongdoers (e.g., Kahneman et al. 1986; Zizzo
and Oswald 2001; Camerer 2003; Fehr and Rockenbach 2003). Using experimental
markets, researchers have found time after time that participants are willing to exact
costly punishment even in cases where these behaviors yield no material gain for
the punisher or gift-giver (Fehr and Gächter 2002). For example, consider the
ultimatum game, a modified form of the dictator game. In this game, the first
individual (the proposer) can again split the money any way he sees fit, but the
second individual (the receiver) can then choose either to accept or reject the
proposer’s offer. If the receiver rejects the offer, then neither the proposer nor the
receiver receives any money. From the perspective of self-interest, one might
expect receivers to accept any nonzero offer. After all, any offer even one of
$0.01 is better than nothing. It, therefore, follows that we would expect extremely
low offers from proposers in anonymous one-shot games. In contrast, researchers
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robustly observe that offers lower than 25% are nearly always rejected, and
proposers frequently offer much higher than this threshold (Güth et al. 1982).
Clearly, neither the proposer nor the receiver is maximizing his own personal
payoffs in this anonymous one-shot game. If they were, then proposers would give
away as little as possible (e.g., $0.01 out of a possible $10), and receivers would be
willing to accept these extremely small offers. Nevertheless, the fact that receivers
deviate from this profit-maximizing strategy indicates that they may actually be
reacting to the relative fairness of proposers’ offers. Indeed, it seems that participants exhibit a cognitive capacity to detect the unfairness of certain rewards
namely, they possess the ability to compare their reward to that of another individual, to react negatively to unfair cases, and to forego small offers in order to punish
those who created the inequity.3
11.4
The Evolution of Primate Economic Strategies: Monkey
Markets
As reviewed above, a wealth of empirical work has established that loss-aversion
which requires sensitivity to others’ rewards, and at least a minimal capacity to
understand the relative quality of a reward is robust in adult humans. A similarly
large body of work has demonstrated that human decision-makers attend to and
obey fairness constraints when making choices. Are these capacities specific to our
own species? Or do other species possess similar tendencies? Unfortunately, less
work to date has addressed whether similar biases are shared across the animal
kingdom and, more specifically, whether they are present in the decision-making
strategies of other species within the primate order. The remainder of this chapter
will review the work that has been done to date exploring whether similar irrationalities exist in the decision-making of closely related primates.
To begin, do primates share our human economic sensitivity to relative rewards
and payoffs? Researchers have known for some time that primates seem to evaluate
their rewards not merely in absolute terms, but also on the basis of expectations. In a
famous early example, Tinklepaugh (1928) presented macaques monkeys with a
memory game in which they had to wait for a hidden reward over a delay period.
During some trials, he switched the kind of hidden reward while the monkey was
waiting. When monkeys found a reward that was smaller than the one they had
originally seen hidden, they reacted negatively (see Watanabe 1996; Santos et al.
2002 for a more recent version of this hiding expectancy task). Monkeys were
happy to accept a small reward (e.g., a slice of lettuce) if they were expecting a
small reward, but refused to eat a small reward if they were expecting a tastier one
3
Interestingly, recent work suggests that the cognitive precursors of inequity aversion emerge
quite early in human development (Fehr et al. 2008; Lobue et al. in press). For example, using a
simplified ultimatum game, Fehr et al. (2008) showed that children as young as eight years exhibit
an aversion to inequity in anonymous one shot games.
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(e.g., a banana). These results suggest that monkeys might also evaluate payoffs in
terms of reference points.
To get at this issue more carefully, we (Chen et al. 2006) decided to explore the
possibility that primates might be reference-dependent in a context more similar to
that of human economic choices. Specifically, we examined whether primates
would evaluate relative payoffs in a real economic market one that used “monetary” gambles similar to the scenarios presented to humans. To this end, we trained
a group of capuchin monkeys (Cebus apella) to trade small metal tokens with
human experimenters in order to obtain food (see Westergaard et al. 1998; Brosnan
and de Waal 2003, 2004; Addessi et al. 2007 for similar token trading methodologies). In each study, we gave monkeys a “wallet” of small metal tokens, and
reinforced them with food rewards for handing these tokens back to a human
experimenter. Once subjects understood that they could “buy” food with tokens,
we looked at whether they exhibited the same kinds of strategies as humans do in
their experimental markets. We gave monkeys a limited budget of tokens and
measured how they allocated these tokens across two experimenters, each of
whom offered different kinds of foods at different prices. In this way, we could
establish a measure of the monkeys’ “preference” for each of the experimenters,
and could thus explore how those preferences changed based on prices and framing.
We first explored whether monkeys paid attention to the “price” of different
kinds of food. In our experiment, monkeys could use each token to purchase either
an apple piece from one experimenter or a grape from a second experimenter. In
this way, the experimenters initially sold different goods at equal prices. Because
monkeys like apples and grapes equally, they spent approximately half of their
token budget on each good. Once monkeys had gotten used to this price, we
introduced what economists call a “price shock,” and observed whether the
monkeys adjusted their spending. Essentially, one of the two goods say, apples
went on sale such that a single token now bought two units of the same item. We
predicted that if monkeys reacted rationally to the price shock that is, if their
purchasing habits were related to the price of the goods, then they should change
their pattern of purchasing in order to buy more of the cheaper good. Our subjects
did just this, showing that their monkey token economy shared at least some
features of human economies.
In a second study, we established that capuchins rationally spend more resources
to obtain a greater reward. To do so, we adapted our experimental market so that the
two experimenters provided the same type of reward for a token apples but
differed in the number of apple pieces they delivered and the frequency that they
provided these rewards. We again gave subjects a choice between two experimenters with whom they could trade their limited budget of tokens. The first experimenter initially displayed two pieces of apple before receiving a token from the
subject, but then when paid sometimes removed one of these pieces and gave
the monkey only one piece of apple. In contrast, the second experimenter always
displayed one piece of apple and delivered exactly this quantity in exchange for a
token (neither adding nor subtracting from it). This resulted in a situation in which
the risky experimenter provided an average payoff that was one and half times the
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payoff of the safe experimenter. Given this choice between a gamble with an
average payoff of one and a half apple pieces and a sure choice of only one
apple, capuchins preferred the gamble with the larger expected value (Chen et al.
2006). In this way, capuchins behaved rationally in at least some aspects of our
trading market: they spent more resources on a cheaper food than an equally
valuable but more expensive alternative, and chose options which maximize
expected value even when this required choosing a risky choice over a safe one.
11.5
Primate Economic Irrationalities: Do Monkeys Exhibit
Human Economic Biases?
Having established these basic similarities with our own markets, we then investigated whether capuchins shared some of the irrational preferences that commonly
influence how people allocate resources in particular, reference dependence and
loss aversion. Specifically, we wanted to see whether capuchins base their choices
not only on the absolute outcome of their choice but also on their initial expectation.
To begin investigating whether capuchins share our human irrational purchasing
tendencies, we again used our experimental two-trader market but this time did not
vary either the kind or amount of food that was given to the subjects; both
experimenters delivered food of the same expected value. Instead, we varied
whether the experimenters added to or subtracted from the amount of food that
they initially showed to the subject. Sometimes the experimenters gave more apples
than they originally showed to the subject, and sometimes they gave less. This
allowed us to explore whether subjects base their choices on the expectation (or
reference point) created before each trade. In one study, we gave the monkeys a
choice between two experimenters who both delivered the same average expected
payoff of one and a half pieces of apples. For one experimenter, this average payoff
was less than the monkeys’ expectation. This first experimenter began every trade
by showing the monkey two pieces of apple, but when paid either delivered two
pieces of apple as shown or removed one to deliver only a single apple piece. The
second experimenter, in contrast, gave more on average than the monkeys expected.
This second experimenter began by showing the monkey only one piece of apple,
but then when paid either delivered that one piece as promised or added a second
piece for a payoff of two apple pieces. Despite the fact that the average payoff was
identical across the two experimenters, our monkeys traded more tokens with the
second experimenter, the one who offered more than the monkeys’ initial expectation. This pattern of performance suggests that capuchins take into account reference points, just as humans do.
But do capuchins also exhibit loss aversion? To investigate this, we again had
both experimenters deliver an identical final payoff (in this case, a single apple
piece). The first experimenter, however, initially began by displaying two pieces of
apple at the start of each trial, but then subtracted one piece before giving the
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monkey his payoff. The second experimenter began each trial with a single piece of
apple, and provided exactly this quantity. Again, despite the fact that the final
outcome of trading was the same across the two experimenters, the monkeys traded
more of their tokens with the second experimenter and avoided the first experimenter who initially offered a larger quantity and then subtracted during the trade
(Chen et al. 2006). Like humans, monkeys appear to avoid choices that involve
perceived losses.
In a final demonstration of how capuchin monkeys share our biased choices, we
gave the monkeys an opportunity to be the “owner” of one good that they could
barter for an equally priced good. Our aim was to see whether monkeys, like
humans, exhibited an endowment effect, overvaluing the goods that they owned.
To do so, we (Lakshminarayanan et al. 2008) set up a new market in which
capuchins received foods that they could either keep and eat, or trade away for an
equally valuable alternative. In the first phase of our experiment, we found two
foods that the capuchins preferred equally. That is, given a small budget of tokens,
they spent approximately half of their tokens on one these goods (e.g., cereal
pieces) and half of their tokens on the other good (e.g., fruit pieces). We then
made the monkeys owners of one of the two goods, say cereal, which they could
then trade for the equally valuable fruit pieces. Like humans, capuchins preferred to
retain the kind of food they owned, and rarely traded away food from their budget
even for an equally preferred kind of food. Even across control conditions in which
monkeys were compensated for any perceived transaction costs, our capuchin
participants failed to trade goods they owned for equally preferred or slightly
more valuable goods. Brosnan and colleagues (Brosnan et al. 2007, 2008) reported
a similar endowment effect in chimpanzees (Pan troglodytes). Chimpanzees also
refused to trade owned objects for equally valued alternatives. Like humans,
capuchins and chimpanzees seem to avoid losing owned objects even if it means
foregoing the gain of an equally valued alternative object.
The capuchin trading studies highlight the fact that at least one species of
primate seems to share our human economic biases. Like humans, capuchin
monkeys tested in an economic market exhibited reference dependence, loss aversion, and endowment effect. They too appear to evaluate their payoffs in relative
terms and weigh losses more heavily than equally sized gains. The demonstration
that other primates react similarly to humans in these market studies raises the
question of whether nonhuman primates also share our relative evaluations in the
social domain. Do primates also compare their payoffs to those of others? Do they
react negatively when they get less than their equitable share? In short, do primates
share our human concerns for equity and fairness considerations?
11.6
Are Primates Irrationally Equity-Seeking?
Excitingly, recent work in primate cognition has devoted considerable attention to
the question of whether primates share a sense of equity and fairness (see reviews in
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V. Lakshminarayanan and L.R. Santos
de Waal 1996, 2008; Brosnan 2006; Silk 2007, Silk and Boyd this volume). Much
of this work has examined whether primates are also averse to inequitable outcomes. In a landmark series of studies, Brosnan and colleagues (Brosnan and de
Waal 2003; Brosnan et al. 2005; Brosnan 2006; van Wolkenten et al. 2007)
investigated whether two primate species capuchins and chimpanzees would
forego an otherwise desired food reward that was delivered unfairly in a trading
task. In the original capuchin study (Brosnan and de Waal 2003), monkeys watched
an experimenter trade with a conspecific and pay that conspecific either a lowvalued (e.g., cucumber) or high-valued food reward (e.g., a grape). After seeing the
conspecific’s payoff, the subject monkey got its own chance to trade and was paid
only the low-valued reward. Although capuchins were happy with a low-valued
cucumber payoff when the other monkey also got a cucumber, capuchins rejected
the cucumber when the other monkey got a grape. Surprisingly, in this and other
studies (e.g., Brosnan et al. 2005; Brosnan 2006; van Wolkenten et al. 2007),
capuchins and chimpanzees were willing to reject an otherwise desired food reward
if they previously observed another conspecific obtaining a better reward for the
same amount of work (but see Dubreuil et al. 2006; Roma et al. 2006 for failures to
replicate this effect with a different paradigm). In a similar vein, capuchins have
been shown to spontaneously share food with the individuals that helped to work for
that food (de Waal and Berger 2000).
Taken together, capuchins and chimpanzees tested in token trading tasks seem to
act in ways that are consistent with notions of inequity aversion, even when doing
so results in losing an otherwise valued piece of food (e.g., Brosnan and de Waal
2003). Like humans, these primate species seem to evaluate their own payoff
relative to that of others and react negatively when others “earn” more than they
do for equal amounts of work. To summarize, then, basic components of equity
for example, judging one’s own reward against the payoffs of others are present in
several primate species.
That said, there does appear to be some potentially important differences
between the equity seeking strategies observed in human and nonhuman primates.
As reviewed above, humans often act in ways that promote equity not just for their
own outcomes, but also for those of others. In the parlance of experimental
economics, humans are averse not just to disadvantageous inequity situations in
which an actor gets relatively less than other individuals but also to advantageous
inequity situations in which the actor gets relatively more than other individuals.
Human decision-makers often act to reduce inequity even in cases in which they
themselves are not negatively effective (e.g., providing equitable offers in one-shot
dictator games, giving to charity, working for social justice, etc.). To date, there is
relatively little evidence that any nonhuman primate does the same. Brosnan (2006)
reported, for example, that capuchins who received the grape in her task never
rejected it or offered to share it with the monkey who received the unfair cucumber
payoff. In fact, she noted that higher-paid monkeys occasionally stole rejected
cucumbers from their lesser-paid cagemates. Failures to observe advantageous
inequity aversion in these and other tasks (see Silk and Boyd this volume for an
elegant review of striking demonstrations of primates’ failures to provide equitable
11
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rewards to others even at low cost to themselves) suggest that humans may be
unique in their advantageous inequity concerns.
11.7
Conclusions About the Evolution of Human
Economic Strategies
Like humans, nonhuman primates also violate at least some of the standard tenets of
expected utility maximization. Several primate species show economic preferences
that are consistent with loss aversion, reference dependence, and disadvantageous
inequity aversion. To summarize, nonhuman primates seem to share many humanlike irrational decision-making biases, despite the fact that making choices in this
way would not lead to economically rational choices in the long run.
The fact that researchers have observed decision-making biases, such as
inequity-aversion, loss-aversion, and prosocial-giving outside of our species, is
striking for at least two reasons. First, these findings suggest that despite the fact
that human irrational decision-making violates the norms of rational economic
behavior, these behaviors seem to represent cognitive strategies that have been in
place for millions of years of evolutionary history. Even though loss-aversion and
inequity norms may not contribute straightforwardly to better economic choices,
these strategies have still been around for quite a long time. Indeed, results
demonstrating economic irrationalities and fairness-like norms in capuchin
monkeys suggest that similar biases may have existed in our common ancestor
with New World primates over 35 million years ago.
Second, the existence of several of these biases in nonhuman primates may
provide a valuable hint for understanding why these biases may be so persistent and
robustly demonstrated in our own species. Although the biases we have discussed in
this chapter are typically considered non-normative from a classical economic
perspective, several researchers have argued that such irrational strategies may in
fact be rather smart in certain contexts (e.g., Gigerenzer and Goldstein 1996;
Gigerenzer et al. 1999; Gigerenzer and Selten 2001). Gigerenzer and colleagues,
for example, have long speculated that seemingly “irrational” decision-making
biases may allow decision-makers to quickly navigate what would normally be
cognitively-taxing complicated computations. In this way, the irrational strategies
we have observed may be better from an evolutionary perspective than originally
thought in part because they are faster and cheaper than a more rational approach.
Our observation that such strategies are evolutionarily ancient provides credence to
this view, and suggests that researchers might need to rethink what should be
considered the best normative model of decision-making.
Finally, we end with an exciting implication of the recent work on primate
irrationalities. Although the nature of nonhuman primate economic strategies is still
far from understood, researchers now have a variety of methods in place that can be
tweaked to investigate many aspects of primates’ social and economic preferences.
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V. Lakshminarayanan and L.R. Santos
These methods create exciting possibilities for future research across human and
nonhuman species. The hope, then, is that the next decade of work in this area can
determine not just whether primates share different human economic behaviors, but
also whether the mechanisms that underlie these behaviors are also shared broadly
across the primate order.
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Part VI
Language, Thought & Communication
Chapter 12
From Whence the Captains of Our Lives:
Ultimate and Phylogenetic Perspectives on
Emotions in Humans and Other Primates
Daniel M.T. Fessler and Matthew Gervais
Let’s not forget that the little emotions are the great captains of our lives and we obey
them without realizing it.
Vincent van Gogh
Abstract We outline an evolutionary approach to emotions intended to spur
further research on the subject in humans and nonhumans alike. Combining adaptationist, comparative, and phylogenetic analyses, we seek to illuminate the functions that emotions fulfill, the reasons why they take the forms that they do, and the
extent to which they are shared across species. Using similar logic, we distinguish
between emotions and attitudes, cognitive representations of other actors that are
both informed by, and potentiate, emotions. Employing select emotions as illustrations, we discuss a taxonomy of emotions. We begin with emotions that address
adaptive challenges common across animals, and which require minimal cognitive
capacities, features that make it likely that they are widely shared across species.
Next, we consider emotions involved in elementary sociality, a category further
elaborated in emotions playing a role in parenting and pair-bonding. In light of the
importance of dyadic cooperative relationships in primate societies, we describe a
set of emotions undergirding such relationships that we expect to be shared by
human and nonhuman primates. To a more limited degree, we expect pan-primate
similarities with regard to vicarious emotions, those wherein the individual experiencing the emotion is affected only indirectly by the eliciting event. The greater
range and complexity of human social relationships, including the human
D.M.T. Fessler (*)
Center for Behavior, Evolution & Culture, Department of Anthropology, University of Anthro
pology, Los Angeles, CA, USA
e mail: dfessler@anthro.ucla.edu
M. Gervais
Center for Behavior, Evolution & Culture, Department of Anthropology, University of California
Los Angeles, Los Angeles, CA, USA
e mail: mgervais@ucla.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 12, # Springer Verlag Berlin Heidelberg 2010
261
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D.M.T. Fessler and M. Gervais
propensity to essentialize cultural groups, extend the class of vicarious emotions
beyond anything evident in nonhumans. Finally, underscoring the importance of
culture in human evolution, we examine moral emotions elicited by norm violations, a pattern unique to humans.
12.1
Introduction
It has long been recognized that an evolutionary perspective is useful in investigating
emotions. In this chapter, we employ two complementary applications of evolutionary theory. One is adaptationism, viewing emotions as discrete adaptations for
behavior regulation that evolved in response to challenges repeatedly confronting
organisms over evolutionary time (Frijda 1986; Nesse 1990; Lazarus 1991; Ekman
1992; Tooby and Cosmides 2008). Each emotion is elicited by cues to the presence of
a recurring fitness-relevant challenge or opportunity, and each coordinates information-processing, motivational, and physiological systems to respond adaptively.
The second application is phylogenetic and comparative, examining (1) the actual or
expected taxonomic distribution of emotion systems based upon patterns of descent,
the distribution of the relevant adaptive problems, and cognitive capacities, and (2)
the logic of each system’s transformation through descent as a result of the interaction
of system evolvability, system affordances, and the structure of adaptive problems.
Neither of these applications of evolutionary theory to the study of emotions has yet
been fully realized, and the synergy resulting from their integration is often overlooked. Our goal is to further develop these approaches, to demonstrate their complementarity, and to employ them in examining some aspects of emotions that
contribute to the “gap” between humans and other primates.
Darwin’s ([1872] 1955) pioneering work on emotional expressions employed a
comparative perspective in order to: (1) substantiate the utility of general explanatory
principles by application to all species; (2) demonstrate human descent from nonhuman ancestors; (3) evaluate the innateness of human expressions by showing similarity to other species; and (4) explain some human characters as vestigial traces of
ancestral forms. The first two goals have been achieved. Ekman and other modern
students of expressions advanced the third goal, and many human facial expressions
are now considered pan-cultural. This leaves Darwin’s fourth goal. While extreme
vestigialism has rightly fallen from favor, many emotions do exhibit evidence of deep
histories of successive modification. We will present several examples of emotional
systems apparently evolved for one purpose that were subsequently modified to serve
a different purpose. In some cases, the properties of an ancestral emotional system
appear to have “preapted” it (Gould and Vrba 1982) to the task demands of a newly
arisen adaptive problem (e.g., Rozin et al. 1997; Fessler 2004; Gervais and Wilson
2005). In a number of cases, an extant emotional action tendency has been put to
novel use by modifying the eliciting conditions (cf. Tooby and Cosmides 1989; Rozin
1996; Keltner and Anderson 2000). Yet, even with subsequent secondary selection,
this process does not necessarily erase the legacy of past selection; substantial
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residues of former function sometimes remain, and it is only with reference to
ancestral traits that such features become intelligible. Applying this consideration
when reverse engineering a trait (“reverse tinkering”; Andrews et al. 2002; Gangestad
and Simpson 2007) illuminates the kludge-like structure of emotions that have served
multiple functions over evolutionary time.
As the above discussion suggests, phylogenetic and comparative approaches
both constrain and inform adaptationist hypotheses (see Maestripieri 2003b; Gangestad and Simpson 2007; Gosling and Graybeal 2007). In addition to controlling for
nonindependence of trait correlations in comparative research (Nunn and Barton
2001), “tree thinking” helps to specify the time frame within which the target
properties of a trait were selected, while suggesting an ancestral state from which
the trait was derived. This refines the task of reverse engineering by rephrasing the
ultimate question as: Why did this form emerge given the form from which it
emerged? A phylogenetically informed comparative approach can help to parse a
trait into aspects that were impacted by selection pressures operating during
particular periods in the lineage. This approach highlights the process of secondary
selection through which (contra Gould and Vrba 1982; Gould 1991) an extant
system is not merely exapted (put to new use), but is further subjected to specializing selection for that new function. This process, which we term cooptation, has
rarely been foregrounded in the study of the mind.
Just as comparative and phylogenetic approaches offer benefits to students of
human emotions, so too can adaptationist perspectives enhance the study of nonhuman primate emotions. Because emotion is difficult to quantify, most primatologists have sidestepped the study of emotion, although this is beginning to change
(e.g., de Waal 1996; Aureli and Schaffner 2002; Maestripieri 2003a). The conjunction of adaptationist and phylogenetic analyses indicates where one can expect to
find homologues of human emotions. It should be possible to predict the taxonomic
distribution of emotion traits through a consideration of (1) the phylogenetic
relationships among species, (2) the selection pressures thought to produce and
maintain a given trait, and (3) the cognitive capacities that constitute prerequisites
for the trait. This generates a taxonomy of emotions, ranging from ancient emotions
expected to be present in all vertebrates, to conserved pan-primate emotions, to
derived emotions expected to be unique to our species. Because explorations of
primate emotions are in their infancy, we cannot conduct rigorous tests of our
phylogenetic hypotheses. However, there is reason for optimism, as investigators
are beginning to employ new methods to probe primate motives (e.g., Silk et al.
2005; Warneken et al. 2007; de Waal 2008; Lakshminarayanan and Santos 2008,
Silk and Boyd, this volume), and obstacles to the study of primate emotions are not
insurmountable. Below, we sketch a set of phylogenetically informed adaptationist
proposals intended to provide starting points for future studies of emotions in both
humans and nonhuman primates. This is not an exhaustive description of these
species’ emotional repertoires. Rather, our goal is to demonstrate the form that we
think such descriptions should take, and to spark further discussion.
A note on terminology: because cultures differentially emphasize or ignore
facets of the pan-human spectrum of emotions, any language’s emotion lexicon is
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but a crude gloss for the underlying phenomena. Accordingly, the emotion terms
used here should be viewed as heuristic labels for adaptations that are not necessarily isomorphic with Anglo-American folk psychology.
12.2
Ancient, Relatively Conserved Emotions
All vertebrates confront a common set of elementary adaptive challenges, including
avoiding injury and disease, and finding mates. The core motivational systems that
evolved in ancestral vertebrates in response to these selective pressures have likely
been largely conserved in descendant taxa. Because of their ancient origins, the
cognitive prerequisites for these adaptations are relatively limited. In general,
attention to simple exogenous or endogenous cues suffices to identify the eliciting
conditions. Many of these eliciting cues are sufficiently uniform across circumstances to allow canalization, obviating learning. Below, we sketch some of the
relevant adaptive challenges and corresponding emotions. Then, we consider how
natural selection refined these basic building blocks
Avoiding imminent threats to life and limb is a fundamental determinant of
fitness; fear marks the presence of such threats, motivating flight as a principal
behavioral outcome, with fighting as a secondary outcome when the threat is
animate and escape is appraised as impossible. The neural underpinnings of fear
appear to be highly conserved, certainly among mammals, and arguably among
vertebrates (Panksepp 1998; Braithwaite and Boulcott 2007; Öhman et al. 2007).
Fear expressions closely paralleling those present in humans are recognizable in
many primates (Parr et al. 2007). This uniformity potentially reflects selection
having conserved an adaptive form. Human experimental results support Darwin’s
([1872] 1955) supposition that many facial expressions are functional beyond their
signal value, as the fear expression enhances perceptual acuity and reaction times to
threat (Susskind et al. 2008).
The avoidance of toxins and pathogens is another elementary adaptive challenge. In humans, disgust is involved in the rejection of ingestible contaminants and
avoidance of cues reliably associated with pathogen risk (e.g., rotting flesh or
feces). Oral contact is a powerful disgust elicitor (see Fessler and Haley 2006),
and disgust reactions involve gastrointestinal rejection, suggesting that preventing
intoxication was the original function of this emotion. Disgust was likely subsequently coopted for prophylaxis by extending elicitors to include contact with cues
of disease (see Kelly in prep). Human disgust can also be elicited by representational contamination (transfer of disgust-eliciting status through contact with an
elicitor absent perceptible changes) and a variety of symbolically mediated events.
The extension of disgust to such cues leads some to argue that disgust is a culturally
constructed defense against existential anxiety (Rozin et al. 2000). However, this
position overlooks design features evident in the avoidance of contact with stimuli
ecologically associated with disease transmission. Because microbes cannot be
detected by the human eye, yet spread through physical proximity, it is beneficial
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that the set of cues that elicits disgust includes representational contamination
(Curtis and Biran 2001; Fessler and Haley 2006, Kelly in prep).
An ingestion rejection system is found in all mammals. Across primates, we
predict that the degree of elaboration of this system (i.e., number of eliciting cues;
flexibility of response) will vary as a function of carnivory, as meat is a potent
source of disease. Similarly, we expect some form of pathogen-avoidance mechanism to be found in all mammals; because sociality is a determinant of disease risk,
these mechanisms should vary as a function of sociality. In contrast, because it
requires abstract conceptual abilities, we expect representational contamination to
be quite rare. In a number of species, investigators have documented behaviors
possibly explicable as prophylaxis, including avoidance of parasitized conspecifics,
and grooming and feces avoidance (see Nunn and Altizer 2006: 159 170 for review
on primates). However, it is not known whether such behavior is motivated by a
disgust-like emotion, as it is in humans.
Ultimately, reproduction, not survival, determines fitness, so we expect all
sexually reproducing species to be equipped with an emotion akin to lust, the
principal motivator of sexual behavior. Although the frequency, form, and elicitors
for sexual behavior vary across mammals, the ubiquity of goal-oriented behavior
leading to copulation is consistent with such an emotion being widespread. There
are several parallels in the task demands associated with mate-seeking and foraging
(appetitive cycles; resource-seeking behavior; discrimination among resource
options), and, in mammals, there is overlap between the neurophysiological systems regulating proceptive sexual behavior and those regulating ingestive behavior
(reviewed in Fessler 2003). This suggests that systems regulating ingestion may
have constituted building blocks from which systems regulating sexual reproduction were subsequently constructed. Disgust, antithetical to both hunger and lust,
plays a central role in human inbreeding avoidance (Lieberman et al. 2003, 2007;
Fessler and Navarrete 2004); likewise, women’s sexual disgust sensitivity increases
around ovulation, possibly functioning to reduce contact with suboptimal reproductive partners at peak fertility (Fessler and Navarrete 2003). Most primates exhibit
marked inbreeding avoidance (Paul and Kuester 2004; Muniz et al. 2006), and it is
possible that a similar emotional system is involved. Indeed, we suggest that both
the ingestion regulation and sexual regulation facets of disgust are pan-mammalian.
12.3
Emotions Associated with Elementary Sociality
As illustrated by disgust at the prospect of sex with individuals identified as close
kin, emotion elicitation is contingent on appraisal, the process of construing the
nature and meaning of a situation (Scherer 1999). Sociality introduces a complex
set of adaptive challenges because there are many ways in which conspecifics can
affect fitness, hence many distinct appraisals and consequent emotional responses
should attend social interaction. At the most elementary level, because competition
for resources is a key determinant of fitness, we expect distinct emotions, with
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corresponding appraisals, to address competition. From the perspective of a resource
holder, an attempt to displace one from a resource should be appraised as a transgression, the infliction of an unwelcome (i.e., fitness-reducing) cost, or threat
thereof. In humans, transgressions elicit anger, motivating aggressive retaliation
when feasible. While varying in degree across species, many social animals exhibit
aggressive responses to actions that investigators plausibly interpret as transgressions. Hence, an elementary anger-like emotion and its eliciting appraisal are
probably both ancient and widely shared. This conclusion is supported by the
extensive conservation across mammals of the neurophysiological systems underlying aggression (e.g., Dierick and Greenspan 2007).
In humans (particularly young men), anger often motivates disproportionate
responses to transgression. Disproportionate responses may be advantageous
because they serve as a signal that transgressing is dangerous, generating reputational
benefits that deter further transgressions (reviewed in Fessler in press). Reputationbased strategies such as this require repeated interactions, the ability to identify and
recall individual actors, and the capacity to acquire information as a third-party
observer. All of these features are likely present among many social primates
(Cheney and Seyfarth 2005), and age-related patterns of impulsive aggression
may parallel those found in humans (cf. Fairbanks et al. 2004). Thus, it is likely
that the central features of the anger adaptation are a shared ancestral trait among
primates.
While the question of applying the appropriate appraisal to a given event is never
trivial, this task is particularly complex in the social domain because the range of
fitness consequences of social interactions is so large. This complexity is multiplied
by the fact that, due to repeat interactions, appraisals of current social actions hinge
on past events. We suggest that the substantial informational demands of applying
the appropriate appraisal to social events are managed, in part, through attitudes,
durable, hierarchically organized representations of previously appraised traits and
relational outcomes that potentiate differential emotional readiness toward others.
Attitudes and emotions are reciprocally related, as attitudes are updated by emotions, and subsequently help to regulate emotions by shaping appraisals. Responsive to the fitness-relevant traits and behaviors of other individuals, attitudes are
continuously adjusted over time, thus constituting summary representations that
proxy the future fitness implications of all past interactions. Attitudes allow individuals to represent their current relations to others without the need for explicit
bookkeeping or recall of all encounters. Via their role in appraisals, these representations adaptively regulate current behavior. Attitudes can thus be conceptualized as “internal regulatory variables,” in the sense that they functionally translate
past appraisals into current behavior regulation (see Tooby and Cosmides 2008).
This proposal obviates the need to posit complex cognitive operations in some
relational domains (the potential complexities of which are discussed in Cords
1997; Silk 2003; see also Aureli and Schaffner 2002; Aureli and Whiten 2003).
Likewise, congruent with extensive social psychological findings on implicit
attitudes (Greenwald and Banaji 1995), our position does not require positing
problematic conscious processes.
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The interaction of appraisals, attitudes, and emotions is evident in the case of
envy, the emotion associated with the goal of displacing a resource holder. In
humans, envy is elicited by the appraisal that, in a zero sum situation, the target
possesses an advantage to which the actor is entitled. The target is therefore
represented as a rival, and there is an enduring attitude of hostility toward the target
(see Smith and Kim 2007) such that the actor is willing to inflict costs on the target.
Likewise, potentiated by this attitude, attempts by the target to retain or increase
control of the resource elicit anger from the actor. Envy is thus usefully contrasted
with admiration, in which pursuit of non-zero-sum advantages enjoyed by another
does not involve an appraisal of entitlement, an attitude of hostility, or anger in
response to the target’s attempts to control the resource (more on admiration later).
Conflict over zero sum resources is central to much social behavior, hence, we
suggest that the core components of the envy system are both ancient and widely
shared. Observations in multiple species of distress at inequitable distributions
(reviewed in Brosnan 2006) are congruent with this suggestion.
Competition and hostile intent are also central to jealousy. Deriving from a
proprietary attitude toward a relationship partner and a corresponding appraisal of
potential interlopers as transgressors, this emotion motivates attempts to maintain
exclusivity by warding off rivals and restricting the partner’s options (reviewed in
Smith and Kim 2007). While sociality does not necessitate the formation of discrete
relationships, well-differentiated relationships exist in many primate species (see
Silk 2007). If such relationships are widespread, jealousy may be widely shared.
Elicitors for jealousy will depend on the nature of the threats posed to a relationship.
The utility of biparental care in humans leads to sex-specific adaptive challenges,
namely the possibility of female extra-pair copulation (leading to misallocation of
male parental investment) and male abandonment (leading to reduced female
access to resources). A growing literature investigates the corresponding sexspecific relative importance of sexual and emotional infidelity as elicitors of mating
jealousy (reviewed in Haselton and Ketelaar 2006). While extensive biparental care
is rare among primates, we expect the same logic of an actor-centered appraisal of
different threats to a relationship to apply across species.
In most social animals, conflicts establish and maintain dominance rank, which
then determines priority of access to resources. As Darwin ([1872] 1955) suggested,
ethology provides clues to both the phylogeny and ultimate function of emotions,
and this is particularly evident in the case of dominance interactions. Displays often
precede, and sometimes obviate, conflict, and threat displays that reduce the costs
of conflict are a pervasive aspect of social behavior in primate groups. In most
species, body size is a key determinant of success in combat, and, correspondingly,
threat displays generally involve an exaggeration of body size. Direct attention
(staring) is often a feature of such displays (reviewed in Fessler 2004). In some
cases, it is beneficial to acknowledge subordinate status, and appeasement displays
are counterpoints to threat displays. Appeasement displays generally involve an
attempt to minimize apparent body size and direct attention away from the aggressor. In humans, threat displays are associated with an emotion we term proto-pride,
and appeasement displays are associated with proto-shame (see Fessler 2007;
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see also Tracy and Matsumoto 2008). Proto-pride is elicited when an actor
appraises herself as occupying a superior position in a social hierarchy and interacting with a subordinate in a context in which the disparity in status is relevant;
conversely, when the complementary appraisal occurs, the subordinate feels protoshame. In each case, the emotion enhances a corresponding attitude that represents
in enduring fashion the disparities in status. For the dominant, this attitude includes
a sense of entitlement relative to the other, while the converse is true for the
subordinate. In keeping with their opposite hedonic valences, this pair of emotions
motivates striving for power and social position (Fessler 2007). Given the clear
homologies with display behaviors in other primates, and the ubiquity of the
relevant adaptive problem across social animals, it is likely that proto-pride and
proto-shame are pan-primate; indeed, the core features of these emotions and
related appraisals and attitudes may be shared by all mammals, and possibly by
most vertebrates.
12.4
Emotions Associated with Parenting and Pair-Bonding
Parental behavior is a defining feature of mammals, although the patterns and extent
of parental care vary greatly across species. Parental behavior must be underlain by
a discrete motivational system on the part of the parent, complemented by a
corresponding system in the offspring (Maestripieri 2003a). We expect these
motivations to be modulated by complementary attitudes, what we term parental
attachment and offspring attachment, that represent the fitness value of offspring
and parent to one another. These attitudes shape appraisals of actions, determining
the circumstances that elicit the corresponding emotions of parental love and
offspring love. For example, positive parental attachment leads a mother to appraise
her infant’s midnight rooting as affiliation, eliciting parental love, rather than as a
transgression that would elicit anger. Offspring develop positive attachment as a
result of the experience of receiving succor. In contrast, because offspring impose
costs on parents without immediate compensation, the building of positive parental
attachment is achieved in part through the pump-priming effects of an emotion
(natal attraction) that makes interaction with infants intrinsically rewarding, a
phenomenon well-documented in primates (e.g., Silk 1999). It appears that, in
humans, natal attraction transforms into parental love in part as a result of positive
infant responses to parental overtures. Given the general primate trend of reductions
in the importance of olfactory cues for parent-offspring bonding, and increases in
the importance of behavioral cues (Broad et al. 2006), similar patterns may obtain
across primates.
Two different systems appear to underlie human mate selection, courtship, and
long-term pair- bonding. Limerance (Tennov 1979) is an emotion characterized by
intrusive ideation concerning a prospective mate, attention to indications of
reciprocation, and a motivation to be near and make a positive impression on
the target individual. Both E. Pillsworth and R. Kurzban (pers. comm.) propose
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269
that the intrusive and obsessive nature of limerance are explicable in terms of
the need to signal commitment to the target party, given the risk of defection. The
most persuasive signals are costly, taking the form of resource provisioning,
time allocation, and the public spurning of alternate potential mates. Once a stable
mateship has formed, the benefits of signaling are reduced: once both parties have
a concrete interest (e.g., offspring) in extended cooperation, it is adaptive to value
the other party’s welfare, and provide benefits noncontingently. Although investigators (e.g., Hatfield 1988) claim that an emotion, companionate love, replaces
limerance in pair-bonded couples, much of the phenomenon thus labeled is an
attitude rather than an emotion
the actual emotion is only present during
punctuated events in which displays of affection reaffirm mutual valuation, reinforcing this attitude. Pair-bonding and biparental investment occur in a variety of
mammals, and there is some commonality in the neurophysiological systems that
underlie these behaviors (Curley and Keverne 2005; Broad et al. 2006). Thus, it is
possible that human limerance and companionate love are complex manifestations
of a basic mammalian potential that has been further developed in pair-bonded
species.
12.5
Emotions Regulating Dyadic Cooperative Relationships
Companionate love and the attitude with which it is intertwined motivate altruistic
behavior toward committed partners. Although pair bonds exist in a limited number
of primate species, long-term affiliative relationships are more common (Silk
2007). In humans, a number of emotions play key roles in motivating behavior in
affiliative and cooperative relationships. Gratitude follows the receipt of a welcome
benefit provided by another party, motivating reciprocation (Trivers 1971), and
enhancing the attitude toward the other. Gratitude thus facilitates the initiation
and maintenance of cooperative relationships (McCullough et al. 2008). Gratitude
is subjectively and behaviorally differentiated from a sense of indebtedness
(McCullough et al. 2008). This is understandable in functional terms, as gratitude
marks an increased estimation (summarized in the attendant positive attitude)
of the potential long-term utility of the relationship, while indebtedness stresses
the short-term burden of repayment, indexing a different type of relationship.
Many primates differentiate among individuals and act in light of past interactions,
exhibiting durable alliances and affiliative behaviors (van Schaik and Aureli 2000).
This suggests that a gratitude-like mechanism may be both widely shared and of
considerable antiquity (Bonnie and de Waal 2004).
Whether by mistake or due to the temptations of short-term rewards for defection, individuals can also inflict costs on their valued partners. If individuals who
commit such acts perceive that they have damaged their partners’ attitude toward
themselves and this is disadvantageous given the utility of the relationship, ameliorative action is called for. Guilt is the prototypical emotion elicited when harm is
done to an ally (Baumeister et al. 1994; Tangney 1998). Guilt motivates apologies
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and, importantly, reparations, compensating the partner for damages and signaling
the individual’s commitment to the relationship (Trivers 1971). Humans employ
theory-of-mind reasoning in contemplating harm done to another; although other
primates may not do likewise, this aspect of human guilt may be an extension of the
basic phenomenon rather than an intrinsic feature, as many animals appear to
calibrate costs inflicted on others. We therefore expect a guilt-like mechanism to
be present in many primates, consistent with the observation that conflict among
allies is sometimes followed by reconciliatory behavior (reviewed in Flack and
de Waal 2000). Lastly, in humans, if reparations are not possible, guilt motivates
penance, infliction of self-imposed costs that signal that the individual does
not pursue self-interest at the expense of partners. Consistent with the symbolic
framing upon which such behavior rests, we expect penance to be absent among
nonhumans.
Selection of prospective partners often precedes the exchange of benefits.
Several emotions mark the positive evaluation of an individual as a prospective
cooperative partner; paralleling pair bonding, an attitude summarizing the value of
the relationship is informed by these emotions. Affiliative liking is a response to
individuals who simultaneously possess valuable attributes and share with the
evaluator sufficient commonalities as to constitute useful partners (commonalities
are important because they facilitate coordination and enhance the likelihood
of shared objectives; Tooby and Cosmides 1996; McElreath et al. 2003). This
emotion builds amicability, an attitude summarizing the potential utility of the
target as a cooperative partner. Admiration resembles affiliative liking, and similarly enhances amicability, but is elicited by individuals who possess a greater
preponderance of valuable attributes relative to commonalities, an asymmetry
that forces the evaluator to invest relatively more in the relationship. Pity may be
elicited by potential allies who are currently incapacitated and cannot contribute to
a cooperative relationship. Pity motivates the actor to provide aid, eliciting gratitude from the incapacitated individual (Cottrell and Neuberg 2005; cf. Trivers 1971
on sympathy).
Alliances that yield extensive benefits over a long period must begin with a
positive appraisal of the other’s value as a cooperative partner, and a motivational
stance that entails willingness to provide benefits. It is therefore likely that affiliative liking and amicability exist in many social animals, constituting fairly ancient
traits. Because dominance hierarchies create asymmetries in power between potential cooperators, we also expect some version of admiration to be present in many
animals capable of calibrating the provision of benefits in light of relative status.
Primates seem to exhibit this capacity, and observers have reported obsequious
affiliation directed at dominants by young subordinates (A. Pusey pers. comm.,
Walters and Seyfarth 1987). Evidence for pity is considerably weaker, as individuals seem more likely to avoid an injured or sick group member than to provide aid
(e.g., Goodall 1986; but see also Preston and de Waal 2002). It is unclear whether
this is because other primates lack the requisite theory of mind capacity, cannot
judge the probability of recovery and future usefulness of potential allies, or other
factors.
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Grief is the emotion felt at the death or loss of partners. The magnitude of grief
appears proportional to the impact of the loss on one’s fitness, suggesting that grief
is adaptive to the extent that it motivates individuals to seek out replacement
partners. Some primates do show marked physiological responses to losses of
preferred companions (Engh et al. 2006), and make efforts to expand their social
networks in the weeks that follow loss.
Not all incapable individuals evoke pity, as not all constitute potential allies
worth rehabilitating (Kurzban and Leary 2001). Additionally, previously beneficial
relationships can later prove unprofitable. Such individuals are excluded from
future alliances and the benefits thus produced, and are often exploited. Although
a considerable literature explores contempt as an emotion, findings are inconsistent.
We suggest this is because contempt is an attitude, not an emotion (Gervais 2009)
it is a representation of the evaluation of an individual as having no value as a
current or future ally. As such, it plays a central role in social event appraisal and
mediates emotion systems. Without any interest in the welfare of another, any cost
imposed by that party is appraised as a transgression, evoking anger. Likewise, any
risk of actual or symbolic contagion stemming from association elicits disgust. A
lack of interest in the welfare of another also undermines guilt, as damage done
does not warrant demonstration of positive valuation; in turn, this mutes anticipatory inhibitions that prevent doing harm. The lack of motivation not to hurt another
is compounded by a lack of empathy (where empathy can be viewed as a transemotional mechanism for assessing the needs of others), as the needs of the
contemned are of no interest to the contemnor. Finally, the contemnor experiences
no grief at the death of the condemned, as this event does not reduce the contemnor’s fitness.
The absence of a prosocial attitude toward some individuals will occur in any
species capable of discriminative affiliation. However, while contempt exists
whenever valuation of another’s welfare has not been raised above zero (what we
term minimal contempt), contempt can also arise through active diminution of
valuation. This occurs when established relationships break down (a possibility in
even minimally cooperative species), or upon unfavorable social appraisal in
species in which baseline conspecific valuation is greater than zero. A positive
default valuation should scale with the possibility of cooperation. For example, we
expect an elevated baseline in male chimpanzees owing to the importance of
alliances and intergroup defense (Watts 2006). Note that evidence of prosocial
behavior in non-human primates (e.g., de Waal 2008; Lakshminarayanan and
Santos 2008; but see also Vonk et al. 2008) does not speak to the question of
baseline valuations, as subjects in such experiments have histories of prior interaction during which valuations may have been raised. In contrast, evidence that
captive chimpanzees spontaneously help unfamiliar humans (Warneken et al.
2007) provides indirect evidence of a positive baseline valuation, at least in this
particular interspecific context. Positive default valuation likely reaches its extreme
in humans, owing to a history of intergroup competition and dependence on
transmitted culture and cooperation (Brewer and Campbell 1976; Richerson and
Boyd 2005). In humans, welfare valuation can be readily downregulated in
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response to derived cues of low value such as cultural difference (see McElreath
et al. 2003) or nonconformity (see Rozin et al. 1999). The combination of the
readiness with which this occurs, the subjectively negative affect that marks such
attitude change, and the role of contempt in potentiating anger and disgust likely
explains why contempt is often miscategorized as an emotion.
12.6
Linked Fate and Vicarious Emotions
Lickel et al. (2005) use the term vicarious to describe emotions elicited when
events that befall another are treated as if they befell the self (see also Rydell
et al. 2008). While possibly phylogenetically linked to emotional contagion, elicitation of vicarious emotions hinges on knowledge of the involvement of others in
events, rather than exposure to others’ emotion displays. Lickel et al. identify two
axes, interdependence and shared identity, that determine vicarious elicitation. We
suggest that Lickel et al.’s criteria for interdependence interaction, joint goals, and
shared norms are constituents of human cooperation. Cooperation links the fates
of the participants: the greater the investment in, and payoffs from, cooperative
ventures, the more the events that affect one member of the venture also impact
other members. Hence, calibrated for degree of cooperative interdependence, it is
adaptive to respond to such events vicariously. This process is likely undergirded by
attitudes that capture the degree of alignment of interests entailed in cooperation; in
turn, these attitudes generate appraisals of events befalling others that parallel
appraisals of events befalling the self.
Reduced by the coefficient of relatedness, it is also adaptive to react to events
affecting kin as if they affected oneself, since one’s own fitness is at least partially
aligned with that of one’s kin. Kinship is separate from cooperation, and hence from
action interdependence in Lickel et al.’s (2005) sense while kin-recognition
mechanisms may rely on propinquity and interaction as cues of relatedness, kin
should experience vicarious emotions even when the level of social interaction and
degree of shared goals and norms is low. While some form of cooperation is found
in many mammals, nepotistic behavior predates this, suggesting that kin-based
vicarious reactions were co-opted long ago for use in the cooperative domain.
We expect that any primate emotion experienced in an individual fashion will
also be experienced in a vicarious fashion given the proper elicitors, just as we
expect to find attitudinal proxies of fitness alignment resulting from either kinship
or cooperative interdependence.
Shared identity, Lickel et al.’s (2005) second axis whereby vicarious emotions
are elicited, can also be understood as a manifestation of an underlying process
whereby the fates of individuals become linked. Humans attribute a shared essence
to members of groups, such as ethnies, that exhibit distinctive cultural markers.
This essence constitutes grounds for inductive reasoning regarding the actions and
attributes of group members, a process thought to occur due to the utility of such
markers in predicting behavior (Gil-White 2001). Because essentializing supports
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inductive generalization, the fates of members of an essentialized group can
become intertwined, as actions by one group member can be taken as indicative
of the character of all group members, influencing outsiders’ attitudes. Essentializing and inductive generalization are likely matters of degree some categories of
individuals are seen as sharing a great deal of essence, while others less so. Like
kinship and degree of cooperation, social identity is thus a variable property that
influences vicarious emotion elicitation.
Noting that essentialism is adaptive in interacting with animals, Gil-White
(2001) suggests that essentialist social cognition derives from this ancestral trait.
We concur, adding that the coexistence of multiple hominid species during human
evolution may have facilitated a process of cooptation, wherein the phenotypic
markers used in essentialist reasoning were expanded to include culturally transmitted traits, as interspecific cultural diffusion might have blurred the line between
biological and cultural phenotypes. Given both the utility of inductive generalizations and evidence of primate antipredator strategies that involve a degree of
learning and are directed at specific species or genera (e.g., Seyfarth et al. 1980), it
is plausible that the ancestral interspecific form of essentialist reasoning is widely
shared. In contrast, the use of symbols to mark cultural affiliation and reinforce
cooperation, the related importance of essentialist reasoning, and the resulting role
of shared identity as an elicitor of vicarious emotions are all uniquely human.
12.7
Norm-Based Emotions
Humans differ from other primates in the extent and importance of cooperation, a
feature tightly linked to the degree to which norms regulate behavior, as socially
transmitted standards define goals, actions, and social relations that promote prosociality and enhance coordination (McElreath et al. 2003). Both our species’
exploitation of cooperation and our reliance on norms are likely undergirded by a
set of uniquely human emotion systems.
Paralleling the extension of eliciting conditions through appraisal modification
that occurs in vicarious emotion systems, in moral outrage, moral disgust, and
moral approbation, events that do not involve the actor elicit emotions as if they
did: moral outrage and moral disgust are, respectively, anger and disgust elicited by
others’ norm-violating actions; moral approbation is a gratitude-like emotion elicited by others’ exemplary performance of normative ideals (Fessler and Haley
2003). Moral outrage motivates inflicting costs on the norm violator as if in
retribution; moral disgust motivates avoiding the norm violator as if in contamination avoidance (Gutierrez and Giner-Sorolla 2007); and moral approbation motivates providing a benefit to the norm-embodier as if in response to a benefit
received. As in vicarious emotions, in each case, the emotion modifies an attendant
attitude toward the target despite no direct interaction. However, whereas in
vicarious emotions a connection to the self is made via another person, in these
emotions, the connection is solely via the norm at issue.
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Socially transmitted standards for behavior play a much more restricted role
in the lives of nonhuman primates, and evidence for norm enforcement by disinterested parties is very limited (see Flack et al. 2005). In contrast, although the proportions in which they occur likely differ across cultures, moral outrage, moral
disgust, and moral approbation are all readily observable in any human society
(e.g., see Henrich et al. 2006).
There are three basic kinds of explanations for the coevolution of cultural
norms and norm based emotions. Boyd and Richerson (2002) contend that punishment can stabilize any behavior, and cultural group selection favors the predominance of group-beneficial norms (see Silk and Boyd, this volume). Norm violators
are punished, as are those who fail to punish norm-violators, and this generates
selective pressure favoring the propensity to treat norms as extensions of the self, as
this reduces the frequency with which the actor is punished. A second view holds
that norm-based emotions are a product of natural selection acting directly on
individuals (e.g., Haley and Fessler 2005; Kurzban et al. 2007). Here, actors
compete in a marketplace of prospective allies; because conformists are predictable, adherence to cultural norms increases an actor’s attractiveness in this regard.
Actors can advertise their adherence to cultural norms by punishing norm violators
and rewarding those who exemplify norms, with the reputational benefits thus
gained outweighing the costs of these actions. The fitness advantages of inclusion
in cooperative ventures thus favor emotion systems in which norms are treated
as extensions of the self. Finally, a third view holds that unique features of human
sociality, such as intergroup conflict and reproductive leveling, provided the necessary conditions for biological group selection in humans. Group selection favored
the evolution of prosocial motivations, including norm-based emotions that
motivate third-party punishing and rewarding behaviors (e.g., Gintis et al. 2003;
Bowles 2006).
Although at present it is difficult to determine which of the above accounts
accurately describes hominid evolution, or whether some additional account is
needed, it is, nonetheless, clear that humans possess the motivational architecture
upon which these perspectives converge. The aversive emotion shame is elicited by
the appraisal that others are aware of one’s failure to conform to important norms,
while the rewarding emotion pride is elicited by the appraisal that others are aware
of one’s success in exemplifying important norms (reviewed in Fessler 2007). Even
holding aside the limited role of norms among non-human primates, we expect
shame and pride to be uniquely human, as they are contingent on sophisticated
theory-of-mind reasoning.
Shame and pride exemplify the evolutionary process of co-optation and modification: despite employing the same display behaviors, qualia, and action tendencies
as proto-shame and proto-pride (see above), the key eliciting conditions differ, as
proto-shame and proto-pride focus exclusively on relative position in a social
hierarchy, do not rely on norms as evaluative criteria for behavior, and do not
involve theory-of-mind reasoning (Fessler 2007). The action tendencies associated
with shame similarly reveal its kludge-like structure. Although parties offended by
norm violations are best mollified by apologies and public commitments to future
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conformity, shame paradoxically interferes with these behaviors by motivating
flight and hiding, tactics more appropriate to dealing with dominants than to
placating judgmental community members. Proto-shame and proto-pride, ancestral
pan-primate rank-related emotions, were modified in the hominid line. With the rise
in importance of norms, coercive force (dominance) was largely replaced by freelygranted deference (prestige) as a determinant of social position (Henrich and GilWhite 2001). Apparently, because prestige is contingent on the opinions of others,
existing emotions that motivated rank-striving behavior were modified by selection
so as to include an assessment of others’ evaluations of the actor. Whether to avoid
punishment, enhance inclusion in cooperative ventures, or generate non-contingent
group-beneficial behavior, these emotions were then further refashioned, shifting
the focus away from rank, and toward norm adherence. Contemporary humans
exhibit all three forms of these emotions the ancestral forms that focus on
dominance-based rank, the intermediate derived forms that focus on prestige, and
the final derived forms that focus on norm adherence.
All three forms of shame and pride can be experienced vicariously. As we would
expect to also be true among primates that engage in coalitional aggression,
vicarious proto-pride or proto-shame should be experienced whenever an ally
enjoys victory or suffers defeat relative to a rival. Vicarious prestige-based pride
and shame, though limited to humans, should exhibit a similar pattern. Finally,
whenever a person linked to the actor via a shared social identity succeeds or fails
with regard to normative standards, to the extent that others will engage in inductive
generalizing, those successes or failures will influence how others treat the actor,
hence the actor should experience vicarious pride or shame (see Lickel et al. 2005).
The primary exception to the latter pattern occurs when identity sharing is incomplete, in which case an actor can distance herself from a norm violator in order to
manage third parties’ assessments; in this case, moral outrage, rather than vicarious
shame, may be elicited (cf. Haley 2002).
Shame and pride illustrate the extensive re-working that can be achieved through
processes of co-optation and modification. However, as the vicarious versions of
these emotions demonstrate, substantial functional changes can also occur merely
by extending an emotion’s eliciting conditions. We propose that an emotion that we
term normative guilt is elicited by norm violations absent a harmed relationship
partner within the worldviews in which they are defined, many sins do not harm
other people, yet the sinner experiences guilt nonetheless. This extension is made
possible by our elaborately developed ability to manipulate representations of
social others. In some manifestations of normative guilt, the represented partner
is a culturally-constructed nonexistent agent (a deity, ancestor, etc.; cf. Darwin
[1871] 1909: 115 116 on remorse); in others, the imagined partner is a representation of the actor’s kin network or cooperative group; and, in the most abstract
manifestation, the (only dimly imagined) partner is a representation of society as a
whole. In each case, violating a norm leads actors to undertake reparations or
penance as if, by doing so, they mitigated the harm done, or signaled their future
reliability. Normative guilt thus illustrates one of the most profound disjunctions
between ourselves and our non-human primate relatives, namely the nature of the
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internal representations that constitute the informational environment in which
emotions operate. Despite our common origins, the complexity of humans’ internal
representations, and the fundamentally cultural nature of those representations,
create a gap between our emotions and those of other primates, a gap that bears
minding indeed.
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Chapter 13
Primate Communication and Human Language:
Continuities and Discontinuities
Dorothy L. Cheney and Robert M. Seyfarth
Abstract Here, we review some questions in the production and perception of
nonhuman primate vocalizations, focusing on three related issues. First, flexible
vocal production separates humans not only from nonhuman primates but also from
most other mammals and birds. The rarity of learned, modifiable call production in
most mammals suggests that important changes in the mechanisms governing
human phonation occurred relatively recently in human evolution, after the divergence of our hominid ancestors from the common ancestors of humans and chimpanzees. Second, while exclusive focus on vocal production reveals clear
differences between humans and most other species, a broader examination of
call usage and perception paints a more complicated picture, with both similarities
and differences between the two groups. Third, an analysis of vocal production and
perception draws attention to the very different mechanisms that underlie the
behavior of signalers and recipients, even when they are involved in the same
communicative event. Nonhuman primates have only a small repertoire of acoustically fixed vocalizations. However, because calls are individually distinctive and
each call type is predictably linked to a particular social context, this limited call
repertoire can, nonetheless, provide listeners with an open-ended, highly modifiable, and cognitively rich set of meanings, allowing them to construct “narratives”
of unseen events. However, although nonhuman primates and other animals seem
capable of thinking, as it were, in simple sentences, this ability does not motivate
them to speak in sentences. Their knowledge remains largely private. We suggest
that long before our ancestors spoke in sentences, they had a language of thought in
which they represented the world and the meaning of call sequences in terms of
actors, actions, and those who are acted upon. The linguistic revolution occurred
D.L. Cheney (*)
Department of Biology, University of Pennsylvania, Philadelphia, PA, USA
e mail: cheney@sas.upenn.edu
R.M. Seyfarth
Department of Psychology, University of Pennsylvania, Philadelphia, PA, USA
e mail: seyfarth@psych.upenn.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 13, # Springer Verlag Berlin Heidelberg 2010
283
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when our ancestors began to express this tacit knowledge, and to use their cognitive
skills in speaking as well as listening.
13.1
Introduction
Darwin (1871) drew attention to a dichotomy in the vocal communication of
animals that had perplexed philosophers and naturalists for at least 2,000 years.
In marked contrast to human language, he wrote, animal vocalizations appeared
to be involuntary expressions of emotion and movement: “When the sensorium is
strongly excited, the muscles of the body are generally thrown into violent action;
and as a consequence, loud sounds are uttered, . . . although the sounds may be of no
use” ([1871] 1981: 83). Two pages later, however, Darwin also acknowledged:
“That which distinguishes man from the lower animals is not the understanding of
articulate sounds, for, as every one knows, dogs understand many words and
sentences. . . . Nor is it the mere capacity of connecting definite sounds with definite
ideas; for it is certain that some parrots, which have been taught to speak, connect
unerringly words with things, and persons with events” ([1871] 1981: 85).
As a contemporary example of the dichotomy first identified by Darwin, consider
Rico, a border collie dog who learned the names of more than 200 different toys
(Kaminski et al. 2004). Rico was able to learn and remember the names of new toys
by process of exclusion, or “fast mapping,” and like small children used gaze
and attention to guide word learning. But of course, Rico never learned to say any of
the words he learned. In this respect, his limited vocal production and extensive
comprehension are similar to those of human-trained sea lions (Schusterman et al.
2002) and dolphins (Herman et al. 1993).
The dichotomy between call production and perception is puzzling because
listeners are also signalers. The same animal that has no problem understanding a
word like “ball” is completely unable to articulate it. Indeed, while there are
fundamental differences between animal communication and human language in
the context of call production, in the context of call perception, these differences
seem much less obvious. So why do not animals articulate their mental representations of the world and other individuals more often, and in more contexts? Why
does their knowledge remain largely private?
Here, we review some questions in the production and perception of nonhuman
primate vocalizations, focusing in particular on the communication of free-ranging
baboons (Papio hamadryas spp.) (for a more detailed discussion, see Cheney and
Seyfarth 2007). For those interested in comparing animal vocal communication
with human language, three conclusions emerge. First, flexible vocal production
separates humans not only from nonhuman primates but also from most other
mammals and birds. The rarity of learned, modifiable call production throughout
most of the Class Mammalia suggests that important changes in the mechanisms
governing human phonation occurred relatively recently in human evolution, after
the divergence of our hominid ancestors from the common ancestors of humans and
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chimpanzees. Second, while an exclusive focus on vocal production reveals clear
differences between humans and most other species, a broader examination of call
usage and perception paints a more complicated picture, with both similarities and
differences between the two groups. Third, an analysis of vocal production and
perception draws attention to the very different mechanisms that underlie the
behavior of signalers and recipients, even when they are involved in the same
communicative event. Nonhuman primates have only a small repertoire of acoustically fixed vocalizations. However, because calls are individually distinctive and
each call type is predictably linked to a particular social context, this limited call
repertoire can nonetheless provide listeners with an open-ended, highly modifiable,
and cognitively rich set of meanings. Listeners can acquire a huge number of
messages from a finite number of signals.
13.2
Call Production and Development
Monkeys and apes have a small repertoire of calls that show relatively little
modification in their acoustic properties during development. The development of
vocal production is largely unaffected by variations in auditory experience or
rearing (see Hammerschmidt and Fischer 2008 for a recent review). For example,
male baboons produce loud two-syllable alarm wahoos when they encounter lions
or leopards. Alarm wahoos are acoustically similar to the contest wahoos that males
give during competitive contests with other males, but the two types of wahoo differ
according to a number of acoustic measures (Fischer et al. 2002). Similarly, the
alarm barks given by females and juveniles are acoustically similar to the contact
barks that baboons give when they become lost or separated from their companions.
Again, however, there are subtle acoustic differences between the two bark types
that allow them to be distinguished by ear (Fischer et al. 2001a).
Unlike predator alarm calls, which depend in a fairly simple way on the type of
predator or the degree of danger, the vocalizations given by animals during social
interactions are elicited by a more complex array of factors that may include both
the immediate social context and the history of interactions between the particular
individuals involved. Baboon grunts offer an example.
Grunts are the baboons’ most common vocalization. They are individually
distinctive, given in a variety of non-aggressive circumstances, and seem to comprise two acoustically graded types (Owren et al. 1997; Rendall 2003). The move
grunt is typically given in the context of group movement. Like similar calls given
by other primate species, move grunts function to alert other individuals to the
signaler’s intentions to travel in a particular direction. By contrast, the infant grunt
is given during many sorts of friendly interactions and functions as a signal of
benign intent. Infant grunts are most commonly given in the context of infant
handling, but they also occur during grooming, other friendly behavior, and reconciliation. The two grunt subtypes thus differ in the specificity of the stimuli that
elicit them. Move grunts are linked to a specific context. By comparison, infant
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grunts are given in a wide variety of friendly situations. Whereas infant grunts
function to signal benign intent toward one specific individual, move grunts broadcast the signaler’s intentions to many individuals. Move and infant grunts exemplify
the variation found in the baboon vocal repertoire, and indeed in the vocal repertoires of many other primates. Some calls are tightly linked to a relatively narrow
context, whereas others are used in a wider variety of circumstances. Some calls are
directed at a specific individual, whereas others are more widely broadcast.
The acoustic intergradation in many nonhuman primate vocalizations may be
caused by gradation in the caller’s arousal or emotional state. Analyses of baboons’
contact and alarm barks, contest and alarm wahoos, and move and infant grunts all
suggest that acoustic variation is consistent with variation in the caller’s emotions
(Fischer et al. 2001a, 2002, 2004; Rendall 2003, see also Jürgens 1995). In each of
these cases, however, graded calls whose production may be determined largely by
the signaler’s emotions are nonetheless perceived as discretely different vocalizations (Rendall et al. 1999). And once listeners have recognized that each of these
discretely different signals is predictably linked to a particular event, calls have the
potential to convey to listeners a meaning that goes far beyond information about
the signaler’s emotional state.
In marked contrast to children, who learn to produce and comprehend thousands
of new words during their first three years of life, monkeys and apes rarely modify
their vocal repertoires by adding new sounds. Although some primates make subtle
modifications in their vocalizations as a result of experience (Hauser 1992;
Elowson and Snowdon 1994; Mitani and Brandt 1994; Crockford et al. 2004) and
can modify the loudness of their calls through auditory feedback (Hage et al. 2006),
a baboon in Kenya produces more or less the same sounds in the same contexts as a
baboon in Botswana. This conclusion follows not only from research on many
primate species (Seyfarth and Cheney 1997a) but also from a cross-fostering
experiment involving two closely related species of macaques. In this experiment,
juveniles that had been fostered into a group of macaques from another species
continued to produce their own species’ calls, despite being physically capable of
producing their adoptive species’ calls (Owren et al. 1993). Monkeys seem genetically predisposed to give particular calls in particular contexts.
This is not to say that nonhuman primate call production is involuntary. In both
the field and the laboratory, nonhuman primates can control their vocalization and
choose to either vocalize or remain silent.. After behaving aggressively toward a
subordinate, for example, a baboon may give a “reconciliatory” grunt to her
opponent or she may not (Cheney et al. 1995b; Cheney and Seyfarth 1997).
Similarly, when capuchin monkeys (Cebus capucinus) find food, they may call or
remain silent (Gros-Louis 2004). Even in highly emotional circumstances like
encounters with predators, some individuals give alarm calls at high rates, others
call less often, and still others remain silent (Cheney and Seyfarth 1990). In more
controlled laboratory settings, the timing, duration, and rate of calling by monkeys
can be brought under operant control (Pierce 1985; Egnor et al. 2007). Clearly,
primates can control whether they vocalize or not depending upon variations in both
the ecological, social, and acoustic environments.
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Within a given context, nonhuman primates can also make subtle modifications
in the acoustic structure of their calls (reviewed by Hammerschmidt and Fischer
2008, Seyfarth and Cheney in press). To cite just one example, wild chimpanzees in
Uganda give long, elaborate pant-hoots either alone or in “choruses” with others.
When two individuals have called together several times, the acoustic features of
their pant-hoots begin to converge (Mitani and Brandt 1994; Mitani and Gros-Louis
1998). Apparently, individuals modify the acoustic structure of their calls depending upon auditory experience (see also Crockford et al. 2004).
Many other studies suggest that, whereas the basic acoustic structure of nonhuman primate calls is innately determined, the fine spectrotemporal features of
vocalizations can be modified (for reviews see Seyfarth and Cheney 1997a, in
press, Egnor and Hauser 2004; Hammerschmidt and Fischer 2008). Some features
are more easily modified than others. As Janik and Slater (1997) first pointed out,
temporal features like call duration and amplitude are more easily modified than
frequency parameters. The distinction between relatively innate and more modifiable components of phonation is important, because it has significant implications
for future research on the neurobiology of primate vocal production (see Egnor and
Hauser 2004; Hammerschmidt and Fischer 2008 for further discussion).
In their relatively fixed vocal production, nonhuman primates are typical of most
mammals and even the great majority of birds. In their 1997 review, Janik and
Slater found evidence for learned, modifiable vocal production in only three orders
of birds, cetaceans, harbor seals, and humans. Although we may yet be surprised by
novel evidence of vocal imitation (e.g., Poole et al. 2005) or creative call combinations (Zuberbuhler 2002; Crockford and Boesch 2003; Arnold and Zuberbuhler
2006), for the moment it appears that the ability to modify vocal production
depending upon experience is comparatively modest.
13.3
Call Perception
Whenever there is a predictable relation between a particular call type and a specific
social context, a vocalization has the potential to inform nearby listeners about
objects or events. The underlying mechanisms are irrelevant. A tone that informs a
rat about the imminence of a shock, an alarm call that informs a vervet about the
presence of an eagle, or a scream that informs a baboon that her offspring is
involved in a fight all have the potential to provide a listener with precise information if they are predictably associated (Rescorla 1988) with a narrow range of
events. The widely different mechanisms that lead to this association have no effect
on the signal’s potential to inform (Seyfarth and Cheney 2003).
But while they have this potential, do animal vocalizations really provide
listeners with information about who is doing what to whom? The study of vocal
perception in animal communication is fraught with both practical and conceptual
difficulties because we cannot interview our subjects. As a result, the only way to
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determine the “meaning” of a signal is to examine the recipient’s response a very
crude measure of whether the recipient has or has not acquired information.
In some cases, animals respond in qualitatively different ways to different
vocalizations, and the characteristics of their responses suggest that each call type
conveys specific information. Consider, for example, the responses of Diana
monkeys (Cercopithecus diana) to a playback of different alarm calls. When
Diana monkeys hear the growl of a leopard, they respond with their own “leopard
alarm”; when they hear the shriek of an eagle, they give their own “eagle alarm”
(Zuberbuhler et al. 1999).
In many other cases, the lack of qualitatively different responses complicates the
interpretation of call meaning. Two methods help to circumvent this problem, at
least partially. First, if observations suggest that some generic response, like looking toward the speaker, will be the only reaction elicited by a playback experiment,
scientists can design a matched pair of trials, alike in all but one respect, and then
compare subjects’ responses under two different conditions. A consistent difference
in the duration of response may permit inferences about the different sorts of
information conveyed by different calls, or by the same call under different conditions. Alternatively, subjects may show no immediate response to playback of a call
but their subsequent behavior may nonetheless be affected. Having heard a particular call from individual X, for example, a subject may be more likely to approach X
in the next 30 min than if no call, or the call of a different individual, had been
played. Such longer-term changes in behavior also allow one to make inferences,
albeit indirect, about the meaning of a specific vocalization (Seyfarth and Cheney
1997b, for examples see Cheney and Seyfarth 2007).
Playback experiments on a wide variety of species have now demonstrated that
many nonhuman primate calls function referentially, providing listeners with information about what is happening and who is involved. This is true both of calls that
are acoustically very different and calls that are acoustically graded. Baboons, for
example, respond differently to alarm and contest wahoos, alarm and contact
barks, and move and infant grunts (Rendall et al. 1999; Fischer et al. 2001b; Kitchen
et al. 2003). Signaler identity and context also play crucial roles in informing
listeners about what is occurring. Whatever the mechanisms underlying call production, therefore, listeners extract meaning from the calls they hear.
The call-meaning relationship in the listener’s mind is interesting in several
respects. First, it constitutes an arbitrary association between a sound and the thing
for which it stands. There is nothing about the sound of a Diana monkey’s leopard
alarm call, for example, that sounds like a leopard, and nothing about the sound of a
monkey’s eagle alarm that would obviously link it to an eagle. In much the same
way, there is nothing in the acoustic details of baboons’ alarm and contest wahoos
that would help a listener learn that one is given in response to predators while
another is given during male male aggression.
Second, the meaning of each call is defined not just by its relation to an object in
the world but also by its relation to other calls in the monkey’s repertoire. A male
Diana monkey’s leopard alarm is similar in meaning to a leopard’s growl and a
female’s leopard alarm, but different in meaning from all of three eagle-associated
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calls. In the monkey’s mind, there exists a kind of semantic space in which the three
leopard-associated sounds are closely linked in one cluster, whereas the three eagleassociated sounds are closely linked in another.
This leads to a third conclusion, that primate calls are acoustic units linked to
particular concepts. When a Diana monkey hears a male’s leopard alarm, she
appears to form a mental representation of the call’s meaning. Then, when she
hears a leopard’s growl coming from the same location shortly thereafter, she forms
a second representation and compares the two calls on the basis of their meaning.
Her lack of response to the now redundant growl is based on this assessment. The
female, in other words, has a concept a kind of mental image of a leopard. The
concept can be activated by any one of three quite different sounds that are linked
together based on their shared meaning (e.g., Cohen et al. 2006). The concept is also
amodal or multimodal, involving a combination of visual and auditory information
(Gil-da-Costa et al. 2004; Ghazanfar et al. 2005).
As another example, consider the phenomenon of individual recognition by
voice, which has been amply demonstrated in many species and underlies many
of our playback experiments (e.g., Snowdon 1986; Rendall et al. 1996; Owren et al.
1997). Baboons clearly recognize other group members from their voices alone,
regardless of whether they are giving a grunt, a contact bark, or a threat-grunt, and
regardless of whether they are vocalizing in a calm or in an agitated manner.
Despite wide variation in the acoustic cues that mark a call as a particular individual’s, and the fact that the calls of one individual may grade acoustically into the
calls of another, listeners still link each call with a unique individual in a discrete,
categorical fashion. Individual recognition occurs in so many contexts, with so
many vocalizations, that it is hard to escape the impression that listeners have a
mental representation, or concept, of each group member as an individual. If
monkeys were human, we would call this a concept of person.
In sum, whereas call production in primates is relatively fixed, the cognitive
mechanisms that underlie call perception are considerably more complex. Underlying primates’ assessment of call meaning is a rich conceptual structure, in which
calls are linked both to objects and relations in the world and to other calls in the
species’ repertoire. When responding to calls, monkeys act as if they recognize
individuals and have concepts like leopard, eagle, close associate, and so on. The
contrast between impoverished production and rich, conceptually based perception
argues strongly against the view that a concept cannot be acquired unless it is
instantiated in one’s language (reviewed by Gleitman and Papafragou 2005).
Monkeys and apes have many concepts for which they have no words.
13.4
Syntax
There is little evidence for rule-governed syntax in the calls of nonhuman primates.
Recent work by Zuberbuhler and colleagues on the alarm calls of forest monkeys
provide intriguing evidence that the presence of one call type can modify the
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meaning of another (Zuberbuhler 2002; Arnold and Zuberbuhler 2006; Clarke et al.
2006), and a study by Crockford and Boesch (2003) suggests that a call combination in chimpanzees may carry new meaning that goes beyond the meaning of the
individual calls themselves, but these rare exceptions meet few of the definitions of
human syntax.
Nonetheless, monkeys and apes hear different calls in combination all the time.
These calls are usually given by different individuals, allowing listeners to construct narratives about what is happening and who is involved. In assessing call
sequences, listeners attend simultaneously to the signalers’ identities, call type, the
rank and kinship of the signalers, and the temporal and spatial juxtaposition of
different individuals’ calls. Baboons, for example, respond much more strongly to a
call sequence that suggests a reversal in the female dominance hierarchy than one
that is consistent with it (Cheney et al. 1995a; Bergman et al. 2003). When played a
threat-grunt, scream sequence in which a high-ranking female say, Sylvia
threat-grunts and a lower-ranking female say, Hannah screams, they show little
if any response. Their responses are much stronger, however, if they hear a
sequence that appears to violate their knowledge of the female dominance hierarchy; for example, Hannah threat-grunts and Sylvia screams. Although the callers’
identities and the call types are the same, subtle changes in the elements of the
sequence cause its meaning to change fundamentally. Listeners also seem to
assume a causal relationship between calls that are closely juxtaposed in time:
Hannah’s threat-grunts caused Sylvia to scream. These assessments are done
instantaneously and probably large unconsciously. They are based on discrete
properties, such as the signalers’ identities, kinship, and rank, that are combined
in a combinatorial system and that encode propositional information: for example,
A is threatening B; A is mating with B; and so on. All of these features are also
present in language.
13.5
Attributing Intentions to Signalers
During conversation, humans routinely make inferences about the motives and
beliefs of their intended recipients. Baboons, too, seem to recognize the intended
recipient of someone else’s calls.
Baboon groups are noisy, tumultuous societies, and a baboon could not manage
her social interactions if she interpreted every vocalization she heard as directed at
her. Inferences about the “directedness” of vocalizations are probably often
mediated by gaze direction and relatively simple contingencies. Even in the
absence of visual signals, however, baboons are able to make inferences about
the intended recipient of a call based on their knowledge of a signaler’s identity and
the nature of recent interactions. For example, when females were played the
“reconciliatory grunt” of a recent aggressor within minutes after being threatened,
they were more likely to approach their former opponent and to tolerate their
opponent’s approaches than after hearing either no grunt or the grunt of another
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dominant female unrelated to their opponent (Cheney and Seyfarth 1997). Call type
was also important, because females avoided their recent opponent if they heard her
threat-grunt rather than her reconciliatory grunt (Engh et al. 2006). Thus, baboons
use their memory of recent interactions to make inferences about whether a call is
being directed at themselves or at some other individual.
In some cases, these inferences are complex and indirect, and call upon baboons’
knowledge of the kinship relationships of other group members. For example, when
female baboons were played the threat-grunts of their aggressor’s relative soon
after being threatened, they avoided members of their aggressor’s matriline. In
contrast, when they heard the same threat-grunts in the absence of aggression, they
ignored the call and acted as if they assumed that the call was directed at someone
else (Wittig et al. 2007a). Similarly, when subjects heard the “reconciliatory” grunt
of their aggressor’s relative after a fight, they were more likely to approach both
their aggressor and the relative whose grunt they had heard (Wittig et al. 2007b).
They did not do so, however, if they had heard the “reconciliatory” grunt of another,
unrelated female, nor did they approach their aggressor’s other relatives. Here
again, subjects behaved as if they believed that a grunt from their aggressor’s
relative must be directed at them, as a consequence of the fight. What is especially
interesting in these experiments is that subjects inferred that they were the target of
the vocalization even though they had not recently interacted with the signaler, but
with her relative. They could only have done so if they recognized the close bond
that existed between the two females.
In primates, faces and voices are the primary means of transmitting social
signals, and monkeys recognize the correspondence between facial and vocal
expressions (Ghazanfar and Logothetis 2003). Presumably, visual and auditory
signals are somehow combined to form a unified, multimodal percept in the mind
of a monkey. In a study using positron emission tomography (PET), Gil-da-Costa
et al. (2004) showed that when rhesus macaques hear one of their own species’
vocalizations, they exhibit neural activity not only in areas associated with auditory
processing but also in higher-order visual areas, including STS. Auditory and visual
areas also exhibit significant anatomical connections (Poremba et al. 2003).
Ghazanfar et al. (2005) explored the neural basis of sensory integration using the
coos and grunts of rhesus macaques as stimuli. They found clear evidence that cells
in certain areas of the auditory cortex are more responsive to bi-modal (visual and
auditory) presentation of species-specific calls than to unimodal presentation.
Although significant integration of visual and auditory information occurred in
trials with both vocalizations, the effect of cross-modal presentation was greater
with grunts than with coos. The authors speculate that this may occur because
grunts are usually directed toward a specific individual in dyadic interactions,
whereas coos tend to be broadcasted generally to the group at large. The greater
cross-modal integration in the processing of grunts may therefore have arisen
because, in contrast to listeners who hear a coo, listeners who hear a grunt must
immediately determine whether or not the call is directed at them.
In sum, when deciding “Who, me?” upon hearing a vocalization, baboons must
take into account the identity of the signaler (who is it?), the type of call given
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(friendly or aggressive?), the nature of their prior interactions with the signaler
(were they aggressive, friendly, or neutral?), and the correlation between past
interactions and future ones (does a recent grooming interaction lower or increase
the likelihood of aggression?). Learned contingencies, doubtless, play a role in
these assessments. But because listeners’ responses depend on simultaneous consideration of all of these factors, this learning is likely to be both complex and
subtle.
Although baboons and other monkeys may be able to recognize other individual’s intentions when inferring whether or not they are the target of another
individual’s call, there is no evidence that they take into account their audience’s
knowledge or beliefs when producing or assessing calls. Both monkeys and apes
give alarm calls, for example, without any apparent recognition of whether listeners
are ignorant or already informed about the presence of a predator (reviewed by
Cheney and Seyfarth 2007). Similarly, although the “food calls” of capuchin
monkeys (Gros-Louis 2004) and the pant hoots of chimpanzees (Clark and
Wrangham 1994) attract others to food, signalers show no evidence of recognizing
whether their audience is already aware of the presence of food. Baboons often give
contact barks when separated from others. When several individuals are calling
simultaneously, it often appears that they are answering each other’s calls in order
to inform others of the group’s location. Playback experiments suggest, however,
that baboons call primarily with respect to their own separation from the group, not
their audience’s. They “answer” others when they themselves are separated, and
they often fail to respond to the calls of even their offspring when they themselves
are in close proximity to other group members (Cheney et al. 1996; Rendall et al.
2000). In this respect, the vocalizations of monkeys and apes are very different from
human speech, where we routinely take into account our audience’s beliefs and
knowledge during conversation (Grice 1957).
13.6
Primate Communication and the Evolution of Language
The vocal communication of nonhuman primates is very different from human
language, especially in the domain of call production. At the same time, however,
comparisons between primate communication and human language have tended to
focus on the differences, ignoring some of the intriguing continuities in perception
and cognition.
As already noted, the striking difference between production and comprehension
in animal communication is puzzling because producers are also perceivers: why
should an individual who can deduce an almost limitless number of meanings from
the calls of others be able to produce only a limited number of calls of her own? The
difference may arise because call production depends on mechanisms of phonation,
which are largely innate, whereas comprehension depends on mechanisms of
learning (classical and operant conditioning), which are considerably more malleable. But this explanation offers no answer to a crucial question: Why has natural
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selection so rarely acted to favor flexible vocal production? Here, we offer some
speculations as they apply to human and nonhuman primates.
At some point in our evolutionary history probably after the divergence of the
evolutionary lines leading to chimpanzees and bonobos on the one hand and
humans on the other hand (Enard et al. 2002) our ancestors developed much
greater control over the physiology of vocal production. As a result, vocal output
became both more flexible and considerably more dependent on auditory experience and imitation (Lieberman 1991; Fitch 2007). What selective pressures might
have given rise to these physiological changes?
Vocal communication in nonhuman primates lacks three features that are abundantly present in human language: the ability to generate new words, lexical syntax,
and a theory of mind. By the latter, we mean the ability of both speakers and
listeners to make attributions about each others’ beliefs, knowledge, and other
mental states (Grice 1957). These are the simplest, most basic features that distinguish human and nonhuman primate vocal production, and it is with these traits that
speculations about the evolution of language must start. At the earliest stages of
language evolution, we need not worry about the more complex properties of
language that probably came later
properties like case, tense, subject verb
agreement, open- and closed-class items, recursion, long-distance dependency,
subordinate clauses, and so on.
How might the ability to generate new words, lexical syntax, and a theory of
mind have evolved: simultaneously, in response to the same selective pressures, or
more serially, in some particular order? We propose that the evolution of a theory of
mind preceded language, creating the selective pressures that gave rise to the ability
to generate new words and lexical syntax, and to the flexibility in vocal production
that these two traits would have required (Cheney and Seyfarth 2005, 2007).
There is no evidence in nonhuman primates for anything close to the large vocal
repertoire we find even in very young children. Similarly, nonhuman primates
provide few examples of lexical syntax. By contrast, there is growing evidence
that both Old World monkeys (Flombaum and Santos 2005; Engh et al. 2006;
Cheney and Seyfarth 2007) and apes (Hare et al. 2001; Tomasello et al. 2005;
Buttelmann et al. 2007) may possess rudimentary abilities to attribute motives or
knowledge to others, and engage in simple forms of shared attention and social
referencing. Taken together, these data suggest that a rudimentary theory of mind
appeared among primates long before flexible vocal production, the ability to
generate new words, and lexical syntax.
A rudimentary theory of mind seems to be crucially important for word learning
in young children. Beginning as early as 9 12 months, children exhibit a nascent
understanding of other individuals’ motives, beliefs, and desires, and this skill
forms the basis of a shared attention system that is essential for early word learning
(Bloom and Markson 1998; Tomasello 2003). One-year old children understand
implicitly that words can be mapped onto objects and actions. Crucially, this
understanding is accompanied by a kind of “social referencing” in which the
child uses other people’s direction of gaze, gestures, and emotions to assign labels
to objects (Baldwin 1991, reviewed in Pinker 1994; Fisher and Gleitman 2002).
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Gaze and attention also facilitate word learning in dogs and other animals. Children,
however, rapidly surpass the simpler forms of shared attention and word learning
demonstrated by animals. Long before they begin to speak in sentences, young
children develop implicit notions of objects and events, actors, actions, and those
that are acted upon. Moreover, in contrast to monkeys, apes, and other animals,
1-year-old children are highly motivated to share what they know with others
(Tomasello and Carpenter 2007). While animals are concerned with their own
goals and knowledge, young children are concerned with making their thoughts
and knowledge publicly available. The acquisition of a theory of mind thus creates
a cognitive environment that drives the acquisition of new words and new grammatical skills. Indeed, results suggest that children could not increase their vocabularies or learn grammar as rapidly as they do if they did not have some prior
notion of other individuals’ mental states (Pinker 1994; Fisher and Gleitman 2002;
Tomasello 2003). In sum, data on children provide an excellent illustration of how a
theory of mind can drive language development.
By contrast, it is much more difficult to imagine how our ancestors could have
learned new words or grammatical rules if they were unable to attribute mental
states to others. The lack of syntax in nonhuman primate vocalizations cannot be
traced to an inability to recognize argument structure to understand that an event
can be described as a sequence in which an agent performs some action on an
object. Baboons, for example, clearly distinguish between a sequence of calls
indicating that Sylvia is threatening Hannah, as opposed to Hannah is threatening
Sylvia. Nor does the lack of syntax arise because of an inability to mentally
represent descriptive verbs, modifiers, or prepositions. In captivity, a variety of
animals, including dolphins (Herman et al. 1993), sea lions (Schusterman and
Krieger 1986), and African gray parrots (Pepperberg 1993), can be taught to
understand and in some cases even to produce verbs, modifiers, and prepositions.
Even in their natural behavior, nonhuman primates and other animals certainly
seem capable of thinking in simple sentences. However, this ability does not
motivate them to speak in sentences. Their knowledge remains largely private.
This limitation may arise because nonhuman primates and other animals cannot
distinguish between what they know and others know and cannot recognize, for
example, that an ignorant individual might need to have an event explained to them.
As a result, although they may mentally tag events as argument structures, they fail
to map these tags into a communicative system in any stable or predictable way.
Because they cannot attribute mental states like ignorance to others, and are
unaware of the causal relation between behavior and beliefs, monkeys and apes
do not actively seek to explain or elaborate upon their thoughts. As a result, they are
largely incapable of inventing new words or of recognizing when thoughts should
be made explicit.
We suggest, then, that long before our ancestors spoke in sentences, they had a
language of thought in which they represented the world and the meaning of call
sequences in terms of actors, actions, and those who are acted upon. The linguistic
revolution occurred when our ancestors began to express this tacit knowledge, and
to use their cognitive skills in speaking as well as listening. The prime mover
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behind this revolution was a theory of mind that had evolved to the point where its
possessors did not just recognize other individuals’ goals, intentions, and even
knowledge as monkeys and apes already do but were also motivated to share
their own intentions, beliefs, and knowledge with others. It led to a mind that was
motivated to make public thoughts and knowledge that had previously remained
private. The evolution of a theory of mind spurred the evolution of words and
grammar. It also provided the selective pressure for the evolution of the physiological adaptations that enabled vocal modifiability. Whatever the selective pressures
that prompted this change, the complex suite of skills that we call human speech
built upon mental computations that had their origins and foundations in social
interactions.
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Chapter 14
Language, Lies and Lipstick: A Speculative
Reconstruction of the African Middle Stone
Age “Human Revolution”
Chris Knight
We have need of lies in order to conquer this reality, this “truth,” that is, in order to live
that lies are necessary in order to live is itself a part of the terrifying and questionable
character of existence. . . if there is anything to be worshipped it is appearance that must be
worshipped. . . the lie and not the truth is divine!
F. Nietzsche (1968: 451, 523)
Abstract Ovulation in humans is well concealed, leaving menstruation salient as
an external sign of fertility. Extant hunter-gatherers package this information in ways
designed to prevent philanderer males from exploiting it to their advantage. This
culminates in human symbolic culture a digital world of institutional facts designed
to conceal and reconstruct selected brute facts of biology. Coalitions of females,
supported by male kin, first began managing the public representation of menstruation, using cosmetics to scramble the fertility information made available to outgroup
males. Selected for their brilliance and redness, ochre pigments such as those found
at Blombos Cave in South Africa match the expectations of this model of symbolic
cultural origins. As costly signals, high-quality cosmetics reliably indicate the
strength of a woman’s commitments and corresponding coalitionary support. Investor
males can benefit by colluding with women’s cosmetic “lies.”
14.1
Digital Minds in an Analog World
Language has been described as a “mirror of mind.” Chomsky attributes this
exciting idea to “the first cognitive revolution” inspired by Descartes among others
in the seventeenth century. “The second cognitive revolution” triggered in large
C. Knight
Professor of Anthropology, Comenius University, Bratislava, Slovakia
e mail: chris.knight@live.com
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 14, # Springer Verlag Berlin Heidelberg 2010
299
300
C. Knight
measure by Chomsky’s own work is taken to have been a twentieth century
rediscovery of these earlier insights into the nature of language and mind. In 1660,
the renowned Port Royal grammarians (Arnauld and Lancelot 1972 [1660], p 27)
celebrated “this marvelous invention of composing out of twenty-five or thirty
sounds that infinite variety of expressions which, whilst having in themselves no
likeness to what is in our mind, allow us to disclose to others its whole secret, and to
make known to those who cannot penetrate it all that we imagine, and all the
various stirrings of our soul.”
For Descartes himself, however, this “marvelous” capacity was no mere human
invention: “the seat of the soul” was the pineal gland (Descartes 1991 [1640],
p 143). In Chomsky’s twentieth century reformulation, the relevant organ becomes
“that little part of the left hemisphere that is responsible for the very specific
structures of human language” (Chomsky in Piattelli-Palmarini 1980, p 182). As
Pinker (1999: 287) puts it: “We have digital minds in an analog world. More
accurately, a part of our minds is digital.”
But if “a part of the mind is digital,” how did it ever get to be that way? Under
what Darwinian selection pressures and by what conceivable mechanisms might a
digital computational module become installed in an otherwise analog primate
brain? Can natural selection acting on an analog precursor transform it incrementally into a digital one? Is such an idea even logically coherent?
If these were easy questions, the origins of language recently dubbed the
“hardest problem in science” (Christiansen and Kirby 2003) might long ago have
been solved. Chomsky accepts Darwinism in principle, but doubts its direct relevance
to this particular problem. In his view (Chomsky 2005, p 12), the “leap” to language
“was effectively instantaneous, in a single individual, who was instantly endowed
with intellectual capacities far superior to those of others, transmitted to offspring
and coming to predominate . . .” He considers the language faculty to be “surprisingly
perfect” just as we might expect had it been designed by “a divine architect”
(Chomsky 1996, p 30). Of course, Chomsky is no creationist. But otherwise supportive Darwinians have criticized him for suggesting an apparent miracle forgetting,
perhaps, that Chomsky’s guiding principle is internal consistency, not conformity
with the rest of science. “In fact,” writes Chomsky (2005, p 12) in justifying his
“Great Leap Forward” narrative, “it is hard to see what account of human evolution
would not assume at least this much, in one or another form.” Chomsky is informing
us that language, as he defines it, cannot gradually have evolved.
Chomsky (2005, pp 11 12) explains: An elementary fact about the language
faculty is that it is a system of discrete infinity. Any such system is based on a
primitive operation that takes n objects already constructed, and constructs from
them a new object: in the simplest case, the set of these n objects. Call that operation
Merge. Either Merge or some equivalent is a minimal requirement. With Merge
available, we instantly have an unbounded system of hierarchically structured
expressions. The simplest account of the “Great Leap Forward” in the evolution
of humans would be that the brain was rewired, perhaps by some slight mutation, to
provide the operation Merge, at once laying a core part of the basis for what is found
at that dramatic “moment” of human evolution...
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Merge, then, is more than an empirical necessity: it is a logical one. It is the
procedure central to any conceivable system of “discrete infinity.” Merge is recursive: it means combining things, combining the combinations and combining these
in turn in principle to infinity. Chomsky suggests that a “slight mutation” may
have allowed a single human ancestor to accomplish this cognitive operation for the
first time. No matter how we imagine the physical brain, according to Chomsky, the
transition to Merge is instantaneous, not gradual. This is because discrete infinity
“the infinite use of finite means” either is or is not. What sense is there in trying to
envisage “nearly discrete” objects being combined in “nearly infinite” ways? A
moment’s thought should remind us that when the objects to be arranged are subject
to even limited blending, the range of combinatorial possibilities crashes to a
limited set. In short, for Merge to work, the elements available for combination
must be abstract digits, not concrete sounds or gestures. Combining a sob with a cry
would not be an example of Merge. Neither would we call it Merge if a chimpanzee
happened to combine, say, a bark with a scream (Crockford and Boesch 2005).
14.2
Analog Minds in a Digital World
One way to escape the conundrums inseparable from Chomsky’s position
conundrums central to the recent explosion of debates on language origins and
very well documented by Botha (2003) might be to keep the essential idea, but
to reverse the underlying philosophy. Humans have analog minds in a digital world.
More accurately, just a certain part of our world is digital. We are at one with our
primate cousins in being immersed in ordinary material and biological reality
Pinker’s “analog world.” But unlike them, we have woven for ourselves an additional environment that is digital through and through. This second environment that
we all inhabit is sometimes referred to as the “cognitive niche” in nature, but the
evolutionary psychologists who invented this expression (Tooby and DeVore 1987)
did so in pursuit of their own particular agenda. Adherents of the “cognitive revolution” but attempting to marry a reluctant Chomsky to their own mentalist version of
Darwin, they are committed to minimizing the intrinsically social, cultural, and
institutional nature of the digital semantic representations made available to our
brains. The expression “cognitive niche” may have explanatory value, but not if the
purpose is to prioritize “nature” at the expense of what social anthropologists and
archeologists term “symbolic culture.” Contrary to Tooby and DeVore (1987), the
“cognitive niche” does not exist anywhere “in nature.” No one has ever found such a
niche “in nature.” As Tomasello (1999) points out, distinctively human cognition is
inseparably bound up with the evolution of culture. The “cognitive niche,” to be
precise, exists only as an internal feature of human symbolic culture.
So what exactly is this thing called “symbolic culture?” Following the philosopher Searle (1996), let us begin by drawing a distinction between “brute facts” and
“institutional facts.” Birth, sex, and death are facts anyway, irrespective of what
people think or believe. These, then, are brute facts. Phenomena such as legitimacy,
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marriage, and inheritance, however, are facts only if people believe in them.
Suspend the belief and the facts correspondingly dissolve. Although institutional
facts rest on human belief, that does not make them mere distortions or hallucinations. Take two five-pound banknotes. Their monetary equivalence to one ten
pound note is not merely a subjective belief: it is an objective, indisputable fact.
But now imagine a collapse of confidence in the currency. Suddenly, the various
bits of paper are worthless the former facts have dissolved.
Institutional facts are not necessarily dependent on verbal language: one can
play chess, use an abacus, or change money without using language. The relevant
digits are then the chess pieces, beads, or coins that function as markers in place of
linguistic signs. Facts of this kind the intricacies of the global currency system, for
example are patently nonphysical and nonbiological. We may think of them as
internal features of an all-encompassing game of “let us pretend.” Needless to say,
institutional facts presuppose a brain with certain innate capacities, syntactical
language being one possible manifestation of those capacities. But as Tomasello
(2006) points out, explaining distinctively human cognition by invoking “language” is circular and unhelpful: it is precisely the language that we need to explain.
When people coin a new word “spam” to mean “bulk e-mail” is a recent
example it becomes established as an institutional fact. Whether linguistic or
nonlinguistic, facts of this kind develop ontogenetically out of the distinctively
human capacity for mindreading, joint attention, and “let us pretend.” The underlying formula is “Let X count for us as Y” (Searle 1996). Using a broomstick to
signify “horse” is, in principle, no different from using “spam” to signify “bulk
e-mail.” When children learn the meanings of words, they succeed not thanks to a
word-learning module dedicated exclusively to this task but by drawing on more
fundamental and empirically verifiable features of social and nonsocial cognition
(Bloom 2000). In particular, learning the meaning of a word presupposes the ability
to correlate perspectives, grasping others’ referential intentions. It is this imaginative ability the ability to infer and share intentions and goals that distinguishes
human cognition so radically from that of apes (Tomasello et al. 2005).
Of course, it is always possible to term this critical ability as “language.” This
might seem helpful if you consider language to be an innate mechanism operating
independently of the rest of cognition or of any institutional setting. Chomsky does
hold this view, treating language as a faculty no different, in principle, from walking
or stereoscopic vision. Pinker sets out from essentially the same position: language,
he says, should be studied on the model of echolocation in bats or stereoscopic vision
in primates. Distancing himself from Chomsky, however, Pinker insists that language is specifically designed for a social function namely, communicating
thoughts. Pinker explores how “words and rules” are continuously invented and
reinvented for this purpose. In Searle’s terms, the entities thereby produced are
“institutional facts.” Pinker calls them “inventions.” But if they are indeed inventions, Chomsky’s foundational assumption must be wrong. Language cannot be
understood simply as a biological object. It operates on an entirely different level of
organizational complexity from walking or stereoscopic vision mechanisms,
which, after all, do not require institutional arrangements in order to work.
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What would language consist of in the absence of institutional facts? What
meaning would language have to a child deprived of “words and rules?” According
to Chomsky, the first human to be endowed with language used it to “articulate to
itself its thoughts.” As he explains (Chomsky 2002: 148): “Actually you can use
language even if you are the only person in the universe with language, and in fact
it would even have adaptive advantage. If one person suddenly got the language
faculty, that person would have great advantages; the person could think, could
articulate to itself its thoughts, could plan, could sharpen, and develop thinking as
we do in inner speech, which has a big effect on our lives. Inner speech is most of
speech. Almost all the use of language is to oneself, and it can be useful for all kinds
of purposes (it can also be harmful, as we all know): figure out what you are going
to do, plan, clarify your thoughts, whatever. So if one organism just happens to gain
a language capacity, it might have reproductive advantages, enormous ones. And if
it happened to proliferate in a further generation, they all would have it.”
But if communication were inessential, what need was there for any kind of
external transmission via phonology? And if there was no such transmission, how
could syntax have interfaced between Phonetic Form and Logical Form? After all,
there would have been no Phonetic Form. Finally, if we accept that language can
exist when stripped of this interface when stripped of syntax as Chomsky (2005)
himself defines it in what sense does the residue deserve to be called “language”?
Why not just call it “mentalese” or “thought”?
Pinker (1999: 287) concludes his book on “the ingredients of language”: “It is
surely no coincidence that the species that invented numbers, ranks, kinship terms,
life stages, legal and illegal acts, and scientific theories also invented grammatical
sentences and regular past tense forms.” Confusing correlation with causation,
Pinker here treats the supposedly digital concepts intrinsic to human cognitive
nature as responsible for the legalistic distinctions so characteristic of symbolic
culture. Note, however, that the digital concepts he actually mentions here whether
linguistic or nonlinguistic belong without exception not to individual cognition but
to the realm of agreements and institutions. This is surely no coincidence after all,
we possess no evidence that language would be possible at all outside such institutional settings. Reversing Chomsky and correspondingly reversing the whole idea
of “digital minds in an analog world” we can conclude that “doing things with
words” (cf. Austin 1978 [1955]) is more than just activating a biological organ. To
produce “speech acts” (Searle 1969) is to make moves in a nonbiological realm a
realm of facts whose existence depends entirely on collective belief.
14.3
The Evolution of Deep Social Mind
Evolutionary psychologists often refer to the evolution of “deep social mind”
(Whiten 1999). By this, they mean the kind of mind that cannot be restricted to
one individual. Deep social mind is recursive mind as represented in other minds
and as it represents to itself such representations. There is a subtle difference
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C. Knight
between this idea and the theory that thought is dependent on language. “No support
can be found for the view that words are necessary for thought,” writes Bloom
(2000) in his exhaustive study of how children learn the meanings of words. But if
words are not necessary for thought, in what sense can “language” be said to be
necessary?
To appreciate why it is so unhelpful to privilege language as the source of
uniquely human cognition, let us take the case of pointing (Tomasello 2006).
Intentional pointing begins in children at about 14 months; chimpanzees never
reach this stage. Pointing would seem to be a relatively simple activity, not
requiring much in terms of computational hardware. Since it appears so simple,
why do not chimps do it?
One answer might be that Universal Grammar is required and chimps do not
have Universal Grammar. But that would surely be absurd: pointing does not depend
on any kind of grammar. It is true that whatever cognitive abilities enable pointing
are necessary to enable talking as well, but that is no excuse for attributing evolutionary priority to language. Tomasello (2006: 520) concludes: “To explain human
cognitive uniqueness, many theorists invoke language. This contains an element of
truth, because only humans use language and it is clearly important to, indeed
constitutive of, uniquely human cognition in many ways. However, . . .. asking why
only humans use language is like asking why only humans build skyscrapers, when
the fact is that only humans, among primates, build freestanding shelters at all. And
so for my money, at our current level of understanding, asking why apes do not have
language may not be our most productive question. A much more productive
question, and one that can currently lead us to much more interesting lines of
empirical research, is asking the question why apes do not even point.”
So why do not apes point? Tomasello offers a social explanation. Regardless of
whatever mindreading abilities apes possess, in their natural environment, they lack
any motive to correlate perspectives or share goals. They are, by nature, incorrigibly
competitive Machiavellian social strategists. Only quite peculiarly cooperative
creatures motivated to share goals and intentions could have any reason to point
or any reason to go yet further and invent “words and rules.”
When fictional representations are given public and observable form as in a
game of “let us pretend” language has started to evolve. Scaled up from the level of
children’s games and extended across society as a whole, “let us pretend” may
generate complex systems of ritual and religion (Durkheim 1947 [1915], Knight
1998, 1999, 2000a,b, 2009b; Power 1999, 2000). The morally authoritative intangibles internal to a symbolic community that is, to a domain of “institutional facts”
are always on some level digital. This has nothing to do with the supposedly digital
genetic architecture of the human brain. The explanation is less mystical. It is simply
that institutional facts depend entirely on social agreement and you cannot reach
agreement on a slippery slope. What would it mean if the Queen in her official
capacity were to “open Parliament,” but only slightly? Or if a couple who had just
made their wedding vows were pronounced man and wife but only “more or less”?
What applies to royal and religious edicts also equally applies to speech acts, in
general. Chomsky notwithstanding, semantic distinctions are social and institutional,
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not individual or innate. Take the classic case of basic color terms. All humans, in all
cultures, discriminate perceptually among an immense variety of different hues. But
while actual colors can be directly perceived and while innate biases play a key role
in determining which regions of the spectrum are picked out it need hardly be
stressed that digital color categories operate on a quite different level. Knowing that
the spectrum is segmented into two, three, or some other limited set of “colors” “the
seven colors of the rainbow,” for example requires access to the relevant institutional conventions. Basic color terms English “red” and “green,” for example map
directly to these simplified abstractions; they do not and could not possibly map to the
vastly more complex features of the human visual system as such (Davidoff et al.
1999; Davidoff 2001; Steels and Belpaeme 2005). To summarize: by definition,
anything perceptible can be evaluated and identified through direct sensory input
in other words, on the basis of innate perceptual mechanisms. But institutional
intangibles are inaccessible to the senses. Being invisible, intangible and in a
fundamental sense unreal, they can be narrowed down and agreed upon only through
a process in which abstract possibilities are successively eliminated. “Discrete
infinity” captures the recursive principle involved.
The sound system of a language its phonology is prototypically digital. It is
no more possible to compromise between the t and the d of tin versus din than to
compromise between 11:59 and 12.00 on the face of a digital clock. Of course,
categorical perception is common enough in nature. But the meaningless contrastive phonemes of human language comprise only one digital level out of the two
that are essential if meanings are to be conveyed at all. Combining and recombining
phonemes “phonological syntax,” as it is called by ornithologists (e.g., Marler
1998) who study the digital phenomenon in songbirds would be informationally
irrelevant if it did not interface with a second digital level, which is the one
necessary if semantic meanings are to be specified. No animal species has access
to this second level of digital structure. It would, therefore, be inconceivable and, in
principle, useless anyway for an animal to make use of syntactical operations
whether Merge or anything else in order to interface between the two digital
levels. The explanation is that animals inhabit just their own biological world, and
therefore do not have access to the extra digital level. It is the nature and evolution
of the entire second level the level of symbolic culture that has proved so
difficult a puzzle. Explaining “the Great Leap Forward” as an outcome of “Merge”
is a parsimonious solution (Chomsky 2005), but only in the sense that explaining it
as an outcome of divine intervention might seem persuasive in terms of parsimony
although less so in terms of relevance or testability.
14.4
A Darwinian Solution
The alternative is to conceptualize the language capacity as one remarkable manifestation of a “play capacity” continuous with its primate counterparts, but let loose
among humans in a manner not open to other animals (cf. Jespersen 1922; Huizinga
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C. Knight
1970 [1949], Knight 2000b). The development of play and the development of
language in children are widely recognized as isomorphic. Pretend-play and language have the same critical period, the same features of intersubjectivity and joint
attention, the same triadic (do you see what I see?) referential structure, and the
same cognitive expressivity and independence of external stimuli. It is unlikely that
these parallels are a pure coincidence (Bruner et al. 1976; Trevarthen 1979;
Tomasello 2003).
“Digital infinity” corresponds to what developmental psychologists might recognize as a children’s game in this case, “let us play infinite trust.” Take any
patent fiction and let us run with it and see where it leads. Metaphorical usage is an
example of this. A metaphor “is, literally, a false statement” (Davidson 1979).
React on a literal level as an autistic person might do and the signaler is
rebuffed, denied the freedom to “lie.” But most of us do not react in this unimaginative and unsympathetic way: by accepting the patent fiction and sharing in it, we
can construct it as truth on a higher level truth for “our own” joint purposes of
conceptualization and communication. A red shirt is not necessarily “blood” but
identifying it that way might pick out one garment from a limited set. As literal
falsehoods become gradually conventionalized, one possible trajectory is that they
crystallize out as “dead metaphors” familiar lexical items whose origins in vivid
metaphor have become forgotten. Grammatical markers and associated constructions are historical outcomes of essentially similar processes that are now well
understood (Meillet 1903; Lakoff and Johnson 1980; Heine et al. 1991; Gentner
et al. 2001; for the best recent overview, see Deutscher 2005).
If all this is accepted, it follows that for words and rules to evolve, humans must
trust one another sufficiently to find value in patent falsehoods. It is for social
reasons that nonhuman primates are unable to do this. Chimpanzees, like other
primates, have solid grounds for attempting to read one another’s minds. But like
devious spies, they have no reason to allow rivals to reciprocally read back into
their own minds. Sometimes, a human may direct a pointing gesture or other
cooperative signal at a captive chimp. Typically, the recipient gets confused, as if
unable to grasp the helpful intent (Hare and Tomasello 2004). In the wild, resistance
to deception would prevent such signals from being accepted on trust. Among other
consequences, such insistence on reliability blocks metaphor and in so doing
blocks the elaboration of abstract analogical thought of the kind so characteristic of
humans (Lakoff and Johnson 1980). If animals do not talk, therefore, it is not
because they lack the requisite digital module installed inside their brains. The
explanation is more simple: they live in a Darwinian world. By “a Darwinian
world,” I mean a competitive world that is not subject to group-level moral
regulation (Knight 2008). Animals value signals to the extent that they are dependable, hence hard to fake. Body language alone has this property. Reflecting cognitive states, intentionally produced symbols are “head language.” If they are not
found in nature, it is because such things are evidently false.
The social factors that allow metaphorical usage in humans are equally the ones
permitting digital concepts to emerge. Hard-to-fake indices such as laughs, sobs,
cries, and so forth must be evaluated for intrinsic quality on an analog scale. It is not
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theoretically possible to evaluate abstract digits in this way. Probe a digit for its
quality and it ceases to function as a digit. Analog evaluation, by the same token,
rules out the possibility of Merge. But now let us examine the reverse situation.
Regardless of innate cognitive architecture, the contrastive possible intentions
behind a communicative fiction are in principle immune to analog evaluation.
“Discrete infinity” becomes unavoidable in this context because linguistic signs
are “honest fakes” literal irrelevancies and falsehoods, significant only as cues to
the intentions underlying them. Since communicative intentions are intangibles,
deciding between them is digital by reason of logical necessity, not because the
brain or any part of it is innately digital.
Durkheim (1947, p 421) long ago observed, “Animals know only one world, the
one which they perceive by experience, internal as well as external. Men alone have
the faculty of conceiving the ideal, of adding something to the real. Now where does
this singular privilege come from?”
Maynard Smith and Szathmáry (1995) offered a bold Darwinian answer to
Durkheim’s question, citing Rousseau and viewing the puzzle of language origins
as inseparable from the wider problem of explaining the emergence of life governed
by morally binding contracts. Their “major transitions” paradigm is ambitious and
conceptually unifying, assuming no unbridgeable chasm between natural and social
science. The same applies to the paradigm being developed by Steels and his
colleagues (Steels et al. 2002; Steels 2006, 2009), who use robots to show how
shared lexicons and grammars patterns far too complex to be installed in advance
in each individual brain spontaneously self-organize through the processes of
learning, recruitment, social coordination, and cumulative grammaticalization.
Steels (2009) emphasizes that language evolves spontaneously but only under
highly unusual conditions conditions of mutual trust and cooperation that are
far removed from Darwinism as usually understood. By maintaining continuity
with primate analog minds while introducing novel social factors factors such as
collective enforcement of “the rule of law” (Knight 2008) we can continue to
apply basic principles of Darwinian behavioral ecology to account for the emergence of distinctively human cognition and communication.
“Analog minds in a digital world” is fully compatible with Darwinian evolutionary theory. “Digital minds in an analog world” is not compatible at all.
Installation of an innate digital mind whether instantaneous or gradual is a
deus ex machina with nothing Darwinian about it. A model of language evolution, to qualify as scientific, cannot invent fundamental axioms as it goes along.
It cannot invoke currently unknown physical or other natural laws. It should be
framed within a coherent, well-tried body of theory; it should generate predictions that are testable in the light of appropriate empirical data; and it
should enable us to relate hitherto unrelated disciplinary fields. While the deus
ex machina approach rejects the accumulated achievements of social science, the
play/mindreading/joint attention paradigm (Tomasello 1996, 1999, 2003, 2006;
Tomasello et al. 2005) has the potential to link the natural and social sciences in
“a theory of everything” a testable theory to explain the origin of language and
symbolic culture as a whole.
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14.5
C. Knight
The Female Cosmetic Coalitions Model
The specific hypothesis I favor involves menstruation. I am well aware that this
topic is usually avoided, being hedged around with our own culture’s unspoken
avoidances and taboos.
Primate sexual signals represent compromise outcomes of varying degrees of
cooperation and conflict between the sexes. Chimpanzee oestrous swellings, for
example, are graded signals representing a compromise between (a) the male
demand for reliable evidence of ovulation and (b) the requirement of females to
withhold accurate information in defense of their own sexual choice (Nunn 1998).
In this battle of the sexes, human females apparently achieved victory in at least one
respect: ovulation has become effectively concealed. However, concealment of
ovulation leaves menstruation salient as the one remaining external evidence of a
female’s (imminent) fertility (Power and Aiello 1997). In extant hunter-gatherer
societies, neither sex is oblivious to the potential philandering opportunities such as
the information represents. The Mbuti “consider that any couple that really wants
children should ‘sleep with the moon’” (Turnbull 1993 [1961], p 169). Hadza
informants generally view conception as occurring immediately after menstruation
(Marlowe 2004). “A woman is considered most fertile during menstruation” writes
Lewis (2002: 109) of the Mbendjele. “Sexual intercourse at this time is widely held
to be a sure way of ensuring pregnancy begins. Menstrual blood is the symbol par
excellence of human fertility.” Adequate statistics are lacking, but on theoretical
grounds, I would predict that hunter-gatherers everywhere will be found to hold
similar views.
Imagine a world in which selfish gene Darwinism was the only law. Menstruation would then act as a starting gun for sexual conflict. Dominant males in
pursuance of individual fitness would be tempted to abandon their current partners
already pregnant or nursing to compete for access to new partners marked out as
imminently fertile by their blood. In real life, hunter-gatherers surround the blood
with elaborate taboos, as if the aim were precisely to keep philanderers at bay
(Knight 1991, 1996). Across Africa, the young menstruant’s senior female relatives
respond to her condition as if to an immediate threat. Mobilizing the entire
community, they subject her to strict supervision and control, celebrating her
fertility but at the same time bonding tightly with her and, above all, controlling
male access to her. Although menstruation is a biological signal, its salience and
significance varying on an analog scale, the logic at work here excludes intermediate states. To acknowledge any blood is to publicize the danger represented
triggering the full collective response.
Contrary to many western misconceptions, menstrual taboos are not necessarily
the evidence of sexist oppression under patriarchal rule. African hunter-gatherers
are not noticeably male-dominated. In fact, women typically have much solidarity
and power (Turnbull 1993 [1961], Lewis 2002, 2009). Although cultural variability
is great, the ubiquity and evident antiquity of menstrual taboos can be explained
on a Darwinian basis as the outcome of an evolutionarily stable strategy pursued
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Fig. 14.1 (a) Females compete for good genes, staggering signs of ovulation. (b) Females
compete for mating effort, synchronizing signs of ovulation. (c) Males abandon females once
ovulation has passed. (d) Females counter this problem by concealing ovulation and extending
receptivity. (e) Menstruation now attracts disproportionate male attention. (f) Coalition members
respond to this threat by controlling male access to the (imminently) fertile female. (g) To prevent
males from picking and choosing between them, members of the coalition join forces and
“paint up.” (h) “The rule of law” is established as an institutional fact.
for millennia by women in pursuit of their own reproductive interests. Childburdened mothers require investment from males, not abandonment, harassment,
or violence. Socially approved posture, clothing, scarification, and cosmetics
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C. Knight
are characteristically human ways of “covering up”
concealing, modifying,
and scrambling those signals which, if left biologically naked and exposed,
might preclude the possibility of rule-governed behavior of any kind. Figure 14.1
presents a schematic outline of the Female Cosmetic Coalitions Model, depicting
how coalitionary resistance to philandering in this scenario culminates in the rule
of law.
14.6
The Human Revolution
Until recently, most archeologists attributed the emergence of language to a “human
revolution” dated to some 45,000 years ago. When Homo sapiens began emerging in
Africa during the Middle Stone Age, according to this model, cognition and behavior was still “premodern” or “archaic.” Only when our species began migrating into
Europe, displacing the resident Neanderthals and triggering the “Upper Paleolithic
Revolution,” did “modern” language and symbolic culture emerge (Mellars and
Stringer 1989; Diamond 1992; Klein 1995, 2000; Tattersall 1995).
Over the past decade, it has become apparent that this scenario was an artifact
resulting from a Eurocentric sampling of the fossil and archeological records
(Mellars et al. 2007). Recent studies by archeologists working in Africa have
shown that almost all the cultural innovations dated to around 45,000 years ago
in Europe can be found at much earlier dates at one or another site in Africa. Blade
and microlithic technology, bone tools, logistic hunting of large game animals,
long-distance exchange networks these and other signs of modern cognition and
behavior do not appear suddenly in one package as predicted by the Upper
Paleolithic “human revolution” theory. They are found at African sites widely
separated in space and time, indicating not a single leap but a much more complex,
uneven but broadly cumulative process of biological, cultural, and historical change
(McBrearty and Brooks 2000; McBrearty 2007).
Such evidence casts doubt on the notion of a single mutation installing the
language faculty in one step and suddenly inaugurating society and history (Bickerton 1990; Chomsky 2005; Klein 1995, 2000). But to reject that particular model
of the “human revolution” is not necessarily to reject the basic idea. I prefer to
update it in the light of recent archeological evidence, extending the timescale and
relocating the whole process within the African Middle Stone Age (Knight et al.
1995; Watts 1999; Mellars et al. 2007; Knight 2009a,b). The human revolution
would then take its place in the history of life on earth as a major transition on the
model of the emergence of multicellular complexity, the origin of chromosomes, or
the first appearance of life itself. In their “major transitions” paradigm, Maynard
Smith and Szathmáry (1995, pp 255 309) present the coevolution of symbolic
ritual, moral regulation, and language as an example of how Darwinan natural
selection may culminate in a revolutionary transition. My aim in this chapter has
been to flesh out a story along these lines.
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Chapter 15
Brain and Behaviour in Primate Evolution
R.I.M. Dunbar
Abstract Primates have unusually large brains for body size compared to all other
vertebrates. Over the years, a number of explanations have been offered for this,
leading to some confusion. I use a systems approach to make sense of these
suggestions, and this suggests that some are constraints (energetic or neural development explanations), others consequences (generated by windows of opportunity),
but only the social hypotheses constitute the real selective pressure for the evolution
of brains. The social hypotheses come in two current forms (bonding social groups
vs. social learning of foraging skills) that differ in whether predation or food-finding
are assumed to be the rate-limiting factor in primate survival. While the standard
form of the social brain hypothesis in primates is a quantitative relationship
between social group size and brain size, comparative analyses for other mammal
and bird taxa reveal that it takes a purely qualitative (i.e., categorical) form in all
nonprimates examined so far: species with pairbonded (i.e., monogamous) mating
systems have larger brains than all others. I suggest that this difference is due to the
fact that anthropoid primates developed bonded social systems early in their
evolutionary history. Finally, I consider briefly the implications of these findings
for human evolutionary history.
15.1
Introduction
Some three decades ago, Jerison (1973) observed that primates have significantly
larger brains for body size than all other vertebrate orders. While considerable
progress has been made in understanding brain evolution since then, three classes of
explanations still have currency as explanations for the evolution of large brains
R.I.M. Dunbar
Institute of Cognitive and Evolutionary Anthropology, University of Oxford, Oxford, England
e mail: robin.dunbar@anthro.ox.ac.uk
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 15, # Springer Verlag Berlin Heidelberg 2010
315
316
R.I.M. Dunbar
Table 15.1 Hypotheses that have been proposed to explain the evolution of brain size in primates
Hypothesis
Explanatory focus
Key source reference
Ecology
Frugivory (ephemeral food sources)
Clutton Brock and Harvey (1980)
Range size (mental maps)
Clutton Brock and Harvey (1980)
Extractive foraging (embedded
Gibson (1986)
foods)
Innovations/tool use (trial and error
Reader and Laland (2002)
learning)
Social
Innovations (social transmission)
Reader and Laland (2002)
Machiavellian behaviour
Byrne and Whiten (1988)
Social bonding
Dunbar (1992, 1998), Barton (1996)
Developmental
Body size (energetic constraints)
Martin (1981)
Structural constraints
Finlay and Darlington (1995)
Longevity (energetic constraints)
Barrickman et al. (2008)
among the primates: ecological hypotheses, social hypotheses, and life history
hypotheses (Table 15.1). Each of these comes in several alternative versions that
can be differentiated by the proximate function that large brains are assumed to
mediate. In other words, the differences lie in the mechanisms whereby fitness is
maximised, and hence in which components of survival and successful reproduction are rate-limiting.
The ecological hypotheses presume that survival is the principal problem that
animals face, and that food acquisition is the rate-limiting process. Traditionally,
this has always been interpreted in terms of food-finding (either in the form of a
mental mapping issue or in the form of the cognitive demands of frugivory) or of
extractive foraging (Parker and Gibson 1979; Clutton-Brock and Harvey 1980;
Gibson 1986). More recently, the role of innovations and tool use has been given
more prominence, and has received considerable empirical support (Reader and
Laland 2002). In addition, the implications of this for extinction risk have not
gone unnoticed, especially in the bird literature (Sol et al. 2002, 2005; Shultz et al.
2005).
In contrast, although the social hypotheses have also emphasised the significance
of survival, their main contention is that survival is maximised by social means. The
key difference between the social hypotheses and the ecological hypotheses is, thus,
how these ecological goals are achieved. The former argue that they are achieved
socially, the latter that it is purely a consequence of individual trial-and-error
learning. Ecologically speaking, survival can be maximised either by solving the
problem of food-finding or by minimising predation, and this generates two alternative versions of the social hypothesis. One (formally known as the social brain
hypothesis) is that, maintaining coherent groups is the key mechanism for minimising predation risk and that it is the cognitive demands imposed by this that lies at
the heart of primate brain evolution (Dunbar 1992, 1998). The other argues that the
constraint lies in the capacity to engage in social forms of learning when acquiring
new foraging skills (Reader and Laland 2002). Thus, these alternative versions of
the social hypothesis differ in whether the rate-limiting process is mortality from
predators or mortality from starvation.
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317
Finally, the life history hypotheses emphasise the fact that growing and maintaining large brains is energetically expensive (Aiello and Wheeler 1995), and
hence that evolving large brains is, in some way, a consequence (or a cause) of
an extended life history. Again, at least two versions can be discerned. One is that
large brains correlate with body size in such a way as to offer savings of scale that
allow the mother to switch spare energy from herself to fetal brain growth (Martin
1981; Hofman 1983; Armstrong and Bergeron 1985; Isler and van Schaik 2006). In
the limiting case, large brains are simply a (nonadaptive) by-product of large body
size and require no evolutionary explanation. The other notes that large brains are
associated with an extended life history (longer life span, longer gestation, etc.), and
argues that large brains are a constraint on the need to evolve a slowed-down life
history (Barrickman et al. 2008). A related series of analyses (Finlay and Darlington
1995; Finlay et al. 2001) have emphasised ontogenetic aspects of brain growth: in
this case, final brain size depends on the number of cell cycles that can be achieved
during the period of brain growth. Since the rate of neurogenesis is constant, the
only way to evolve a larger brain is to slow down the rate of development to allow
time for more neurogenic cycles. Life history explanations tend not to identify (or
even consider) functional explanations as to why large brains might be selectively
advantageous. If anything, large brains are often viewed as an accidental by-product
of changes in life history.
Evidence has been adduced for all of these hypotheses, but in most cases
individual hypotheses have been tested without considering the implications of
other hypotheses. General principles of parsimony make this rather confusing
situation particularly puzzling: we would not normally expect so many radically
different selection pressures to influence the same phenomenon equally. While
some previous analyses (e.g. Dunbar 1992; Deaner et al. 2000) have attempted to
test between alternative hypotheses for large brain size in primates, they have
invariably done so using a series of bivariate analyses (one for each possible
independent variable) without forcing the hypotheses into direct competition.
Since many of these variables are confounded not least because some may be
emergent properties of others it was, not too surprisingly, difficult to distinguish
between alternative hypotheses. In fact, without an attempt to integrate them into
a single explanatory framework, little real progress is likely to be made. One
solution to this is the “critical tests approach” advocated by van Schaik (1983):
this seeks to put alternative hypotheses into direct contest with each other in
such a way that the data are forced to support one and only one hypothesis (e.g.
see van Schaik and Dunbar 1990; Dunbar et al. 2002; Calhim et al. 2006).
However, a “critical tests approach” assumes that all competing explanations are
at the same explanatory level. This need not always be so: some may be functional
explanations, while others are concerned more with mechanisms (e.g., constraints). A systems approach will usually be a better approach in such cases. In
particular, path analysis offers an alternative way of testing more complex relationships between a set of variables, especially when the variables themselves
form part of a nested set of explanations, not all of which are in competition with
each other.
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15.2
R.I.M. Dunbar
A Systems Approach to Brain Evolution
Given the number of factors that have been advocated as possible (or, indeed, sole)
causes of brain evolution, it is perhaps surprising that no serious attempts have been
made to integrate all these into a single analytical model in such a way as to test
between them and establish why it is that so many factors seem to correlate with
brain size. The fact that many variables correlate with brain size ought to prompt us
to think systemically in terms of causes, constraints, and consequences. In the
interests of parsimony, one ought to assume that there is only one core selection
pressure that is principally responsible for the evolution of a given trait (unless and
until demonstrated otherwise). An important distinction, therefore, needs to be
drawn between (1) the selection pressure that gives rise to a trait (and without
whose presence the trait would simply not evolve) and (2) constraints (that have to
be resolved in order to allow the trait in question to evolve), (3) secondary selection
factors (that push the trait in the same direction, but which of themselves are
insufficiently powerful to give rise to the trait in the face of the costs that militate
against it), (4) tertiary selection pressures (that impose selection on individual
subunits, but not the brain as a whole), and (5) emergent properties (“windows of
opportunity” that arise once the trait is in place). All of these are well-known
aspects of evolutionary systems, but do not seem to have been kept as well
separated as they should have been in discussions of brain evolution.
In considering the factors acting on brain size, it is important to bear in mind
both the costs and the benefits. Brain tissue is exceptionally costly (Mink et al.
1981; Aiello and Wheeler 1995; Kaufman 2003; Isler and van Schaik 2006;
Karbowski 2007), and any benefits must exceed the costs of evolving and maintaining additional brain tissue. Since the costs are high in this case, the benefits must
be as well. Thus, not only do we need to understand which variables are costs
(and, thus, act as constraints on brain evolution) and which benefits, but we also
need to determine which of the many potential benefits is the core selection factor
that has driven brain evolution and which are by-product benefits (emergent properties that provide a supplementary benefit to having a large brain once you have
evolved one). This is especially important in the context of primate brain evolution,
since it seems often to be assumed that the forces selecting for large brains are
constant across taxonomic groups (i.e., one explanation fits all). However, this
overlooks Jerison’s (1973) original point that primates have significantly larger
brains than all other taxa. In other words, we have a size issue that itself needs
explaining.
One way of approaching this problem would be to cost out the relative costs and
benefits that derive from different sources and calculate the rate of change in payoff
against changing brain volume. The rate of change in costs can be calculated fairly
easily, since it is, in principle, simply the added energetic costs of a unit of neural
tissue. However, calculating the fitness gains from different possible benefits is
much less straightforward. Indeed, this may be especially problematic if some of
the benefits arise through multilevel selection rather than through direct impact on
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individual fitness: calculating these indirect fitness components is much more
difficult and almost never done in behavioural ecology, except in the special case
of kin selection.
Multilevel selection is particularly important in the case of the social brain
hypothesis because this explicitly claims that the benefits accrue through reduced
predation as a result of more cohesively bonded groups. The proximate goal driving
brain size evolution (and hence cognitive ability) in this case is not reduced
predation as such (though this may play some role even so: Shultz and Dunbar
2006), but how well bonded the social group itself is. The argument is explicitly a
two-step process: large brains allow larger groups to be better bonded, and better
bonded groups are more effective at reducing predation (or solving any other
ecological survival problem). As with cooperative hunting, group members have
to be able to work effectively together if they are to gain the benefit that living in a
large group offers. It is unlikely that this takes the form of deliberate cooperation
(indeed, it is even questionable whether this occurs among cooperative hunters);
rather, it probably takes the form of mechanisms that allow group members to
ameliorate the stresses that close physical proximity invariably imposes. These
include both direct and indirect costs. Direct costs include the impact of dominance
on the foraging efficiency of low-ranking animals as well as the temporary infertility
that even mild harassment can create by disrupting low-ranking females’ menstrual
cycles (Dunbar 1980; Abbott et al. 1986). Indirect costs include the familiar ones of
longer-day journeys and higher foraging costs (e.g., Lehmann et al. 2008a,b).
Emergent properties (secondary or tertiary benefits) often create an analytical
problem in evolutionary explanations: in many cases, these are secondary benefits
that are easily confused with the primary benefits that have been responsible for the
trait’s evolution. Thus, it might be that social coherence was the selection pressure
that led to the evolution of large brains; but, once large brains were in place, their
generic computational capacities could be used for any number of other purposes
(including the kind of smart foraging identified by the innovations hypothesis). In
this case, however, we at least have an alternative hypothesis to create a critical test,
since the converse causal sequence is, in principle, equally plausible (i.e., that the
social benefits of bonding are an emergent property of having a large brain that
originally evolved for direct individual-level ecological problem-solving). One way
to approach this question is to undertake a phylogenetic analysis in which the two
traits and brain size are mapped onto a phylogenetic tree so that the order in which
changes in all the traits occurred can be mapped. The aim is to search for correlated
evolution between brain size and one trait (the principal selection factor) and lagged
evolution between these two and the second trait (the emergent property). It is now
possible to do this using statistical methods such as Pagel’s (1997) DISCRETE.
A full such analysis has yet to be undertaken, but Pérez-Barberı́a et al. (2007)
have demonstrated that among primates, in contrast to carnivores or ungulates,
changes in brain size and sociality are so tightly correlated that there are almost no
lag or indirect effects. Although cognitive skills as such were not included in this
analysis, it seems safe to conclude on this evidence that it is very unlikely that
sociality is a mere by-product of evolving a large brain; rather, one variable must be
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R.I.M. Dunbar
acting as a constraint on the other and the two are locked in a tight coevolutionary
cycle you cannot change one without forcing a change in the other at the same
time. Given this, the most likely interpretation is that brain size evolution is driven
by the demands of sociality, and that any other benefits in terms of foraging are a
by-product of the added computational power offered by having a large brain.
One reason for assuming the causal arrow must be this way round is that living in
large groups incurs costs (as noted above), and a species is unlikely to live in large
groups simply because its brain allows it to do so when larger brains have evolved
to permit some other behavioural adaptation. In other words, large groups do not
come for free if the brain evolves to facilitate smarter foraging, but smarter foraging
probably does come for free if the brain evolved to facilitate large groups because
smart foraging capitalises on the same inferential cognitive processes as are used in
the social domain, but does not in itself incur additional costs.
An alternative is to use a systems approach that explicitly integrates all the
components into a single model that specifies directly which factors are causes,
which are constraints, and which are emergent properties. Path analysis is particularly helpful in this respect because it allows a number of alternative models to be
evaluated against each other using an information index criterion. With group size
as the dependent variable, Dunbar and Shultz (2007a) used path analysis to show
that the best model tested consisted of just three key predictors: neocortex size,
activity pattern (nocturnal vs. diurnal), and range size (Fig. 15.1). It seems unlikely
that range size determines group size, so we may assume (as the primate literature
Fig. 15.1 Path analysis of the functional relationships influencing social group size in primates.
Path analysis allows us to separate out primary selection factors from those that act indirectly
(i.e., constraints). Reprinted with permission from Dunbar and Shultz (2007a)
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has always done: Dunbar 1988) that the causal direction in this case is from group
size to range size. Thus, only activity period and neocortex volume explicitly
influence group size. Since neocortex size and group size are locked in a tight
coevolutionary spiral (Pérez-Barberı́a et al. 2007), the causal arrow, thus, runs both
ways in this case. It is important to be clear about what this means: in an evolutionary timescale, it is the demands of increasing group size that select for a larger
neocortex, but from the contemporary perspective of primates actually on the
ground, it is neocortex size that limits group size.
In principle, path analysis allows us to test between alternative models that
rearrange the variables into all possible permutations and combinations of relationships between the variables. However, with the number of variables under consideration in Fig. 15.1, it would be prohibitively time-consuming (and largely
unproductive) to consider every conceivable permutation and combination of the
structural relationships. Dunbar and Shultz (2007b) looked at the most plausible set
and structured models on the basis of findings generated by a minimum adequate
model (MAM) analysis, which determined the set of variables that were most
predictive of neocortex size. These established the set of primary or first-order
relationships for the path analysis. Doing so then allowed them to identify a second
layer of factors that function as constraints on brain size evolution: these included
several life history variables (total brain size, lifespan, body size) and diet (which
might act as a proxy for a number of the ecological hypotheses). Including these in
the model, or using them as the only independent variables predicting group size,
yields a significantly poorer fit, suggesting that they are constraints that have to be
resolved rather than factors that influence group size directly.
In effect, as Finlay and Darlington (1995) pointed out, if you want to have a
large neocortex (for whatever reason), you need to find a way to evolve a larger
brain to support it. This may involve a shift in diet (to more digestible or higher
energy diets) and larger body size, which, in turn, may necessitate an extended
life history (mainly because it seems that brain tissue can only be laid down at a
slow, constant rate). In other words, it is important to be clear about this: neither
the fact that brain tissue is costly (Isler and van Schaik 2006) nor the fact that brain
size correlates with a suite of life history variables are explanations for brain
evolution; they are constraints that a lineage has to overcome if it is to be able
evolve a larger brain.
In sum, then, these analyses suggest that the functional demands of large
neocortices relate principally to maintaining large social groups. In primates at
least, large social groups are demanded by occupancy of predator risky environments (primates: van Schaik 1983; Dunbar 1988; Shultz et al. 2004; ungulates:
Adamczak and Dunbar 2008), one contributor to which was the adoption of a
diurnal lifestyle. Thus, the causal sequence seems to be as suggested in Fig. 15.2.
It is important to understand that the changes indicated by the double arrows
represent obligatory coevolutionary changes: the downstream effect cannot be
produced unless the upstream effect also occurs. Solving the predation problem,
thus, requires a whole suite of variables to be changed more or less simultaneously
(Pérez-Barberı́a et al. 2007).
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R.I.M. Dunbar
Fig. 15.2 Causal
relationships in primate brain
evolution, as revealed by the
path analysis of Fig. 15.1.
Double arrows: causal
relationships; Single arrows:
windows of opportunity
This functional benefit of large brain size is given added emphasis by Barrickman et al. (2008), who show that, in primates, reproductive success (in effect,
fitness) is mainly a function of longevity rather than instantaneous reproductive rate
(see also Clutton-Brock 1988). The capacity to survive is, in part at least, a function
of avoiding predation. The important finding here is that instantaneous fecundity
and the factors that influence this (such as dominance rank and body condition) play
only a secondary role. However, it might still be possible to construe the causal
relationship in two alternative ways: maximising fitness requires an extended lifespan and either (1) a large brain is an unavoidable correlate of an extended lifespan
or (2) a large brain is necessary to ensure an extended lifespan. The first flies in the
face of the costs of brain tissue (~20% of total energy consumption: Aiello and
Wheeler 1995), since it offers no reason why brain size should have to correlate
with longevity. The second makes sense in the light of Charnov’s (Charnov 1993;
Charnov and Berrigan 1993) suggestion that minimising predation risk is the main
constraint on longevity in primates. But an extended lifespan is a risky gambit in the
absence of mechanisms that allow animals to avoid the costs of premature mortality.
Although the latter has often been seen in terms of foraging costs, in reality, there is
little evidence to suggest that foraging skills influence longevity (at least, above a
certain threshold of competence: Altmann 1998) and a lot of evidence to support the
claim that predation has a major impact on life expectancy, and that, in primates at
least, the principal mechanism for avoiding predators is the formation of cohesive
social groups.
15.3
The Structure of Primate Social Groups
Although the social brain hypothesis has often been construed in terms of a
quantitative relationship between social group size and some measure of brain
volume (Dunbar 1992, 1998; Barton 1996; Barton and Dunbar 1997), in fact it is
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properly constituted in terms of social complexity, with the group size effect being
an emergent property of how well animals handle complex relationships. Indeed, a
number of analyses have reported correlations between various indices of behavioural complexity and brain size (Pawłowski et al. 1998; Lewis 2000; Kudo and
Dunbar 2001; Byrne and Corp 2004). An important issue arising out of the social
brain hypothesis for primate brain evolution thus centres around its implications for
how social groups are structured.
Individuals do not have intimate social relationships (i.e., groom) with everyone
else in a primate group. Rather, as Kudo and Dunbar (2001) and Lehmann et al. (in
press) have shown, each individual has a small number (seldom more than four) of
core social partners, and that the size of these grooming cliques correlates very
closely with the total group size (Fig. 15.3: r ¼ 0.81, N ¼ 15, p < 0.001). More
importantly, although total time devoted to grooming increases more or less
linearly with group size in primates (both between and within species: Dunbar
1991; Lehmann et al. 2007a), the additional grooming time in large groups is not
distributed proportionately around more group members; rather, individuals often
have fewer grooming partners in very large groups than they do in small groups,
even though they devote more time to social grooming (Dunbar 1984, 2003).
Evidence in support of this has come from a seminal field study of the endocrinological consequences of network size in baboons: Wittig et al. (2008) showed that
females with smaller grooming networks (or those who reduced the size of their
networks) showed significantly lower stress responses (indexed as serum corticosteroid titres) to stressful events, such as invasion by males, than those that with
larger grooming networks (or those who did not reduce their networks).
Fig. 15.3 Mean group size plotted against mean grooming clique size for individual primate
genera. Open circles: Prosimians; solid triangles: New World monkeys; open squares: Old World
monkeys; solid circle: Apes. Source: Kudo and Dunbar (2001)
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R.I.M. Dunbar
I interpret as reflecting the fact that alliances need to work more effectively in
large groups because of the escalating pressure that living with many individuals
imposes. As we noted above, female fertility, in particular, declines rapidly with
group size in primates (gelada: Dunbar and Dunbar 1977; colobus: Dunbar 1987) as
a consequences of stressors that accumulate on lower-ranking females. In order to
mitigate these, animals have to invest more heavily in their core relationships as
group size increases so as to ensure that, when needed, these relationships do their
job of protecting the individuals involved. This might involve active protection
(coalitionary support when an ally is attacked) as clearly happens in gelada (Dunbar
1980, 1989), but in many cases may have as much to do with passive support: an
observed relationship might imply retaliation by allies later (Datta 1983; Cheney
and Seyfarth 1999, 2007; Bergman et al. 2003), and this may be enough to keep
other individuals at a sufficient distance to minimise the likelihood that they will
impinge on one’s wellbeing. A poorly bonded grooming clique may mean that an
individual is not buffered against the costs of living in large groups, and these then
become unstable and liable to fragmentation.
The fact that the size of these small grooming cliques turns out to be linearly
(and rather tightly) related to social group size across the primates may reflect the
fact that, as group size increases, it is necessary to have a grooming coalition that is
proportionally large enough to balance the stresses that large groups inevitably
impose. In effect, primates are engaged in a difficult balancing act that uses
grooming cliques as a mechanism to keep other group members just far enough
away to avoid these kinds of stresses without driving them away altogether.
Without such a mechanism, social groups would fission rapidly and find an equilibrium at a lower size as these stresses drove individuals away. Ultimately, it may
be that the real cognitive constraint lies at the level of this inner layer of the
social network, and that the cohesion and size of the conventional group is an
emergent property of how well animals solve the problem of bonding at this very
intimate level.
The intensity of bonding in primate social groups seems to be of a different order
to that in most other mammals, and certainly that found in carnivores and artiodactyl ungulates (Shultz and Dunbar 2007). This is clear from Pérez-Barberı́a
et al.’s (2007) analysis of the coevolution of brain size and sociality in these three
orders, which showed that the evolutionary coupling of these traits was significantly
tighter in primates than the other two mammalian orders. This, combined with
Shultz and Dunbar’s (2007) finding that anthropoid primates (in particular) have
significantly more female-bonded groups than carnivores or ungulates, suggests
that primatologists might have been right in their insistence that primate sociality is
of a qualitatively different kind to that found in other taxa, the longstanding
scepticism of those who study other mammals and birds notwithstanding.
There are obviously two questions here. One is why this transition from a
qualitative to a quantitative effect should have happened: what was it about early
primate ecology that made bonded groups of this kind especially valuable? The
second is why only (anthropoid) primates seem to have made this transition. We
have no real idea how to answer either question at present, but it would have to be
15
Brain and Behaviour in Primate Evolution
325
something that sets anthropoid primates apart from all other taxa, including most
(but perhaps not all) prosimians. (In general, prosimians resemble carnivores rather
than anthropoid primates in their distribution of social types [Shultz and Dunbar
2007], but there may, nonetheless, be some convergence with anthropoid primates
among the more social lemurs).
One plausible explanation for the timing of the evolution of bonded social
groups is the switch to a more frugivorous diet in the transition between ancestral
prosimians (the Eocene primates) and the lineage leading to modern anthropoid
primates, not least because this transition occurs at exactly the right time. In
contrast to the characteristic diets of most prosimians (predominantly insects),
ungulates (grass) and carnivores (mobile prey), fruits offer the opportunity for
spatial concentration and, as a result, a significantly increased risk of competition
when individuals are forced to cluster around small but, at least in the short term,
relatively stable rich resource packets. The increased within-group competition that
would be inevitable under these conditions increases the risk of group fragmentation (thereby neutralising the antipredator benefits of grouping demanded by the
simultaneous shift to a more diurnal lifestyle). Frugivorous birds may be able to
cope with this by being able to disperse in a form of individual- or pair-based
fission fusion sociality, something that may be possible only because their aerial
habit allows them to reduce predation risk. In addition, their small body mass means
that relatively large numbers of birds can gather at a fruiting tree before exceeding
the local carrying capacity. Anthropoid primates face two disadvantages in this
respect: their large body size and the fact that, being plantigrade, they cannot so
easily escape predators. Bonded relationships that create functional alliances may
have solved this problem by reducing the levels of competition (see also van Schaik
1989), so allowing primate groups to remain together despite these disruptive
effects. This might then also explain Barton’s (1998) the finding that the grade
shift in brain size between the prosimian and anthropoid lineages is associated with
the greater importance of colour vision (associated mainly with foraging for fruits).
15.4
Implications for Human Social Evolution
The human lineage has its roots in the ancestral primate lifestyle. It, thus, inherits
the same kinds of intense bondedness that, generally speaking, characterises other
monkeys and apes. However, human social groups differ from those of most other
primates in two key respects: their size and the fact that they have a dispersed
(or fission fusion) form. Although the latter trait is shared with a small number of
other primates (spider monkeys and chimpanzees, and the hamadryas and gelada
baboons), it is rare enough among the anthropoid primates to appear to be a derived
feature and so to require explanation.
There seems little reason to doubt the suggestion that group size has increased
steadily over time within the hominid lineage. Modern humans demonstrably live
in much larger groupings than any other primate. However, although the size of
326
R.I.M. Dunbar
groups predicted for modern humans from the primate brain size regression equation is close to those actually observed (Dunbar 1993), the changes in brain size
within the human fossil record suggest that the kinds of very large groups found in
modern humans are a relatively late evolutionary feature (post-0.5 MYA) (Aiello
and Dunbar 1993; Dunbar 2004; Dunbar et al. in press). They may well coincide
with the major exoduses out of Africa.
On the other hand, the human lineage shares with chimpanzees (and a small
number of other mammalian taxa) the distinctive feature of having a series of social
groupings that are nested within a large dispersed community. In both cases, these
layers have a scaling ratio of approximately 3 (Zhou et al. 2005; Hill et al. 2008).
The fact that the largest grouping level is dispersed (often over a wide geographical
area) raises questions about the function of these groupings, since they lack the
cohesiveness that seems to form such a crucial component of monkey social groups.
The provision of protection against predators seems an implausible function for
these kinds of dispersed grouping. Indeed, since female body size is a major factor
in reducing predation risk, chimpanzees and humans should be able to manage with
group sizes that are approximately half those found in baboons (~50 on average:
Dunbar 1992). This suggests that the community is not an antipredator defence, but
has evolved (or been coopted) for some other function.
The functionality of chimpanzee communities remains unclear, though some
form of resource defence (of food trees or of females) remains a likely explanation.
One finding of importance for our understanding of human evolution, however, is
the fact that, owing to the time costs of travel, chimpanzees can only survive in most
of their current geographical range by opting for fission fusion sociality (Lehmann
et al. 2007b, 2008b). Since it is difficult to imagine hominids radically solving this
particular problem even with bipedalism, some form of fission fusion sociality is
likely to have continued to be unavoidable throughout human evolution, just as it
still is among contemporary hunter-gatherers. While resource defence remains a
plausible explanation for large dispersed communities among modern humans, an
alternative possibility might be the opportunities created for trading access to
keystone resources over a very wide area (Dunbar 1996), especially in the ecologically less predictable habitats at high latitudes. Some evidence to support this
comes from analyses of human language distributions. Nettle (1999) showed that
both language community size (the number of people who spoke the same language) and language area (the geographical range within which a language was
spoken) correlate linearly with latitude in both the New and Old Worlds. He argued
that this reflected the need to be able to engage in reciprocal exchange (mediated by
a common language) over much wider area in the more unpredictable habitats
found at higher latitudes.
Since Zhou et al. (2005; see also Hamilton et al. 2007) showed that human
societies form a nested series of grouping levels, it might be possible to deploy the
same argument suggested for nonhuman primates as to how these large communitylevel social groupings are maintained as stable units through time. In other words,
the cohesiveness and coherence through time of the large outer community layer
(~150 in humans) may depend on the effectiveness with which some key inner
15
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327
grouping layer is maintained through face-to-face interaction. In humans, the tiers
of the grouping layers take values that approximate 5, 15, 50, and 150 individuals
(Zhou et al. 2005). On the basis of relative size compared to chimpanzees, Kudo
and Dunbar (2001) identified the second tier (the 15-person layer) as the equivalent
to the nonhuman primate grooming clique in this respect. What is special about this
grouping level in humans is not clear, although it has long been recognised in social
psychology that groups of size 12 15 have a particular significance where close
bondedness (e.g., in team sports) or emotional closeness is required (juries, cabinets, etc.) (Buys and Larsen 1979). As yet, we understand almost nothing of either
the dynamics or the function of these grouping levels in humans, and this remains a
rich area for future development.
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Chapter 16
The Gap is Social: Human Shared
Intentionality and Culture
Michael Tomasello and Henrike Moll
Abstract Human beings share many cognitive skills with their nearest primate
relatives, especially those for dealing with the physical world of objects (and
categories and quantities of objects) in space and their causal interrelations. But
humans are, in addition, biologically adapted for cultural life in ways that other
primates are not. Specifically, humans have evolved unique motivations and cognitive skills for understanding other persons as cooperative agents with whom one
can share emotions, experience, and collaborative actions (shared intentionality).
These motivations and skills first emerge in human ontogeny at around one year of
age, as infants begin to participate with other persons in various kinds of collaborative and joint attentional activities. Participation in such activities leads humans to
construct during ontogeny, perspectival and dialogical cognitive representations.
16.1
Introduction
The gap we are trying to explain is obvious: we humans live in complex societies
dependent on complex technologies, symbol systems, and social institutions
whereas other primate species do not. We, humans, are scientifically investigating
and writing about them, not they about us.
Following Vygotsky (1978) and Tomasello (1999), the general proposal here is
that the human gap is best explained in terms of, ultimately, social (or cultural)
factors. That is, human beings are especially sophisticated cognitively not because
of their greater individual brainpower, but rather because of their unique ability to
put their individual brainpowers together to create cultural practices, artifacts, and
institutions underlain by skills and motivations for shared intentionality which
M. Tomasello (*) and H. Moll
Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany
e mail: tomasello@eva.mpg.de, moll@eva.mpg.de
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 16, # Springer Verlag Berlin Heidelberg 2010
331
332
M. Tomasello and H. Moll
are then passed along to youngsters as a second line of inheritance in the species,
resulting in a ratcheting up of cultural and cognitive complexity over historical
time. A child raised alone on a desert island, or even by chimpanzees, would
cognitively not be very different from the apes, as its unique adaptation for
absorbing culture would be intact but there would be nothing there to absorb.
But it turns out that identifying the cognitive and social-cognitive factors that
enable developing human beings to take advantage of the cultural practices,
artifacts, and institutions around them is not so easy. The approach in our research
group for some years has been to focus on situations and phenomena that are mostly
simpler than those in modern adult life, but still have all of the key characteristics
for exploring the gap between human and ape cognition. Toward this end, we have
focused on young children as representative of the human species, with the idea that
they help us to abstract away from the complexities of adult human life and get
down to the essentials. For comparison, we have focused on great apes, especially
chimpanzees, as humans’ closest primate relatives.
What we will do here is three things. First, we will present evidence that the gap
is indeed social, drawn especially from a recent large-scale study of the full range of
cognitive skills in great apes and human children. Second, we will review some
recent studies specifying in more detail the nature of the difference between apes
and human children in situations involving shared intentionality in (1) collaboration, (2) communication, and (3) social learning. Finally, we will offer some
speculations as to how these small-scale cooperative abilities of shared intentionality scale up into uniquely human cognition and culture.
16.2
Human and Great Ape Cognitive Skills Compared
An obvious hypothesis about the human difference is that human beings simply
have bigger brains and so more “general intelligence” than other animals: more
memory capacity, greater inferential skills, faster learning, further foresight and
planning, finer skills of perceptual discrimination more and better of everything.
And this quantitative difference somehow translates into a qualitative difference in
cognitive abilities.
In a recent study, we tested this hypothesis in comparison with what we called
the cultural intelligence hypothesis by giving a very large test battery (a kind of
nonverbal IQ test) to two of humans’ closest primate relatives, chimpanzees and
orangutans, along with 2-year-old human children (Herrmann et al. 2007). If
humans simply have more cerebral computing power and general intelligence,
then the children should have differed from the apes uniformly across the different
types of tasks. But that was not the result. The result was that the three species were
very similar when it came to cognitive skills for dealing with the physical world
problems having to do with space, quantities, and causality but the human
children were much better than the two ape species when it came to cognitive skills
for dealing with the social world
problems of imitative learning, gestural
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Human Shared Intentionality and Culture
333
communication, and reading intentions.1 When correlational analyses were done,
no general intelligence, or g-factor, was found (Herrmann et al. in press). These
findings are not compatible with the hypothesis that the cognitive difference
between humans and apes is a simple function of more cerebral horsepower.
There have also been a few hypotheses about more specific computational
capacities that might make human cognition unique though they cannot explain
these cross-species test battery results either. In one hypothesis, for example,
humans are better able to perform multiple tasks simultaneously. In another,
humans are better with relational categories and making analogies across different
materials. These hypotheses may have some validity, but it is difficult to see how
multitasking alone or analogy-making could account for the results of our crossspecies study of intelligence, much less such things as linguistic symbols, social
institutions, and cultural norms. These are all collective cultural products that are
not easily accounted for by simply adding up the computational power of individuals either generally or in specific skills.
Uniquely, human cognitive skills are not simply the result of greater computational power overall, or of some increase in a specialized cognitive ability. Rather,
they result from an ability enabling humans to put their heads together, so to speak,
in cooperating and communicating with one another in ways that led to the creation
of complex cultural products, including both material and symbolic artifacts, such
as linguistic symbols. Human children grow up in the midst of these material and
symbolic artifacts, and by learning to use them in interaction with others (as well as
internalizing these interactions cognitively), they actually create, during ontogeny,
evolutionarily new ways of thinking. This hypothesis sets the problem that must be
addressed if we want to provide a plausible evolutionary account of how human
cognition became, in effect, a collective enterprise.
16.3
Cultural Activities in Humans and Great Apes
Humans’ adaptation for living and exchanging information in cultural groups
manifests itself in many ways. What we would like to focus on here is the three
main classes: (1) small-scale collaborative activities (translating ultimately into
large-scale social and cultural institutions); (2) cooperative communication (transforming ultimately into language); and (3) cultural learning (resulting ultimately in
cumulative cultural evolution).
1
One might object that the human subjects had an advantage over the non human ones in the social
tasks, because they interacted with a conspecific vs. a member of a different species. However: (1)
many of the tasks were chosen based on previous research showing no difference in performance
as a function of conspecific or human interactants (e.g., ape vs. human demonstrators in social
learning); and (2) the comfort level of all subjects in the testing situation was assessed and, if
anything, the children were most shy (and this measure did not correlate with overall cognitive
performance in the main tasks).
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16.3.1
M. Tomasello and H. Moll
Collaboration
Individuals of virtually all primate species engage in group activities on a daily
basis. These activities may be considered cooperative in the sense that they coordinate their behavior temporally and spatially with the other animals in the group.
However, as in previous theoretical work (Tomasello et al. 2005), here, we want to
single out for special attention “shared cooperative activities” a subtype in which
humans routinely engage. In our modified version of Bratman’s (1992) characterization, joint or shared cooperative activities are mainly characterized by the three
features: (1) the participants in the cooperative activity share a joint goal, to which
they are jointly committed; (2) the participants take reciprocal or complementary
roles in order to achieve this joint goal; and (3) the participants are generally
motivated and willing to help one another to accomplish their role, if needed.
Joint commitment to a goal. One group activity that has been posited as being
especially complex is chimpanzee group hunting. Boesch and colleagues (Boesch
and Boesch 1989; Boesch and Boesch-Achermann 2000; Boesch 2005) have
observed chimpanzees in the Taı̈ forest hunting in groups for arboreal prey, mainly
monkeys. In the account of these researchers, the animals take complementary roles
in their hunting. One individual, called the driver, chases the prey in a certain
direction, while others, the so-called blockers, climb the trees and prevent the prey
from changing directions. An ambusher then silently moves in front of the prey,
making an escape impossible. Of course, when the hunting event is described with
this vocabulary of complementary roles, then it appears to be a joint cooperative
activity: complementary roles already imply that there is a joint goal, shared by the
role-takers. But the question really is whether this vocabulary is appropriate at all.
A more plausible characterization of the hunting event, from our perspective, is as
follows: each animal fills whatever spatial position is still available at any given
time so that the encircling is accomplished in a stepwise fashion, without any kind
of prior plan or agreement to a shared goal or assignment of roles. Then, without
pursuing a joint goal or accomplishing a certain role within a higher-order framework, each individual chases the prey from its own position (see also Moll and
Tomasello 2007a). This event clearly is a group activity or group action, because
the chimpanzees are “mutually responsive” as they coordinate their behaviors with
that of the others in space and time (see also Melis et al. 2006). But what seems to
be missing is the “togetherness” or “jointness” that distinguishes shared cooperative
activities from other sorts of group actions.
This interpretation is strongly supported by studies that have investigated chimpanzees’ abilities to cooperate in experimental settings. In one study, Warneken
et al. (2006) tested three juvenile human-raised chimpanzees with a set of four
different cooperation tasks. In two of these tasks, a human tried to engage the
chimpanzee to cooperate in order to solve a problem (e.g., extracting a piece of food
from an apparatus). In the other two tasks, the human tried to engage the ape to play
a social game. The authors looked at two things: the chimpanzees’ level of
behavioral coordination and the chimpanzees’ behaviors in the so-called
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interruption periods, in which the human suddenly stopped participating in the
activity. The results were very consistent: in the problem-solving tasks, chimpanzees coordinated their behaviors quite well with that of the human, as shown by the
fact that they were mostly successful in bringing about the desired result, as, for
instance, extracting the piece of food from the apparatus. However, they showed no
interest in the social games, and so the level of coordination in these tasks was low
or absent. Most important was what happened when the human suddenly interrupted the activity. In none of the tasks did a chimpanzee ever make a communicative attempt to reengage the partner. Such attempts were missing even in cases in
which they should have been highly motivated to obtain the desired result, as in the
problem-solving task involving food. The absence of any efforts by the chimpanzees to reengage their human partner is crucial: it shows that the chimpanzees did
not cooperate in the true sense, since they had not formed a joint goal with the
human. If they had been committed to a joint goal, then we would expect them, at
least in some instances, to persist in trying to bring it about and in trying to keep the
collaboration going.
For humans, the situation is different from very early on in ontogeny. Warneken
et al. (2006) conducted an analogous study with 18- and 24-month-old human
children. Unlike the chimpanzees, children cooperated quite successfully and
enthusiastically not only in the problem-solving tasks, but also in the social
games. For example, these infants enjoyed playing a “trampoline” game together,
in which both partners had to simultaneously lift up their sides of a small trampoline
with their hands, such that a ball could bounce on it without falling off. Most
importantly, when the adult stopped participating at a certain point during the
activity, every child at least once produced a communicative attempt in order to
reengage him. In some cases, the children grabbed the adult by his arm and drew
him to the apparatus. The older children of 24 months of age also often made
linguistic attempts to tell the recalcitrant partner to continue. Unlike the chimpanzees, we thus find in human infants the ability to cooperate with joint commitment
to a shared goal: the children “reminded” the recalcitrant partner of their shared
goal and expected him to continue in order to achieve it. There was even some
evidence that the children already understood the normativity behind the social
games and the way they “ought to be played.” For example, in one of the games,
they always used a can in order to catch a toy when it came falling out of one end of
a tube after their partner had thrown it in from the other end. They could have also
caught it with their hands, but they preferred to do it the way it had been demonstrated to them. This implies that they perceived the can as a constitutive element of
the game, and they wanted to play the game the way it “ought” to be played. The
chimpanzees, on the other hand, never used the can in order to catch the toy if they
engaged in the game at all, they simply used their hands. It, thus, seems that human
infants by the age of 18 months, in contrast to apes, are able to jointly commit to a
shared goal.
Role reversal. The second criterion for cooperation, as we define it, is roletaking. True cooperation should involve that the partners perform reciprocal roles
and also understand them, in the sense that they coordinate their actions and
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intentions with the possibility of reversing roles. This form of role-taking would
suggest that each partner represents the entire collaboration, its shared goal and
reciprocal roles, holistically from a “bird’s eye view” instead of just from within
whatever role they happen to be taking at the moment. One study purporting to
show role reversal in chimpanzees is that of Povinelli et al. (1992). In that study,
chimpanzees were trained in one of two roles of a cooperative hiding game with a
human. Some chimpanzees were trained in the role of a communicator, who
indicated to the human where a piece of food was located. The other chimpanzees
were trained in the complementary role of the “operator,” who extracted the food
from the location indicated by the human. When the chimpanzees had learned their
initial role to criterion, a role switch was initiated and the question was whether the
chimpanzees would spontaneously reverse the roles. One of the chimpanzees,
whose initial role was that of the communicator, was immediately successful as
operator after the switch. But the problem is that this individual most likely
comprehended human indicating gestures before the study as this animal had
extensive interactions with humans. The two individuals that switched to be a
communicator also seemed to reverse the roles effectively, as they were reported
to provide the human with cues about the location of the food fairly quickly.
However, the problem in this case is that it is not clear that the chimpanzees
actually produced any communicative signals at all, but instead the humans simply
interpreted their natural bodily orientation to the food.
A better controlled investigation of role-reversal skills in chimpanzees was done
by Tomasello and Carpenter (2005) with the same three young human-raised
chimpanzees which participated in Warneken et al.’s (2006) study. In this study,
a human demonstrated to the chimpanzee various actions with each of four pairs of
objects. For each pair of objects, one functioned as a “base” and the other as an
“actor.” The human then demonstrated to the chimpanzee how the two, the actor
and the base, are put together. For instance, she put a “Tigger” figure on a plate
and “Winnie the Pooh” figure in a little toy car. Then E gave the actor (e.g., Tigger)
to the chimpanzee and held out the base (the plate) towards the chimpanzee,
thus offering that the chimpanzee put the actor on the base to complete the act.
If chimpanzees did not perform the role of putting the actor on the base spontaneously, E encouraged them to do so by vocalizing and, and if they still did not
respond, by helping them put the actor on the base. To test for role reversal, E then
handed the chimpanzee the base and held out the actor to see whether she would
spontaneously offer the base. Two of the three chimpanzees held out the base object
at some point. But, crucially, none of these responses occurred spontaneously, and
more importantly, in none of these responses was the holding out of the base
accompanied by a look to E’s face. A look to the partner’s face while holding out
the object is a key criterion of “offering” used in all studies with human infants
(Bates 1979; Camaioni 1993). Thus, in Tomasello and Carpenter’s (2005) study,
there was no indication that the chimpanzees offered the base to the human, and so
there were no acts of role reversal.
An analogous study with human infants of 12 and 18 months of age was
conducted by Carpenter et al. (2005). As in the study with the chimpanzees,
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situations were set up in which an adult did things like hold out a basket in which
the infant was asked to place a toy. After the infant complied, in the test for role
reversal, the adult placed the basket within the infant’s reach and held up the toy
herself. Impressively, even some of the 12-month olds spontaneously held out the
basket for the adult while at the same time looking to her face, presumably in an
anticipation of her placing the toy inside. Thus, the infant’s handing behaviors, in
contrast to those of the chimpanzees, were clearly the acts of offering learned
through role reversal. It, thus looks as though chimpanzees, in contrast to young
human children, do not fulfill either of the first two criteria of cooperation: sharing a
joint goal and understanding the roles of a joint activity in some general way.
Mutual support. The third criterion is that, if needed, the partners of a joint
cooperative activity help one another do their part successfully. This criterion is
inherently linked to the other two: the commitment to the joint (and not just an
individual) goal implies a responsibility not just for the successful completion of
one’s own role, but also to some degree for that of the other participants, and so
helping them fulfill their goal (or, in some instances even replacing them) is an
integral part of true collaboration. In two recent studies, chimpanzees did not take
an opportunity to “help” another individual to obtain food (Silk et al. 2005; Jensen
et al. 2006). But food is a resource over which apes are used to compete, and so
helping might be better investigated in situations that do not revolve around food.
Given our interest in helping as a constituent of collaboration, the most important
form of helping is “instrumental helping,” in which one individual helps another
instrumentally to achieve a behavioral goal. We know of only one study investigating instrumental helping in nonhuman primates. Warneken and Tomasello (2006)
had three human-raised juvenile chimpanzees watch a human attempt, but fail to
achieve, different kinds of individual goals. Reasons for her failure were that her
desired objects were out of reach, that she ran into physical obstacles, clumsily
produced wrong results, or used ineffective means. The chimpanzees helped the
human in some cases. However, the range of situations in which they helped was
very limited: only when the adult effortfully reached but failed to grasp objects did
the chimpanzees help by fetching them for her.
An analogous study was conducted with 18-month-old human infants, who also
saw an adult fail to reach her goals for the same reasons (Warneken and Tomasello
2006). In this study, infants as young as 18 months of age helped the adult in various
scenarios: for instance, they spontaneously removed physical obstacles that hindered the adult (e.g., they opened a cabinet so that the adult could place books
inside) and showed him means that they knew were effective to bring about the
intended result. It thus seems that, even though some helping behavior can be found
in nonhuman primates, only human infants display helping actions in a variety of
situations, providing whatever help is needed in the given situation.
What we conclude from these experimental studies is that, despite their group
hunting in the wild, chimpanzees do not have “we-intentionality” (see Bratman
1992; Searle 1995; Tuomela 2002). They do not form a joint commitment to a
shared goal and they do not perform reciprocal roles in the true sense as they do not
generally understand roles from a bird’s eye view, in the same representational
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format. Finally, they seem to be limited in their abilities to help another individual
which is a necessary prerequisite to engage in cooperative activities narrowly
defined. Human infants and young children, in contrast, have this we-intentionality
and act cooperatively from at least 14 to 18 months of age. They “remind” their
partner of the joint commitment to a shared goal, as they reengage her when she
suddenly interrupts the activity (Warneken et al. 2006; Warneken and Tomasello
2007); they begin to reverse and understand their roles as early as 12 months of
age (Carpenter et al. 2005); and they help others in the fulfillment of their individual
roles in various ways by at least 14 18 months (Warneken and Tomasello
2006, 2007).
16.3.2
Cooperative Communication
A related domain, which also requires some form of cooperation is communication.
As noted above, chimpanzees usually perform poorly in experiments that require
some understanding of cooperative communication. Here, we address this issue in
more detail by first looking at nonhuman primates’ own production of communicative gestures, and then at their comprehension of such gestures produced by others.
Chimpanzees gesture to one another in different contexts. Some of these gestures are clearly intentional, in the sense that they are not just triggered by certain
environmental conditions, but used flexibly to do such things as elicit play in the
other (by an “arm-raise”) or to request nursing (by a “touch-side”). That these
gestures are indeed used flexibly is illustrated by a number of phenomena, for
instance, the fact that visual gestures are only used in instances in which the
recipient is visually oriented towards the sender (e.g., Tomasello et al. 1997a;
Kaminski et al. 2004). One might think that if chimpanzees can gesture flexibly
and understand some things about visual perception (see Call and Tomasello 2008),
they should also use gestures to direct another chimpanzee’s attention to a certain
event or object by pointing. There are certainly occasions in which it would be very
helpful if one ape pointed for another ape to indicate the locus of some relevant
event. It must, therefore, seem somewhat surprising that, in fact, there has not been
a single reliable documentation of any scientist in any part of the world of one ape
pointing for another. But captive apes which have had regular interactions with
humans point for their human caretakers in some situations. Leavens and Hopkins
(1998, 2005) conducted a study with chimpanzees in which a human experimenter
placed a piece of food outside of the ape’s reach and then left. When another human
came in, the chimpanzees pointed to the food so that the human would get it for him
(pointing was usually done with the whole hand, but some points were produced
with just the index finger; see also Leavens et al. 2004). Human-raised chimpanzees
have also been found to point to humans in order to obtain access to locations where
there is food (Savage-Rumbaugh 1990), and some orangutans point for humans to
the location where they can find a hidden tool, which they will then hopefully use to
obtain food for the orangutans (Call and Tomasello 1994).
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We thus find that apes do sometimes point for humans given that they have
had some contact with humans in the past. Importantly though, they use this
manual gesture imperatively only. That is, they point for humans either in order
to obtain a desirable object from them directly, as in the studies by Leavens and
Hopkins (1998, 2005), or indirectly by requesting from the human to provide the
necessary conditions for them to get the object themselves, as in Savage-Rumbaugh’s (1990) study. It, thus seems that what the apes have learned from their
experience with humans is that the human will help them, and that they can use the
pointing gesture instrumentally in order to make him help them. They, thus, “use”
the human as a “social tool” in order to get things they otherwise could not get, and
they have learned that pointing gets this tool to work (the term “social tool” was
first used by Bates et al. 1975). However, no ape has ever been observed to point
for another ape or for a human declaratively that is, just for the sake of sharing
attention to some entity or event, or to inform others cooperatively, as humans
often do.
Liszkowski et al. (2004, 2006) have shown in a series of experiments that even
when they first begin to point at around 1 year of age, human infants do this with a
full range of different motives including the motive to share attention and interest.
In one study (Liszkowski et al. 2004), an adult reacted differently towards infants’
points, and the infants’ responses to the adult reaction were investigated. The main
finding was that if the adult did not jointly attend to the event with the infant (by
alternating gaze between infant and event and commenting on it) but instead
either (1) just “registered” the event without sharing it with the infant or (2) only
looked and emoted positively to the infant while ignoring the event the infants
were dissatisfied and tried to correct the situation. In contrast, in the joint attention
condition, infants appeared satisfied with the response. Using the same basic
methodology, Liszkowski et al. (2006) found that beyond the classic distinction
of imperative and declarative pointing, 12-month-olds point for others also to
inform them about things that are relevant for them. In that study, they directed
an adult’s attention to the location of an object for which that person was searching.
What this suggests is that in human ontogeny, pointing is used from the very
beginning not just in order to obtain certain objects via helpful adults as social
tools, but with the motivation to help/inform others or to just jointly attend to things
in the world with them.
The question is thus why apes do not point to share interest and inform others as
human infants do from very early in development (see also Tomasello 2006). They
clearly have the necessary motor abilities to do so. And again, it would surely be
useful if they spatially indicated important events for one another. So why do they
not do it? To answer this question, one needs to look at apes’ understanding of
pointing. One of the main paradigms that has been used to assess chimpanzees’
comprehension of pointing is the Object Choice task. In the task designed by
Tomasello et al. (1997b), one human, the hider, hides a piece of food for the ape
in one of several containers. Then another human, the helper, shows the ape where
it is by tilting the container so that she can look inside and see the food. After this
“warm-up,” the hider again places a piece of food in one of the containers, but now
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the helper indicates the location of the food for the ape by pointing at the baited
container with his index finger (or by gazing at it). Variations of this method
involve other kinds of communicative cues (Call and Tomasello 2005) and a trained
chimpanzee instead of a human as the provider of the cue (Itakura et al. 1999). The
results were the same in all these studies: the apes performed poorly, that is, they
chose the correct container at chance level. They often followed the human’s point
(or gaze cue) to the container with their eyes, but they did not make any inferences
from there about the location of food. That is, they cannot use or exploit the
information that is conveyed to them via the pointing gesture they do not know
what it means. When following the human’s point with their gaze, all they perceive
is a useless bucket. To understand that the point is not directed at the bucket as such,
but at the bucket qua location or qua container of a desired object, the apes would
need to understand something about cooperation or communication. They would
need to understand that the other is trying to communicate to them something that
might be relevant for the achievement of their goal. In other words, an understanding of the meaning of the pointing gesture presupposes a more general understanding that others might want to help or inform us about the things which they assume
are relevant for our purposes. And this understanding obviously goes beyond the
apes’ social-cognitive skills.
The view that the challenge of the Object Choice task does, indeed, lie in its
cooperative structure is supported by recent studies using a competitive version of
the task. In one version, Hare and Tomasello (2004), instead of pointing to the
baited container, reached unsuccessfully for it. Superficially, this reaching behavior
is very similar to the pointing gesture: the human’s hand is oriented towards the
container in which the food is hidden (the difference being that when pointing, only
the index finger is stretched out, whereas in the case of reaching, all fingers point at
the container). However, the chimpanzees’ response in the reaching version was
very different, as they successfully retrieved the food from the correct container.
The reason for this must be that, even though the two tasks are superficially highly
similar, their underlying structure is very different. Our interpretation is that in the
case of reaching, the chimpanzees just need to perceive the goal-directedness of
the human’s reaching action and ‘see’ that there must be something desirable in the
container. This task can, thus be solved with some understanding of the individual
intentionality of the reaching action. In contrast, to understand pointing, the subject
needs to understand more than the individual goal-directed behavior. She needs to
understand that by pointing towards a location, the other individual attempts to
communicate to her where a desired object is located; that the other tries to inform
her about something that is relevant for her. So the ape would need to understand
something about this directedness towards itself ‘this is for me!’ and about the
communicative intention behind the gesture in order to profit from it. Apparently,
apes do not understand that the cue is “for them” used by the other in a helpful,
informative and communicative way. Even though they are quite skillful in understanding intentional behavior that is directed at objects in the world (see Tomasello
et al. 2005, for a review), they do not understand communicative intentions, which
are intentions that are not directed at things or behaviors but at another individual’s
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intentional states (with the embedded structure: “I intend for you to know that I
intend for you x”).
In order to explain why the apes fail to understand communicative intentions,
one needs to broaden the perspective and focus on what we call the “joint attentional frame.” The joint attentional frame or common ground (Clark and Brennan
1991) is what gives a pointing gesture its meaning it is what “grounds” the
communication in the shared space of meaning. To illustrate the point, imagine
you are walking down the aisle of a hardware store and all of a sudden a stranger
looks at you and points to a bucket standing in one of the shelves. You see the
bucket, but, with a quizzical look on your face, look back at the stranger, because
you do not know what is going on. The reason why you do not know what is going
on is that you lack a joint attentional frame with the stranger, which would give the
point its meaning. The pointing as such, in this frameless scenario, does not mean
anything. But if, instead, you are walking down the same aisle with a friend because
you are looking for a bucket to use for cleaning purposes, and your friend points out
the bucket to you, you would know immediately what he means: “Here is one!” The
presence of the joint attentional frame, which could be described by something like
“we are searching for a bucket,” grounds the point in the ongoing activity and gives
it its meaning. Another possible scenario could be that you and your friend are
looking for anything that is made of a certain kind of plastic because you like it so
much. In this case, your friend’s point would have a different meaning, namely
something like: “Here is an item which is made of that plastic that you like so
much!” The referent of the pointing gesture thus varies as a function of the joint
attentional frame in which the pointing is anchored. One can imagine an endless
number of joint attentional frames for the same basic scenario, with the referents of
the pointing gesture being, for instance, “item with texture of kind x,” “item which
is similar to that other item we just saw,” and so forth. The pointing gesture does not
just indicate some spatial location, but instead it already contains a certain perspective from which the indicated object or location is to be viewed. And the perspective
is carried by the joint attentional frame.
Humans can read pointing gestures based on joint attentional frames from as
early as 14 months of age. Behne et al. (2005) found that 14-month-olds choose the
correct container in the Object Choice task significantly above chance, thus demonstrating that they understand the pointing gesture cooperatively. Infants also know
that the validity of a joint attentional frame is limited to those people who share it.
Liebal et al. (2009) had 18-month-old infants clean up with an adult by picking up
toys and putting them in a basket. At one point, the adult stopped and pointed to a
ring toy, which infants then picked up and placed in the basket, presumably to help
clean up. However, when the adult pointed to this same toy in this same way but in a
different context, infants did not pick up the ring toy and put it in the basket;
specifically, when the infant and adult were engaged in stacking ring toys on a post,
children ignored the basket and brought the ring toy back to stack it on the post. The
crucial point is that in both conditions the adult pointed to the same toy in the same
way (and everything else in the room was the same), but the infant extracted a
different meaning in the two cases based on the two different joint attentional
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frames involved, and the jointness is, indeed, crucial here. Thus, in a control
condition, the infant and adult cleaned up exactly as in the shared clean-up
condition, but then a second adult who had not shared this context entered the
room and pointed towards the ring toy in exactly the same way as the first adult in
the other two conditions. In this case, infants did not put the toy away into the
basket, presumably because the second adult had not shared the cleaning context
with them. Rather, because they had no shared frame with this adult, they seemed
most often to interpret the new adult’s point as a simple invitation to note and share
attention to the toy.
We, thus find that apes “communicate” individualistically, to get others to do
things, and without joint attentional frames to ground the communicative intentions
in a preexisting space of shared meaning. Human infants from as early as 14 months
of age, on the other hand, communicate cooperatively to share interest in things
and inform others of things and they construct and participate in joint attentional
frames, which give cooperative gestures their meaning. Without a foundation in
cooperative communication of this type, human language is not even thinkable
(Tomasello 2008).
16.3.3
Cultural Learning
Human behavioral traditions have a cumulative history, with some of them showing
a kind of “ratchet effect” of accumulating complexity over time (Tomasello et al.
1993). There is no convincing demonstration of the ratchet effect or any other form
of cumulative cultural evolution for chimpanzees or any other nonhuman animals.
The explanation for this difference involved four components. First, although
chimpanzees learn much socially (see for example the recent work of Whiten and
colleagues as summarized in Whiten in press, this volume), humans seem to be
more focused on actions than are chimpanzees, who are mainly focused on outcomes and goals. Humans are better and more accurate social learners: they are
cultural learners. This special focus on actions enables them to socially learn
activities from others in a much more accurate fashion, which not only contributes
to the ratchet effect over time, but also enables the acquisition of cultural conventions that are only arbitrarily related to any causal relations in the world (such as
linguistic symbols), since in this case faithful copying of actions is required.
Second, humans rely on teaching as a complement to their natural skills of social
and cultural learning. Gergely and Csibra (2006) have recently elaborated an
account explaining why the existence of relatively “opaque” cultural conventions
(there is no causal structure or else it is difficult to see this structure) requires that
human adults be specifically adapted for pedagogy toward children and human
children be specifically adapted for recognizing when adults are being pedagogical
(what Tomasello et al. 1993, called “instructed learning”). Engaging with others in
this way is a kind of shared intentionality relying on cooperative and communication, in which the learner trusts the information given by the teacher. There has been
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no systematic study of chimpanzees engaged in anything resembling teaching since
the observations of Boesch (1991), which have multiple interpretations.
Third, humans imitate one another not simply when they are aimed at acquiring
more effective behavioral strategies in instrumental situations, but they also imitate
for purely social reasons to be like others. The tendency of human beings to
follow fads and fashions and to conform are well known and well documented, and
the proposal here, following Carpenter (2006), is that this represents a different and
important motivation for social learning that may produce qualitatively different
behaviors. For example, human infants have a greater tendency than do chimpanzees for copying the unnecessary “style” of an instrumental action (Carpenter and
Tomasello, unpubl. data), and in acquiring linguistic conventions, children are not
just driven by communicative efficacy but also by the desire to do it the way the
others do it (Tomasello 2003). This analysis would also explain why children in the
studies cited above sometimes imitated poor demonstrators when it would have
been to their advantage to ignore them, and, in general, why children copy the
actual actions of others more readily than do other apes. This so-called “social
function” of imitation (Uzgiris 1981), the urge to be like others, is clearly an
important part of human culture and cultural transmission.
Finally, human culture persists and has the character it has, not just because
human children do what others do, but also because adults expect and even demand
that they behave in certain ways: children understand that this is not just the way that
something is done, but rather the way it should be done. This normative judgment is
another aspect of shared intentionality, as it is essentially a judgment based on the
perspective of the group how “we” do things. In a recent study, Rakoczy et al.
(2008) found that 3-year-old human children not only copied the way that others did
things, but when they observed a third party doing them in some other way they
objected and told them they were doing it “wrong” that is not how “one” does it.
Kelemen (1999) has also shown that young children learn very quickly that a
particular artifact is for a particular function, and other uses of it may be considered
“wrong” this is not how “we” use this artifact. This normative dimension to human
cultural traditions serves to guarantee their faithful transmission across generations
in a way that supports further ratcheting up in complexity across historical time.
It may very well be, then, that it is these processes and aspects (cultural learning,
teaching, normativity) that give human cultural traditions their extraordinary
stability and cumulativity over time. Integral in all of these is a kind of social
engagement depending on skills and motivations for shared intentionality.
16.4
Joint Attention and Perspective
It is, thus clear that human infants, before they are fully participating members of a
culture, already have a special motivation for sharing experiences with other
persons, and they possess special skills for creating with others joint goals, joint
intentions, and joint attention. They learn from others in unique ways as well.
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However, our claim goes further. Our claim is that participation in interactions
involving shared intentionality transforms human cognition in fundamental ways.
Most importantly, it actually creates new forms of cognitive representation,
specifically, perspectival or dialogic cognitive representations (see also Tomasello
1999; Tomasello et al. 2005). In understanding and internalizing an adult’s intentional states, including those directed towards her, at the same time she experiences her own intentional states towards the other, the child comes to
conceptualize the interaction simultaneously from both the first and third person’s
perspective (see Barresi and Moore 1996) forming a bird’s eye view of the
collaboration in which both commonalities and differences are all comprehended
within a single representational format. Such perspectival representations are
necessary not only for supporting cooperative interactions online, but also for
the creation and use of certain kinds of cultural artifacts, most importantly linguistic and other kinds of symbols, which are socially constituted and bi-directional in
the sense of containing simultaneously the perspective s of both speaker and
listener (see Mead 1934).
These perspectival cognitive representations pave the way for later uniquely
human cognitive achievements. Importantly, following Harris (1996), Tomasello
and Rakoczy (2003) argued and presented evidence that coming to understand
false beliefs the fact that someone else’s cognitive perspective about a state of
affair is different from what I know to be true depends on children’s participation over a several year period in perspective-shifting discourse. In such linguistic
discourse, including such things as misunderstandings and requests for clarification, children experience regularly that what another person thinks is often
different from what they think, and the understanding of false beliefs which,
in almost everyone’s account, is fundamental to mature human social cognition
is apparently unique to humans (Call and Tomasello 2008). And at age 4,
children not only come to understand that others might hold false beliefs, they
develop a sophisticated understanding of perspectives more generally. That is,
they appreciate that different people might see or conceptualize a given event or
object in different ways and also, that one and the same person can view or
construe an object differently at different times. This ability to simultaneously
“confront” perspectives becomes manifest in a variety of tasks besides the
standard false belief task. For example, children now understand that one and
the same object might (1) look like a rock but really be a sponge (the so-called
appearance-reality distinction, Flavell et al. 1986), (2) be both an animal and a
rabbit (as shown by their acceptance of alternative labels for a given object,
Doherty and Perner 1998), and (3) be seen right-side-up from one perspective but
upside-down from another (the so-called level 2 perspective-taking, Masangkay
et al. 1974).
We would argue that young children come to understand and operate with the
concept of perspective only after first experiencing the sharedness of attention on
one and the same thing (see also Barresi and Moore 1996). From thereon, they can
later begin to understand the differences in perspectives which converge on the
shared target or object of interest. The foundations again can be found in infancy.
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Evidence for this comes from a series of studies in which infants must determine
what an adult has experienced and has not experienced. Tomasello and Haberl
(2003) had 12- and 18-month-old infants play with an adult with two toys in turn.
Before a third toy was brought out by an assistant, the adult left the room. During
her absence, the infant played with the third toy together with the assistant. Finally,
all three toys were held in front of the infant, at which point the adult returned into
the room and exclaimed excitement followed by an unspecified request for the
infant to give her that toy (without indicating by gazing or pointing which specific
toy she was attending to). Surprisingly, infants of both ages selected the toy the
adult had not experienced (the one which was new for her). In order to solve this
task, infants had to understand (1) that people get excited about new, not familiar
things and (2) which of the toys was new for the adult and which she was already
familiar with from previous experience.
In this study, infants knew what was familiar for the adult after they had
participated with her in joint attention around two of the objects (but not the
third). This suggests the possibility that infants need to attend to another person’s
experience in joint attention with her in order to register the other as knowing the
object in question. And this is what was basically found in the two studies by Moll
et al. (2007), Moll and Tomasello (2007b). Following the basic procedure of
Tomasello and Haberl (2003), 14- and 18-month-old infants either (1) became
familiar with the first two objects in a joint attentional frame together with the
adult or (2) simply witnessed the adult become familiar with the known objects
individually. In each case, infants themselves became equally familiar with all three
objects, as in the original study. The result was that infants knew which of the three
objects was new for the adult, and thus captured her attention only when they had
explored the known objects in a joint attentional format with her. They could not
make this distinction when they had just witnessed her exploring them on her own,
outside of a joint attentional frame. The shared attention to the known objects thus
highlighted the fact that the third object was not jointly experienced. It is, thus
inside of joint attentional frames, that infants first begin to realize differences in
people’s experiences and perceptions. This early understanding of other’s experiences is the foundation for the later developing understanding of divergent perspectives on one and the same thing an understanding which is, just like the ability
to jointly engage with others, uniquely human.
Our argument is thus that basically all species-unique aspects of human cognition reflect their cooperative roots in fundamental ways. The ability to take the
perspective of others which spawns the understanding of false beliefs, perspectival cognitive representations, and collective/institutional reality is only possible
for organisms that can participate in social interactions involving shared intentionality, especially involving joint attention. Let us be very clear on this point.
Participation in these interactions is critical. A child raised on a desert island would
have all of the biological preparations for participation in interactions involving
shared intentionality, but because the child did not actually participate in such
interactions, she would have nothing to internalize into perspectival cognitive
representations. Ontogeny in this case is critical.
346
16.5
M. Tomasello and H. Moll
From Collaboration to Culture
We, thus find that human infants in their second year of life are much more skilled,
and much more motivated, than are great apes at participating in collaborative
problem solving and cooperative communication, and their skills of social/cultural
learning have unique qualities as well. Following Tomasello et al. (2005), our claim
is that the reason for this difference is that human infants are biologically adapted
for social/cultural interactions involving shared intentionality. Even at this tender
age, human infants already have special skills for creating with other persons joint
goals, joint intentions, and joint attention, and special motivations for helping and
sharing with others and for communicating with and learning from others within
these special interactions as well.
It would seem that we are still a long way from such things as governments and
religions and sciences and other large-scale cultural institutions. But actually we are
not. These kind of cultural institutions arose only very recently in human evolution,
after, and partially as a result of, the agricultural revolution when people began
living in large cities. There, many new demands arose for collaborating with
strangers and in much larger groups and across longer spans of time than previously.
Our simple proposal is that the collaborative and communicative skills we see in
young children form the necessary foundation for beginning to participate in such
large-scale collaboration. Virtually no one believes that there were any genetic
events at the agricultural revolution that led to the unprecedented population
explosion and flowering of cultural institutions associated with that great event.
So what is required for scaling up small group collaboration into large group
collaboration is mainly certain sociological conditions, as a first step, and then
cultural-historical processes over time and generations in which such institutions
could be formed.
What we have called the cultural intelligence hypothesis, or the Vygotskian
intelligence hypothesis, reflects the idea that the incredible complexities of modern
human culture and its institutions are the result of a qualitatively new process that
arose in human evolution. Although there may be some cultural transmission in
some animal species, nothing like the human creation of cultural artifacts and
institutions which ratchet up in complexity over time and within which children’s
ontogeny proceeds and on which it depends takes place in other species. This is
because human beings collaborate with, communicate with, and learn from their
groupmates based on unique skills and motivations for shared intentionality and
cultural learning. We do not believe that the human gap can be explained by any
appeal to individual cognitive skills, but rather it can only be explained by the
social cultural processes for which they are specially adapted and within which
each generation of modern humans has evolved.
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Chapter 17
The Evolution and Development of Human
Social Cognition
David F. Bjorklund, Kayla Causey, and Virginia Periss
Abstract Humans’ ancestors experienced intense selection pressures to develop
enhanced social-cognitive abilities, facilitating the coevolution of an extended
childhood, larger brain, and increased social complexity. This chapter describes
the emergence of human social-cognitive abilities from an evolutionary developmental perspective, focusing on the importance of social interaction and epigenetic
inheritance. The development of shared attention and referential communication,
empathy, social learning, and theory of mind is discussed as it occurs in human
children, and research demonstrating the importance of parent child interactions
and individual differences in maternal behavior during the development of these
abilities is highlighted. A discussion of how these abilities are expressed in motherreared and enculturated chimpanzees is also included and indicates that these
animals possess substantial social-cognitive competencies, which, under certain
rearing conditions, can be modified to resemble a more Homo sapiens way of
thinking. This is strongly suggestive that our common ancestor with chimpanzees
also possessed the neurological plasticity to adapt its behavior and cognition in
response to changes in environmental conditions.
17.1
Introduction
Biologists since Darwin have believed that there is a continuity of mental functioning in evolution, so that the cognition of a species such as Homo sapiens should
share many features with species with which they recently shared a common
ancestor. In a quest both to understand human evolution and the cognition of extant
animals, scientists have assessed the cognitive gap between humans and other
D.F. Bjorklund (*), K. Causey, and V. Periss
Department of Psychology, Florida Atlantic University, Boca Raton, FL, USA
e mail: dbjorklu@fau.edu, klabeth@mac.com, vperiss@gmail.com
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 17, # Springer Verlag Berlin Heidelberg 2010
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D.F. Bjorklund et al.
primates. In fact, the size of the gap between humans and our great-ape relatives has
been a point of contention among scientists and laypeople for decades, perhaps
centuries. Some view chimpanzees, for instance, as “almost human,” whereas
others see them as just “clever animals.” How people view the great apes is also
related to how they view humans, as “next to angels” (literally or figuratively) or
simply as “the thinking animal.”
The urge to either increase or decrease the cognitive and evolutionary distance
between apes and humans varies both with one’s scientific and philosophical (and,
perhaps, political and theological) viewpoints. The scientific debates hinge on what
unique attributes evolved, over the past 5 8 million years when modern humans last
shared a common ancestor with Pan troglodytes and Pan paniscus, in the line that
led to H. sapiens. Identifying the set of selective pressures that is the best candidate
for the driving force in human cognitive evolution has also provoked much discussion. Many have been nominated, and we believe there is no single domain that can
be pointed to as “the” cause of human evolution. However, we, as many others (e.g.,
Humphrey 1976; Alexander 1989; Dunbar 1995) have concluded that human
intellectual evolution was driven by the need to deal with conspecifics. As such,
human social cognition can be viewed as the more primary form of thought, with
H. sapiens’ impressive suite of intellectual abilities being essentially the derivative
of cognition initially evolved to cooperate and compete with fellow hominids.
Humans obviously share many social-cognitive abilities with other social primates,
and so, even for those that are unique to humans, we are able to see their roots in the
behaviors of our close genetic relatives.
There are many different aspects of human social cognition and its development,
but at their core is the understanding that other people are intentional agents
beings whose behavior is based on knowledge and desires, and whose actions are
deliberate in achieving a goal (i.e., they do things “on purpose”; see Bandura 2006;
Tomasello and Carpenter 2005). Although signs of intentional representation appear late in the first year, children’s understanding of others as intentional agents
develops over childhood, culminating in the ability to pass false-belief tasks around
4 years of age, the benchmark for attaining theory of mind.
Although most aspects of social cognition follow a regular developmental
pathway, there are substantial individual differences in the rate at which some
contents are acquired or the level they are expressed. Given the wide range of social
environments in which humans live, it is necessary that children’s social cognition
be flexible, matched to the demands of their particular culture. Although there are a
host of potential factors that contribute to individual differences in social cognition,
parents likely play a particularly important role. Mothers (and, sometimes, fathers)
are children’s first social partners, and humans’ extended period of dependency
requires prolonged parental care. Although peers and other adults in a community
certainly influence the children’s development (e.g., Harris 1995; Bronfenbrenner
and Morris 2006) increasingly so as they become more independent from their
parents, beginning around 6 or 7 years of age the foundation of social cognition is
laid down during the preschool years, and it is this time when parents’ influences
tend to be greatest (e.g., Scarr 1993).
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The focus of this chapter is on the development of social cognition in human
children. In addition to describing the typical course of development, we also
examine the influence that parental behaviors have on the development of socialcognitive abilities of humans as well as on the social-cognitive abilities of chimpanzees (P. troglodytes). Chimpanzees provide the best model for what the social
cognition of our ancestors may have been like. Like humans, chimpanzees and
bonobos (P. paniscus) have big brains, an extended juvenile period, and live in
socially complex communities, conditions that some have proposed were necessary
ingredients for the evolution of the human mind (e.g., Bjorklund and Rosenberg
2005; Bjorklund 2006). We should, thus, see hints of some of the social-cognitive
abilities that we see in young humans in our close genetic relatives. Moreover, just
as individual differences in rearing environment affect the course of social cognition in children, they may also influence the development of social cognition in
young apes. We, therefore, include in our review evidence that human rearing
(enculturation) of chimpanzees can alter ape cognition in a more human-like
fashion. Enculturation in this context refers to apes being reared by humans
much as human children are reared. This typically includes language, encouraging
shared attention, and deliberate teaching (Call and Tomasello 1996).
We start our review with the early-developing foundational social-cognitive
abilities of shared attention, referential communication, and empathy. We then
examine more advanced forms of social cognition, specifically, social learning
and theory of mind.
17.2
Shared Attention and Referential Communication
Shared (or joint) attention involves a triadic interaction between the child, another
person, and an object. For example, parents often draw their child’s attention to an
object by pointing or gazing at the object, a form of referential communication,
which indicates that the “pointer” understands that he or she sees something that the
observer does not.
17.2.1 Development
Although parents may engage in this type of behavior from the earliest days of a
baby’s life, it takes infants a while to fully catch on. However, infants are oriented
to social interactions from birth and “hit the ground running,” so to speak, regarding
the acquisition of information through social interaction. For instance, neonates
orient to the human face and quickly learn to seek their mothers’ faces (Feldman
and Eidelman 2004). By 2 months, infants begin to engage in dyadic exchanges,
reciprocating the social patterns of a partner in an intimate one-on-one context (e.g.,
Lavelli and Fogel 2005), and by 2 or 3 months, they can recognize self-produced,
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biological motion (Bertenthal et al. 1987) and soon turn to look in the same
direction of another (Tomasello et al. 2003). These behaviors are necessary for
shared attention, which is typically first seen in infants around 9-months of age and
increases over the next year. For example, 12-month olds will point to inform others
about unknown events (Liszkowski et al. 2007), and between 12 and 18 months,
infants acquire the ability to reference others’ eye gaze as a cue to direct their own
attention (Brooks and Meltzoff 2002) and to point to objects to direct an adult’s
attention to something he or she is searching for (Liszkowski et al. 2006). Between
18 and 24 months of age, toddlers use eye gaze along with other directional cues,
such as pointing and head orientation, for word learning and social referencing
(Poulin-Dubois and Forbes 2002). These findings indicate that beginning about 9
months of age, infants view other people as intentional agents, an understanding
that other peoples’ behavior is based on their goals and intentions (see Tomasello
1999; Tomasello et al. 2007).
By the end of the first year of life, infants extend shared attention to social
referencing to guide their behavior when encountering ambiguous events (e.g.,
Feinman 1991; Vaish and Striano 2004). Infants can make use of a parent’s facial
expression, tone of voice, gestures, or combinations of these sources to determine
their actions in an uncertain situation (Hornik et al. 1987; Vaish and Striano 2004).
In these contexts, infants exploit the triadic interaction between themselves, an
object, and an adult to derive information from the adult’s responses to the object or
situation. Differences in the amount and quality of the information that is transmitted by the mother may have significant, and sometimes dire, consequences for
her child. Moreover, as a situation in which infants must infer the emotional
response of an individual, social referencing may provide the stepping-stone
necessary to develop empathy.
By being drawn into the social relationship and taking the perspective of the
parent, children are able to extend their attention beyond the dyad to the object
being referenced. This ability to take the perspective of others is at the core of social
cognition, and, as such, deficits in shared attention are taken as early indicators of
autism (e.g., Adamson et al. 2001).
17.2.2
Parental Effects
While the development of shared attention seems highly canalized, a supportive
social partner is required and this is typically a child’s parent, frequently his or her
mother. As such, characteristics of the parent may lead to individual differences in
parent infant interactions.
For example, dysphoric (depressed mood) mothers spend less time engaging in
shared-attention activities with their infants than nondysphoric mothers (Goldsmith
and Rogoff 1997). But it is not only the quantity of shared attention that can be
affected, but also the quality. For example, Deák et al. (2008) reported that 15- and
21-month-old infants were more likely to look at a parent (mostly mothers) who
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elicited their attention verbally and to follow the shifting gaze of a parent who also
pointed or used directing verbalizations, indicating that differences in maternal
behaviors can influence how effective parents are in attracting and redirecting their
child’s attention.
Individual differences in parental behavior that lead to variations in dyadic
interactions may also have effects on infants’ later social-cognitive development.
For instance, Carpenter et al. (1998b) found that the amount of time 9-month-old
infants spent in shared attention with their mothers and a mother’s use of language
containing some reference to an object predicted infants’ communication skills at 16
months. Other research found that 12-month olds’ initiations of, and responses to,
shared attention predicted their social competence at 30 months, even after accounting for infant temperament, cognition, language, and demographic variables (Van
Hecke et al. 2007). Taken together, these findings underscore the important role of
early social interaction in the development of later social-cognitive abilities.
17.2.3 Abilities of chimpanzees
Although chimpanzees, and even monkeys, will follow the gaze of another individual in some contexts (Bering and Povinelli 2003; Bräuer et al. 2005), most
researchers argue that there is no evidence that either mother-reared or enculturated
chimpanzees engage in shared attention (Tomasello and Carpenter 2005; Herrmann
et al. 2007). Others disagree, noting, for example, that captive chimpanzees point to
food only in the presence of a caretaker (Leavens et al. 2005). Moreover, evidence
that wild chimpanzees will scratch themselves in an exaggerated way to direct the
action of a grooming partner (Pika and Mitani 2006) suggests that even apes in the
wild may engage in some form of referential communication, albeit a type that is
centered on a chimpanzee’s body and not on distant objects.
In contrast to mother or nursery-reared chimpanzees, clear evidence of referential pointing is observed in each of the great ape species for enculturated individuals: orangutans (Pongo pygmaeus, e.g., Miles 1990), gorillas (Gorilla gorilla,
e.g., Patterson 1978), chimpanzees (P. troglodytes, e.g., Povinelli et al. 1992), and
bonobos (P. paniscus, e.g., Savage-Rumbaugh et al. 1986).
17.3
Empathy
As evolutionary primatologist Sarah Hrdy (1999, p. 392) wrote: “What makes us
humans rather that just apes is the capacity to combine intelligence with articulate
empathy.” Generally, empathy refers to the ability to recognize, perceive, and feel
the emotion of another and requires the ability to take the perspective of another.
Here, we distinguish between empathy that is insightful (cognitive empathy),
sympathy (emotional recognition), and emotional contagion (emotional empathy).
When considering its development, it is important to consider different types of
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empathy as reflecting different underlying mechanisms. For example, recognizing
that Mom is angry based on her facial, vocal, or postural expression can be
characterized as sympathy. The sympathizer can recognize the emotion and may
even be moved to change it, but lacks insight into the nature or cause of the
emotion. Sympathy (emotion recognition) is less sophisticated than the ability to
recognize that Mom is angry because she got cut off in traffic (understanding the
object of emotion) and is running late (nonpsychological context) and is quick to
anger, in general (character traits). These insights are considered truly empathic
because the observer gets an immediate sense of what it would be like “in Mom’s
shoes” and understands the perspective of Mom (cognitive empathy). It is also
important to distinguish empathy from emotional contagion, an automatic or
unconscious process by which the observer shares the emotions of another, but
that does not require insight on the part of the observer (Preston and de Waal 2002).
17.3.1 Development
There is evidence that empathy is rooted in an early ability to associate the
behaviors, vocal cues, and facial expressions of another with the proprioceptive
feelings that occur when producing the corresponding movements or expressions
oneself (e.g., Singer et al. 2004). For example, even newborn infants display
emotional contagion, engaging in contagious crying in response to the cries of
other newborns, although not to the cries of older infants or to their own cries when
played back to them (Martin and Clark 1982; Dondi et al. 1999).
Cognitive empathy, in contrast, is described as a secondary, or self-conscious,
emotion and emerges in the second half of the second year of life (Lewis 1993). For
instance, in one study, researchers examined 12- to 24-month-old children’s responses
to the distress of other people (Zahn-Waxler et al. 1992). They reported that empathic
responses increased with age, such that 2-year-old children often made facial expressions or other gestures indicative of sadness, tried to comfort distressed people, and
sometimes sought information about the person’s distress (What’s wrong?). BischofKöhler (1988, 1991) proposed that cognitive empathy, insofar as it involves emotional
insight, is a mechanism that aids understanding of another’s belief or desire state by
allowing the observer to vicariously share in this state. In other words, fully developed
empathy requires theory of mind, while emotional contagion does not. As such, true
empathic insight relies on self-recognition, the ability to make a clearcut distinction
between the emotional domains of self and other, which develops typically in the latter
half of the second year. Thus, much of the “empathic” behaviors observed in infants
can be explained as resulting from emotional contagion.
In a series of experiments, Bischof-Köhler (1988, 1991, 1994) demonstrated that
self-objectification (measured by mirror self-recognition) and empathy (objectified
as prosocial behavior) emerge almost simultaneously in development: only those
children capable of passing the “rouge test” for self-recognition tried to help a
person in need who demonstrated grief, whereas nonrecognizers stayed indifferent.
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Bischoff-Köhler’s depiction of empathy development is stage-like: initially, infants
are victims of emotional contagion, but emergence of self-objectification serves as
a switch that “turns on,” or allows, empathy. The process requires that infants orient
to emotional cues early on and their ability to do so is partly due to the manner in
which their caregivers direct them to relevant information.
17.3.2
Parental Effects
Kuchuk et al. (1986) showed 3-month-old infants static pictures of faces that varied
on the intensity of their smiling expression. Infants were more sensitive to the subtle
variations of smiles in the pictures if their mothers more often directed attention to
their own faces when smiling. Importantly, this relationship was strongest for
mothers who displayed relatively low levels of smiling, suggesting that it is not
the number of smiles that a mother shows her infant, but the quality of the dyadic
interaction during these experiences that affects infants’ ability to discriminate
between subtly different expressions. Kuchuk et al. suggested that events that are
made salient during atypical experience might be more distinctive to infants,
increasing their sensitivity to corresponding subtle differences as a result.
In support of this, although most infants look longer at sad faces relative to
happy faces (Field et al. 1998), infants of depressed mothers tend to experience
disproportionately high levels of exposure to sad, angry, or neutral faces, and as a
result seem to be desensitized to sad expressions and do not display the typical
looking-time preference for sad faces but rather prefer smiling faces (Striano et al.
2002). This and related work with maltreated children (e.g., Pollak and Kistler
2002; Pollak and Sinha 2002) suggest that young children possess mechanisms that
allow them to become sensitized to particular types of emotional expressions and
heighten their sensitivity to other types. For example, it would be advantageous for
maltreated children to quickly recognize anger in order to avoid negative consequences, but quickly detecting sad faces would not provide any additional advantage to children of depressed mothers (de Haan et al. 2004).
While the existing data suggest that the quality of early mother child interactions affects children’s differential attention to emotional expressions, we view this
ability as a stepping stone from emotional contagion toward fully developed
empathy that requires the ability to take the perspective of another, and presumably
to treat the other individual as an intentional agent, whose emotions affect his or her
goals and behaviors. This is often referred to as possessing a “theory of mind,” and
we discuss maternal effects on these abilities later in this chapter.
17.3.3
Abilities in Chimpanzees
As we noted, cognitive empathy seems to require the ability of the observer (or
empathizer) to take the perspective of another in order to identify with or to
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understand another’s feelings. However, Preston and de Waal (2002) proposed that
simpler forms, such as emotional contagion and sympathy, are observed in a wide
range of social animals.
At least one study provides evidence that enculturated apes will spontaneously
provide help to a familiar human. In a study by Warneken and Tomasello (2006),
18- and 24-month-old human children helped an adult when he or she, for example,
accidentally dropped a marker on the floor or when misplacing a book on a stack.
They failed to help (e.g., retrieve the fallen marker or book) when the adult
intentionally threw the marker on the floor or placed the book beside the stack.
Warneken and Tomasello reported similar results for three human-reared chimpanzees, although only when the human was reaching unsuccessfully for an object, not
for other types of tasks. These young, enculturated apes displayed evidence of
“helping” behavior, something not previously observed in unrelated chimpanzees.
A more recent controlled study reported that semi-free ranging and nonenculturated
chimpanzees engaged in some helping behaviors to both humans and conspecifics,
suggesting that the roots of altruism may be part of chimpanzee, as well as human,
nature (Warneken et al. 2007).
De Waal (1997, 2005) has argued that great apes display more advanced forms
of empathy under certain circumstances, including cognitive empathy, and he
provides substantial anecdotal evidence in support of this. For example, de Waal
(2005) relates the episode in which Binti Jua, an 8-year-old female gorilla, helped a
3-year-old boy who fell into the primates’ cage at the Chicago Brookfield Zoo. In an
equally provocative example, a long-time researcher at the Stuttgart Zoo introduced
her newborn baby to the bonobos, upon which the alpha female disappeared for a
short time and returned with her own newborn (de Waal 1997).
Yet, in laboratory settings, chimpanzees seem to be indifferent to the welfare of
others. For example, chimpanzees will not provide food to other familiar but
unrelated chimpanzees, even though there is no material cost to themselves (Silk
et al. 2005, see also Jensen et al. 2006).
17.4
Social Learning
Social learning is broadly defined as occurring in a situation “in which one
individual comes to behave similarly to another” (Boesch and Tomasello 1998),
and is further differentiated based on presumed underlying mechanisms (see
Tomasello and Call 1997; Tomasello 1999).
Consider the various ways in which a young child might go about learning as a
consequence of observing his of her sister push a chair over to the refrigerator,
climb on top, and retrieve the cookies hidden above. Perhaps the simplest form of
social learning is local enhancement, in which an individual demonstrates an
increased interest in the particular location at which he or she observed another
individual performing an action (in this case, the refrigerator). As a result, the child
moves to that location and independently discovers a useful outcome (cookies).
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In a similar process, stimulus enhancement involves an observer’s increased interest
in manipulating a particular object (a chair) as opposed to moving to a particular
location. Mimicry involves learning about the causal actions of an individual, as
opposed to the object or goal, and replicating those actions without any insight as to
why those actions are being performed or that a goal is even present (pushing the
chair to the refrigerator and climbing on it just because your sister did it without
knowing ahead of time that it helps you reach the cookies on top). Goal emulation
involves recognizing that a particular goal exists in the environment (there are
desirable cookies on top of the refrigerator) and setting to reach that goal by one’s
own means (climbing on the counter to reach the top of the refrigerator rather than
climbing on a chair). Imitation involves understanding and reproducing the goal
(get cookies) and the means by which it was achieved (push the chair over to the
fridge and climb up on it).
While imitation might be the most sophisticated form of social learning someone
can demonstrate, perhaps the most sophisticated component of the learning process
is teaching, which involves the teacher modifying his or her behavior in order that
the “student” acquires new knowledge. We define teaching following Tomasello
et al. (1993a), as requiring that the learner appreciate the perspective of the teacher
and that the teacher be sensitive to the knowledge, motivations, and emotions of the
learner.
17.4.1
Development
Although neonates will match the facial expression of a model (Meltzoff and Moore
1977), infants are learning nothing new via such matching behavior, and thus, this
likely does not reflect social learning (Jacobson 1979; Bjorklund 1987). Rather,
infants and caregivers are engaging in social mirroring that helps foster social
interaction between the infant and caregiver (Byrne 2005). Such early facilitation
of social interaction is important for establishing patterns of later social interaction
and, as a result, social learning. In fact, Heimann (1989) found that neonatal
“imitation” of tongue protrusions positively predicted the levels of mother infant
social interaction 3 months later.
Infants’ social learning clearly increases over the first year of life, although it is
difficult to ascertain with certainty if infants are merely mimicking an adult model
or engaging in other more sophisticated forms of social learning. Early in the
second year of life, however, infants are sensitive to the model’s intentions, as
reflected by their selective imitation of a model’s intended behavior rather than his
or her actual behavior (Meltzoff 1995; Carpenter et al. 1998a). For example, in one
study 14- to 18-month-old infants watched adults perform complex behavior
sequences, some of which appeared intentional as reflected by the model’s vocal
behavior and others that, based on what the model said, were accidental. When they
were later given the chance to imitate the model, the infants reproduced twice as
many “intentional” as “accidental” behaviors (Carpenter et al. 1998b).
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Although young children’s social learning is flexible, most research indicates
that they are more apt to engage in mimicry or imitation, copying relatively
precisely the actions of a model, rather than goal emulation. For example, Horner
and Whiten (2005) presented 3- and 4-year-old children with an opaque puzzle box
and demonstrated a series of three actions, two of which were necessary and one of
which was not, to retrieve a gummy bear from inside the box. The children copied
all three of a model’s actions to retrieve a gummy bear. However, when presented
with an identical transparent puzzle box, children still copied all three actions, even
when the irrelevancy of some actions was readily observable. These findings
suggest that children blindly imitate the behavior of a model (see also Nagell
et al. 1993). Young children may be motivated to mimic the actions of others in
order to maintain a social interaction, and such social motivation may be as
important, or more so, as an understanding of intentions in some contexts of social
learning. Relatedly, young children’s “overimitation” may be due to the belief that
all of an adult’s actions are goal-directed, making imitation of those actions a
reasonable course to take (Lyons et al. 2007).
Despite a tendency toward “overimitation,” even 14-month olds will engage in
goal emulation rather than mimicry or imitation in some situations. For example,
extending an earlier study by Meltzoff (1988), Gergely et al. (2002) had 14-monthold infants watch as a model used her head to press a button to turn on a light in one
of two conditions: (a) her hands were free, as in the earlier Meltzoff study, or (b) her
hands were wrapped in a blanket, that is, her hands were occupied and thus not
available to turn on the light. Most of the infants (69%) in the “hands-free”
condition used their heads to turn on the light, just as Meltzoff had found. But the
pattern was reversed in the “hands-occupied” condition. Now, most of the infants
(79%) used their hands to turn on the light. That is, when there was a reason why the
model did not use her hands (they were wrapped in a blanket), the babies focused on
the goal (turn on the light), not the means (use your head), and used their hands
as the most efficient way to turn on the light. When no such reason was available,
they copied the model’s behavior exactly, reflective of imitative learning. Gergely
and colleagues referred to this as rational imitation. Other researchers have similarly shown that young children’s social learning is flexible, with children between
12- and 26-months of age displaying both imitation (focusing on the means of a
model, as well as the ends) and goal emulation (focusing only on the ends), depending
on the context (e.g., Carpenter et al. 2005; Nielsen 2006). This is important because
it indicates that children possess the plasticity to modify their learning based on the
type of information they receive.
17.4.2 Parental Effects
Parents’ role in social learning begins early. For example, newborns not only copy
the facial gestures of an adult but also provoke imitation from them and demonstrate different patterns of heart-rate change that correspond to these different
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behaviors (increasing heart rate when imitating and decreasing heart rate when
provoking, Nagy 2006). Thus, although infants are not engaging in any form of
social learning yet, these behaviors serve a unique function during the early months
of life. When members of a dyad match each other’s behavior, they are engaging in
a nonverbal dialog, expressing mutual identification and maintaining mutual attention between the two interactants.
Mothers and infants continue this reciprocal imitative relationship over the
course of infancy. For instance, Masur and Rodemaker (1999) observed mothers
and their infants during free play or bath time when the babies were 10-, 13-, 17-,
and 21-months old. Rates of imitation were about one episode per minute, with
mothers imitating infants more than infants imitating mothers. And unlike neonatal
imitation or mutual imitation described by Piaget (1962), these older infants are
learning many new things through social learning. For example, in a study in which
parents were asked to keep diaries of their children’s imitative behavior, 12-, 15-,
and 18-month olds learned, on average, one or two new behaviors a day simply by
watching (Barr and Hayne 2003).
Being engaged in imitation and social learning may facilitate infants’ and young
children’s abilities to learn that others are “like me” and aid children in the
acquisition of perspective-taking abilities (Meltzoff 2005). As children learn from
and imitate a model’s behavior, they become self-aware and better able to reason
about the relationship between their own behaviors, knowledge, and desires and the
behaviors, knowledge, and desires of others.
17.4.3 Abilities in Chimpanzees
Chimpanzees are impressive social learners. They have been found to transmit
nongenetic information across generations, including forms of greeting, grooming,
and foraging, a characteristic of culture (e.g., Whiten et al. 1999; Whiten 2007).
Orangutans (van Schaik et al. 2003) and cetaceans (Rendell and Whitehead
2001; Bender et al. 2009) have also been observed to transmit information across
generations, although no species does so with the ease, fidelity, and to the extent
that humans do.
It is unequivocal that chimpanzees engage in various forms of social learning,
including goal emulation, in which new behaviors are acquired (e.g., Call et al.
2005; Horner and Whiten 2005). For example, we described earlier a study in which
3- and 4-year-old children imitated irrelevant actions to open a puzzle box to obtain
a treat (Horner and Whiten 2005). In contrast, chimpanzees given the same task
used goal emulation, imitating only relevant actions (see also Nagell et al. 1993).
There is less unequivocal evidence that chimpanzees engage in imitation, in which
the observer must understand the goal of the model and reproduce important aspects
of the model’s behavior to achieve that goal (see Tomasello and Call 1997; Whiten
et al. 2004).
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An exception to this seems to be for enculturated apes that have demonstrated
imitation both immediately after viewing a model’s behavior (e.g., Tomasello et al.
1993a; Buttelmann et al. 2007) and following a significant delay (e.g., Tomasello
et al. 1993b; Bering et al. 2000; Bjorklund et al. 2002). For example, Buttelmann
et al. (2007) used a procedure similar to that described earlier by Gergely et al.
(2002), in which enculturated chimpanzees observed a human model using an
unusual body part to operate an apparatus (such as using his forehead to turn on a
light), both with his hands free and occupied. Similar to Gergely et al.’s findings
with 14-month-old human infants, the enculturated chimpanzees imitated rationally,
using the unusual body part when the model’s hands were free and using the more
convenient hands when the model’s hands had been occupied.
With respect to teaching, there have been a handful of observations of chimpanzees in the wild, all involving foraging behaviors of mothers with their infants (e.g.,
Boesch 1991; Greenfield et al. 2000). For example, Boesch observed mother
chimpanzees making exaggerated movements while cracking nuts when in the
presence of their infants, which he interpreted as teaching. More recently, Lonsdorf
(2006) observed mother and infant chimpanzees with respect to the activity of
termite fishing and reported that the amount of time mothers spent alone or with
maternal family members (which is highly correlated with time spent on termite
fishing) was related to their offspring’s skill at specific components of termite
fishing, suggesting that the young animals were, indeed, learning these skills
from their mothers. However, she reported no evidence of active facilitation of
the offsprings’ actions by the mothers, suggesting that teaching, though possibly
practiced by some mothers, is not the primary means by which young chimpanzees
learn foraging skills (Bering and Povinelli 2003).
To our knowledge, there is no evidence from controlled studies that either
laboratory-reared or enculturated chimpanzees engage in teaching one another.
There is anecdotal evidence, however, that some enculturated apes may engage in
teaching. For example, Fouts (1997) reported that the language-trained and enculturated chimpanzee, Washoe, taught her adopted son, Loulis, about 50 signs, which
the younger animal used to communicate both with Washoe and with his human
caretakers.
17.5
Theory of Mind
Advanced forms of social cognition in modern humans, such as cheater detection
and the negotiation of social contracts, are argued to have their basis in belief-desire
reasoning (Cosmides 1989). This reasoning involves understanding that one’s
actions and the actions of others are based on what one knows and what one
wants, and can differ across time, and that one’s own knowledge and desires can
differ from others. Possessing the ability to reason in this way is often referred to as
having a theory of mind under the assumption that people develop a theory of how
their own and others’ minds work.
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17.5.1 Development
While the foundations of theory of mind are observed earlier, belief-desire reasoning
is not reliably observed in children younger than 4 years old, as measured by passing
false-belief tasks (see Wellman et al. 2001). The false-belief task is used to measure
children’s awareness that an individual can believe something that is not actually
true. This is assessed in the unexpected-transfer task (Wimmer and Perner 1983),
sometimes referred to as the Sally-Anne task (Baron-Cohen et al. 1985). Children
watch as an item (a cookie, for instance) is hidden in one location. Sally and Anne
also watch, and then Sally leaves the room. Anne then moves the cookie from one
location to another. Children are then asked where Sally will look for the cookie.
Most children under 4 years of age say that Sally will look in the new location, where
it is actually hidden, whereas most children 4 years of age and older understand that
Sally has a false belief and that she will look for it at its original hiding location.
A second frequently used task to assess false belief is the Smarties task, in which
a researcher presents children with a box of Smarties (candy familiar to British
children), for example, and then reveals that it really contains something else,
pennies for instance. Children are considered to pass this task if they understand
that they falsely thought there were Smarties in the box before or that someone else
who has not seen what is in the box will believe there are Smarties in the box, not
pennies (Hogrefe et al. 1986).
Although most 3-year-old children cannot solve false-belief tasks, they have
some understanding that other people have desires and knowledge different from
their own (see Wellman and Liu 2004). For example, in one study, Repacholi and
Gopnik (1997) tested 14- and 18-month-old infants for their preference for two
types of food (Pepperidge Farm Gold-fish and raw vegetables). The infants then
watched as a woman tasted the food, expressing happiness for one type and disgust
for the other. When the woman then requested food from the infants, the 18-month
olds gave the woman the type of food she preferred, regardless of their own
preferences, presumably recognizing that their likes were different from those of
another person. In contrast, the 14-month olds gave the woman the type of food they
liked, regardless of what food the woman liked.
The developmental patterns observed in variants of false-belief tasks have been
found universally (e.g., Avis and Harris 1991; Tardif and Wellman 2000; Sabbagh
2006), although the timetable for passing false-belief tasks may be slower for
children in some cultures than others (see Lillard 1998; Wellman et al. 2006; Liu
et al. 2008 for a discussion of cultural variations in theories of mind).
17.5.2 Parental Effects
While findings are fairly robust regarding the universal developmental trajectory of
theory of mind in humans (e.g., Wellman et al. 2001), individual differences related
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to factors in the child’s environment significantly impact the rate of acquisition
and sophistication of these abilities (Carpendale and Lewis 2004). Among them
are quality of attachment, parenting styles, parent child communication (e.g.,
Carpendale and Lewis 2004), language skills (e.g., Milligan et al. 2007), maternal
warmth, the extent to which mothers use mental state talk (i.e., talking about what
they and their children are thinking) (e.g., Ruffman et al. 2006), and family size and
structure (e.g., Perner et al. 1994; Jenkins and Astington 1996).
For example, Ruffman and colleagues showed that mothers’ frequent use of
mental-state utterances increases children’s likelihood of passing a subsequent
false-belief task (Ruffman et al. 2002; Ruffman et al. 2006). In related work
(Taumoepeau and Ruffman 2006, 2008), mothers’ use of desire versus belief
terms was measured when their children were 15, 24, and 33 months old. The
researchers reported that mothers increased their talk about belief/knowledge
during this time, while talk about desire/emotional states remained the same.
Mothers’ talk about beliefs and knowledge preceded their children’s use of such
terms and was a strong predictor of children’s social understanding at 33 months.
Together, these findings suggest that mothers who, at some level, monitor their
children’s theory-of-mind development and fine-tune their own linguistic behaviors
in response, facilitate the acquisition and sophistication of belief-desire reasoning.
Other research has shown a significant relation between individual differences
in preschool children’s performance on a theory-of-mind scale and performance
of their parents on an adult theory-of-mind task (Sabbagh and Seamans 2008).
Although it is impossible at this point to determine the extent to which this
transgenerational effect is attributed to genetics or to ways in which parents interact
with their children, given the impact of parental behavior on theory-of-mind
development (e.g., Carpendale and Lewis 2004; Ruffman et al. 2006), it would
seem that at least some of this relationship can be attributed to individual differences
in parental behavior.
In addition to parental effects, siblings also seem to influence children’s theoryof-mind development (e.g., Jenkins and Astington 1996; Ruffman et al. 1998).
Specifically, having an older, not a younger, sibling is associated with better theory
of mind (Ruffman et al. 1998). One interpretation of this finding is that competition
with an older sibling hastens the development of theory-of-mind skills in the
younger child (Cummins 1998).
17.5.3
Abilities of Chimpanzees
Chimpanzees and other great apes have been shown to deal in deception on
occasion, for example, leading a more dominant group member to a location
where food is not hidden and then running to the actual source of the food, or
inhibiting a distinctive cry during orgasm while copulating with a favorite female
so as to not have to share the estrous partner with others (see Whiten and Byrne
1988), actions suggestive of theory of mind. In a recent experiment involving
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competition for food between mother-reared chimpanzees and humans, chimpanzees were able to deceive the human by selectively reaching through tunnels to
retrieve food so that the human competitor could either not see the chimpanzee’s
hand or could not hear anything when the tunnel was opened, behaviors indicative
of deception, as well as inferring that eyes (and ears) possess “knowledge” (Melis
et al. 2006). These are sophisticated social behaviors that require substantial control
of one’s actions and the inhibition of prepotent responses, but they do not, in and of
themselves, require an understanding of what another being “knows.”
Other research supporting the idea that chimpanzees understand that the “eyes
have knowledge” has been reported in a food-competition task (Hare et al. 2000,
2001). In these studies, lower-ranking chimpanzees would retrieve food placed in a
room only if it were placed out of sight of a higher-ranking animal. This implies that
chimpanzees understand that a conspecific that is looking at something “sees” that
thing (i.e., has knowledge of it), and they can use this information adaptively, in this
case to retrieve the food (or not). This ability is more limited than it is in humans,
however. For example, in other research, chimpanzees had to make a “begging
response” to one of two human caretakers who stood on either side of a treat placed
on an out-of-reach table in front of the chimpanzee. One caretaker’s eyes were
occluded (e.g., due to wearing a blindfold, having a bucket over her head), whereas
the other’s were not (Povinelli and Eddy 1996; Reaux et al. 1999). The chimpanzees chose randomly, suggesting an ignorance of the role of seeing (and eyes) in
“knowing.”
These various research projects, plus anecdotal reports of impressive socialcognitive performance of chimpanzees (e.g., de Waal 1982; Fouts 1997), suggest
that these animals possess at least the rudiments of theory of mind. However, there
is no evidence that chimpanzees, either mother-reared or enculturated, pass falsebelief tasks, at least under controlled conditions (Call and Tomasello 1999;
Herrmann et al. 2007). Nonetheless, even if the social-cognitive abilities of chimpanzees are less than those of human preschool children (e.g., Herrmann et al.
2007), research cited in this and earlier sections (e.g., Horner and Whiten 2005;
Buttelmann et al. 2007; Warneken et al. 2007) makes it clear that chimpanzees have
some understanding of the psychological states of other individuals. The task of
researchers is to determine which states these are and the extent of their understanding (Tomasello et al. 2003).
17.6
Conclusion
We have included only a partial list of the development of important human socialcognitive abilities. Omitted from the list are autobiographical memory, language,
cognitions associated with attachment, as well as general intelligence, all of which
are influenced by individual differences in parental, mainly maternal, behavior (see
Bjorklund et al. 2009). We hope that our brief description of the development of
several aspects of social cognition makes it clear that parents shape the course of
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social intelligence in their offspring. Parents, however, do not create these tendencies in their children out of whole cloth, but serve as a supportive environment for
the emergence of species-typical and universal patterns of social cognition. Yet,
these abilities are not observed in other species, at least to the extent they are in
humans, and they must have evolved sometime in human phylogeny following the
separation of the line that leads to modern-day humans from those of contemporary
chimpanzees and bonobos. However, as our review of chimpanzee social cognition
indicates, these animals possess substantial social-cognitive competencies, which,
under certain rearing conditions, can be modified to resemble a more H. sapiens
way of thinking. This is strongly suggestive that our common ancestor with
chimpanzees also possessed the neurological plasticity to adapt its behavior and
cognition in response to changes in environmental conditions.
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Chapter 18
Deceit and Self-Deception
Robert Trivers
Abstract Deception is a universal feature of life, at all levels and in all relationships both within species and between species, inside individuals and outside, with
strong effects on both deceiver and deceived. Being detected often results in a sharp
reversal of fortune for the deceiver thereby intensifying selection to deceive
successfully. In encounters between human strangers, nervousness, signs of control
and of cognitive load can all serve as cues of deception but cognitive load appears to
be the most important. Self-deception is defined as hiding true information from the
conscious mind in the unconscious, and is illustrated by classical experimental
work. Selection to deceive can favor self-deception, the better to hide the deception
and separately to reduce its cognitive costs. Four examples are described. There is a
general tendency toward self-inflation in humans, the better to give off a positive
image. Conscious thought suppression, studied via fMRI, shows that one area of the
brain has been coopted to suppress memory formation elsewhere in the brain. When
people reach age 60, they fail to attend to negative social reality, and this old-age
positivity may give immune benefits. Across primates there is a strong positive
association between relative size of the neocortex and frequency of deceptive acts
in nature. If the relationship holds within species, we may expect relatively intelligent humans to be prone to self-deception. There is such a thing as imposed selfdeception, in which we act out the system of self-deception of another. Likewise,
there is parasitized self-deception in which our system of self-deception makes us
more vulnerable to deception by others. Con artists are given as an example. One
could model the evolution of deceit and self-deception as a multiplayer game,
which can then be analyzed mathematically, modeled via computer simulations
or tested experimentally. One promising possibility is a variant of the Ultimatum
Game, in which deception and detection of deception are permitted and given
quantitative values.
R. Trivers
Department of Anthropology, Rutgers University, Rutgers, 131 George St, NJ, USA
e mail: trivers@rci.rutgers.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 18, # Springer Verlag Berlin Heidelberg 2010
373
374
18.1
R. Trivers
Introduction
In our own species, deceit and self-deception are often two sides of the same coin
best seen together. If by deception, we limit ourselves to consciously propagated
deception lies we will miss the larger category of unconscious deception,
including active self-deception. On the other hand, if we look at self-deception
and fail to see its connection to deception, we will miss its major function. The
central claim is that self-deception evolves in the service of deception (1) the better
to avoid its detection and (2) to reduce the immediate cognitive costs. In the first
case, the self-deceived person fails to give off the cues that go with consciously
mediated deception, thus escaping detection. In the second, the actual process of
deception is rendered less expensive by keeping part of the truth in the unconscious
mind. But this turns out to be a very complicated matter. Suppressing the truth may
have short-term cognitive benefits as well as almost immediate immunological
costs, or for that matter, longer-term cognitive ones. In addition, deception and
self-deception are also being directed at you by others, so that there is such a thing
as imposed self-deception, in which you act out the system of self-deception of
another person, presumably often to your own detriment. In this account, we will try
to fuse the two topics of deceit and self-deception into a single coherent account,
with special attention to the associated costs and benefits, that is, with their selective
effects.
18.2
Deception is Everywhere, at All Levels of Life
Deception (misleading others) is a very deep feature of life. It occurs at all levels
and, it would seem, by any means possible. It tends to hide from view and is, by its
nature, difficult to see and to study. Self-deception is even worse, hiding itself more
deeply in our own unconscious minds. It is, thus, important to build up our logic
carefully, with a full view of the enormous variation that has evolved for various
reasons (Trivers 2000). Unfortunately, there has been very little formal theory on
the subject, so I will limit myself to making a series of general comments, with
special focus on cognitive load and self-deception.
When I say that deception occurs at all levels of life, I mean that viruses practice
it, bacteria do, so do plants, and so do the insects preying on plants, and a wide
range of other animals. It is everywhere. Even within our genomes deception may
flourish, as selfish genetic elements use deceptive molecular techniques to overreproduce at the expense of the larger genome (Burt and Trivers 2006). Or, when a
selfish paternal orientation collides with an oppositely oriented maternal one
(Haig 2002). Deception infects all the fundamental relationships in life, parasite
and host, predator and prey, plant and animal, male and female, neighbor and
neighbor, parent and offspring (including mother and fetus), and even the relationship of an organism to itself.
18
Deceipt and Self Deception
375
Viruses and bacteria actively deceive to gain entry in to their hosts, by mimicking body parts for example. Or, as in HIV, the virus deceives by changing coat
proteins so often as to make mounting an enduring defense almost impossible.
Predators gain from being invisible to their prey or resembling items attractive to
them while prey gain by being invisible to their predators or mimicking items
noxious to them, e.g., poisonous species or their predator’s predator.
Deception within species requires only imperfect (<1) degrees of relatedness
between individuals (so that their self-interests are not identical) and imperfect
information (so that the other party can be fooled). Both are easily satisfied. Clonal
species are rare in nature and imperfect degrees of relatedness the rule (under outbreeding, 1/2, 1/4 down to near-zero). Perfect information is impossible. Deception
can allow you to steal or induce the transfer of food and other resources, engage in
extra-pair copulations undetected, manipulate your parents, your mate, your offspring, your neighbors even the maternal (or paternal) half of yourself.
And deception always takes the lead, while detection of deception plays catchup. As has been said regarding rumor, the lie is halfway around the world, before
the truth goes to work. When a new deception shows up, it starts rare in a world that
lacks a proper defense. As it increases in frequency, it selects for such defenses in
the victim, so that eventually its spread will be halted by the appearance and spread
of countermoves, but new defenses can always be bypassed and new tricks
invented.
The adaptive potential of deception is chronically overlooked by those with an
attachment to the truth. Bill Gates told the world confidently, in 2004, that the
problem of spam “would be solved by 2006” (N.Y. Times 12/06/06). He saw that
defenses could easily be erected against the set of spamming devices then in use but
he could not imagine that these defenses could easily be bypassed while yet newer
forms of spamming were continually being invented. Spam is now at least ten-fold
as frequent as it was in 2004, and correspondingly costly. One inevitable cost is the
destruction of true information by spam-detectors too stringently set, including by
oneself. This is a universal problem in animal discrimination. Greater powers of
discrimination will inevitably increase the so-called false negatives, rejecting
something as false which is, in fact, true.
It always amuses me to hear economists saying that the costs of deceptive
excesses in our economy (so-called “white” crime robbery) will be naturally
checked by “market forces.” Are these the same forces that force us to add one
unit to every price we see in order to know the true price? Why should the human
species be immune to the general rule that where selection is strong, deception can
be generated that extracts a substantial net cost every generation. Consider the
following.
Deception is such an important feature of life that it can entrain the evolution of
entire groups of organisms, as well as the evolution of specialized deceitful morphs
within species. For example, the Phasmatodea, or stick insects, is a group that has
given itself over to the imitation of either sticks (~3,000 species) or leaves (~30
species) (Markle 2007). In the case of sticks, there is apparently a tremendous
evolutionary pressure to produce a long, thin (stick-like) body, even if this forces
376
R. Trivers
the individual to forego the benefits of bilateral symmetry. Thus, to fit the internal
organs into a diminishing space, one of two organs have often been sacrificed, only
one kidney, one ovary, one testis, etc. This shows that selection for successful
deception has been powerful enough not only to remold the external shape of the
creature but also to remold its internal organs as well even when this is otherwise
disadvantageous to the larger organism, as loss of symmetry, in principle, must
usually be.
Likewise, selection for deception has been strong enough to mold morphs that
are obligately committed to deception; that is, morphological forms whose strategy
depends entirely on deception of others. A classic example occurs in the blue-gill
sunfish, where a specialized male form has evolved that mimics a female in
appearance and behavior, being 1/6th the size of a territorial male and roughly
the size of an actual female (for a recent reference see Stoltz and Neff 2006). This
female-mimic seeks out a territorial male, permits himself to be courted, and
responds enough to keep the other male interested, so that when a true female
spawns the pseudo-female is ready nearby, along with the territorial male, to
fertilize the eggs. It is as if the territorial male imagines he is in bed with two
females when, in fact, he is in bed with one female and one male. The two kinds of
males appear to be distinct forms that never turn into each other. To have persisted
for so long their long-term reproductive success must be identical; that is, the
deceiver is doing exactly as good as the deceived and this equality must, in
turn, have been produced by frequency-dependent selection. That is, when the
female-mimic is relatively rare, he will do relatively well; when common, less so.
18.3
Detection of Deception Often Leads to Negative
Consequences, Including Punishment
In our own species, we hardly need convincing that when our deception is detected,
we may receive some harsh feedback, a beating, a public humiliation, a lost love, or
at the very least, a withdrawal of some trust and affection. This appears to be true of
many other animals. That is, in insects and birds, deceivers are often punished
aggressively, and most often by those whose status is being threatened by the
deceiver (for some examples see Trivers 1985; Møller 1987; Hauser 1992; Tibbetts
and Dale 2004). If we take it as generally true that detection of deception often leads
to an unfortunate reversal in fortune, then a deceiver will be under pressure when
deceiving and these signs of pressure are available for detection by others. Indeed,
the lie detector test is based on this fact.
But here a caution is worth mentioning. The lie detector test continuously
measures a series of physiological parameters in response to a series of posed
questions designed to detect unusual signs of stress in particular places. For example, the most useful device is the “guilty knowledge” test, which suddenly introduces some pertinent fact known only to the guilty (was the gun red?). Here, any
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deviation from the norm high reactivity or suppressed reactivity shows guilty
knowledge.
Daily life is very different from this. At one extreme, such as intimate relations
between husband and wife, or partner and partner, each may indeed have a detailed
behavioral template of the other, honest and dishonest, against which to evaluate
the ongoing behavior, while at the other extreme, perfect strangers are interacting
with no prior knowledge of each other.
18.4
Cognitive Load is a Key Factor in the Detection
of Deception in Humans
In anonymous or infrequent interactions, behavioral cues can not be read against a
background of known behavior, so more general attributes of lying must be used.
Three such attributes have been emphasized (Vrij 2004):
1. Nervousness: because of the negative consequences of being detected, including
being aggressed against, nervousness can reveal deception (DePaulo et al. 2003)
2. Control: in response to concern over appearing nervous (or concentrating too
hard), people exert control, trying to suppress both, but this requires additional
effort, with possible side effects, and there is the danger of over-acting, overcontrol, a planned or rehearsed impression
3. Cognitive load: lying is cognitively demanding: you must suppress the truth and
construct a falsehood, one that will not contradict anything known by the
listener, or likely to be known, you must tell it in a convincing manner and in
such a way that you can remember the story. This takes time and concentration,
both of which may give off secondary cues.
The most recent work suggests that cognitive load is the critical variable among
the three, with a minor role for control and very little for nervousness. At least this
seems to be true in real criminal investigations as well as experimental situations
designed to mimic them (Mann and Vrij 2006). People who are lying have to think
too hard and this causes several effects, some of which are opposite to those of
nervousness.
Consider, for example, blinking. We blink our eyes more often when nervous,
but we blink them less under increasing cognitive load (e.g., while solving arithmetic problems). Recent studies of deception suggest that we blink less when deceiving, that is, cognitive load rules (Vrij 2004; Vrij et al. 2006). Nervousness makes us
fidget more but cognitive load has the opposite effect. Again, contra usual expectation, people fidget less in certain deceptive situations. Again consistent with
cognitive load effects, men use less hand gestures while deceiving and both sexes
show longer pauses while they speak.
The effects of control are illustrated by pitch of voice and (separately) displacement activities. Deceivers tend to have higher-pitched voices (DePaulo et al. 2003).
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This is a natural consequence of any effort to suppress behavior by becoming more
rigid. Tensing up the body inevitably raises the pitch of voice.
Another effect of suppression is the production of displacement activities. As
classically described in other animals, these are irrelevant activities often seen
when two opposing motivations are simultaneously aroused. Since neither impulse
can express itself, the blocked energy easily activates irrelevant behavior, such as a
twitch. For this reason, displacement activities in primates are a reliable indicator of
stress (Troisi 2002).
Nervousness is almost universally cited as a factor associated with deception both
by those trying to detect deception as well as by those trying to avoid detection, yet
surprisingly enough, it is one of the weaker factors predicting deception. This is,
perhaps, precisely because we are conscious of our nervousness so that mechanisms
of suppression may be almost as well developed as the nervousness itself. The point
about cognitive load is that there is no escape. If it is cognitively expensive to lie,
there is no obvious way to reduce the expense. Mechanisms of denial and repression
may serve to reduce immediate cost, but alas, as we shall see, with ramifying costs
later on.
18.5
What is Self-Deception?
Before going any further it would be useful to define self-deception, and give an
example. Some people, especially philosophers, imagine that self-deception is a
contradiction in terms: how can the self deceive itself, does that not require that it
knows what it does not know? But the contradiction is easily resolved by defining
the self as the conscious mind, so that self-deception occurs when the self is being
kept in the dark, when the larger organism preferentially keeps true information out
of consciousness, and when it misleads the conscious mind. Sometimes, this may
involve activities of the conscious mind itself, which arrange e.g., via active
memory suppression for later mental states to be biased in a particular way but
usually the processes are unconscious and bias conscious mentation in a great
variety of ways (see Gilbert 2006). So the key to defining self-deception is that
true information is preferentially excluded from consciousness and, if held at all, is
held in (varying degrees of) unconsciousness.
A very dramatic example of self-deception was demonstrated experimentally
almost 30 years ago, with true and false information simultaneously stored within
individuals but showing a strong bias toward the true information hidden in the
unconscious mind. Gur and Sackeim (1979) also showed that they could manipulate
an individual’s self-deception so defined by the simple device of increasing or
decreasing the individuals’ opinion of themselves.
The experiment was based on a simple fact of human biology: we are physiologically aroused by the sound of the human voice but more so to the sound of our
own voice (as reproduced, for example, on a tape recorder). We are unconscious of
these effects. Thus, you can play a game of self-recognition, in which people are
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asked whether a voice is their own or not (conscious self-recognition) at the same
time recording whether unconscious self-recognition was achieved, via higher
arousal.
Here is how it worked. People were matched for age and sex and asked to read
the same paragraph from Thomas Kuhn’s “Structure of Scientific Revolutions,”
then these recordings were chopped into 2, 4, 6, 12, and 24 s segments, and a master
tape was created consisting of a mixture of own and other voices. Meantime, the
individual is hooked up to a machine measuring his or her galvanic skin response
(GSR), which is normally twice as great for hearing one’s own voice as hearing
someone else’s. People are asked to press a button to indicate if they think the
recording is of self and another button to indicate how sure they are.
Several interesting things were discovered. First, some people denied their own
voices some of the time, this was the only kind of mistake they made, and they
seemed to be unconscious of making it (when interviewed later, only one was
aware of having made this kind of mistake), yet the skin had it correct, that is,
showed the large increase in GSR expected upon hearing one’s own voice. By
contrast, another set of people heard themselves talking when they were not they
projected their voice and half were aware later that they had sometimes made
this kind of mistake, but the skin once again had it correct. There were two other
categories, those who never made mistakes and those who made both kinds,
sometimes even fooling their skin; but in what follows, we neglected these two
categories.
This is unconscious self-recognition shown to be superior to conscious recognition, but Gur and Sackheim also showed that one could affect the tendency toward
self-denial or projection by manipulating the person’s opinion of himself/herself.
Made to feel bad by a poor score (in fact, randomly administered) on a pseudoexam, individuals started to deny their voices more often. Made to feel good by a
good score, individuals started to project their voices. It was as if self-presentation
was expanding under success and contracting in response to failure.
Another interesting feature never analyzed statistically was that deniers also
showed the highest GSRs to all stimuli. It was as if they were primed to respond
quickly, to deny the reality and get it out of sight, while inventing the reality
(projecting) seemed a more relaxed enterprise. Perhaps, reality that needs to be
denied is more threatening than is the absence of reality one wishes to construct
or, in any case, denial can be dealt with more quickly.
Although this example is nice in showing clear, strong effects, it is not the only
paradigm for self-deception. It is not necessary, for example, for the true information to be simultaneously stored. We act very early in information-generating
processes to produce biased results. For example, we will choose not to read factual
articles on a position we oppose (e.g., that marijuana consumption has negative
personal effects) but to read those consistent with our views (e.g., marijuana
consumption is beneficial). Somewhere in our body may be stored facts regarding
preferential attention but not the content of the missed articles.
Alas! except for a couple of trivial articles quibbling with aspects of Gur and
Sackeim’s work (e.g., Douglas and Gibbons 1983), no follow-up work has
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appeared. How do those who make no mistakes differ from those who deny or
project? And why do some individuals make both kinds of errors? Are deniers really
more aroused, in general, than the other categories of people? And which voices of
one’s own do we deny and which expropriate from others, and why? Unfortunately,
we have no answers to these or a host of related questions. In part, this reflects
the difficulty of the work itself, very painstaking and demanding, especially, in the
pre-computer era, but mostly it reflects the degree to which self-deception has not
been seen as a major subject in psychology, for which this was a significant
methodological breakthrough. In any case, it is a shame: Gur and Sackeim created
a whole new line of work which then failed to develop. The fact that is has failed to
develop is cited as evidence against the original work. One prominent social
psychologist told me that the results were so uncertain in his field that unless
there was at least one replication, no one paid attention to solitary results.
18.6
One Needs a Separate Theory of Reality
Since deceit and self-deception must always be judged against the truth, one needs a
separate theory of reality that is reliable against which to test self-deception.
Evolutionary theory, of course, pretends to provide exactly that. The dangers of a
false theory of reality are illustrated by the failure of both Freud and Marx’s theories
of self-deception. Although Freud was able to describe such phenomena as denial,
repression, and projection, his own unfounded theory of human development led
him to deny a thing and project the other. For example, he denied that sexual
advances from male relatives or step-relatives were a common problem for females,
and projected on these women, the desire for exactly such encounters. An evolutionary approach is not congenial to the notion that women should have an inborn
desire for sexual congress with close male relatives. Quite the contrary, but with a
critical asymmetry, males are more likely to benefit genetically from such
inbreeding than are the investing females. Here Freud’s absence of any plausible
view of human development allowed him to twist his self-deception argument any
which way. Marx, in turn, provided an analysis of bourgeois deceit and selfdeception but his naı̈ve theory of inevitable economic evolution only encouraged
socialist self-deception.
18.7
Is Self-Deception the Psyche’s Immune System?
The immune analogy is very popular within psychology. The argument goes as
follows. Just as our body is under constant threat from parasites, so is our psyche
under threat from factors that reduce happiness. Hence, we have psychological
defense mechanisms, just as we have immunological ones, the one to keep us
healthy and disease-free, the other to keep us happy. In one formulation, people
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are seen as having a “psychological immune system that defends the mind against
unhappiness in much the same way that the physical immune system defends the
body against illness” (Gilbert 2006).
This is said to be an “unusually appropriate” analogy because immune response
and the degree of psychological defensiveness both share the trait that too little is
bad but so is too much. Alas, this is true of all biological systems. Otherwise, we
would not have stable phenotypes. Too little oxygen is bad, and so is too much. Too
little food is bad as is too much. And right down the line, height, weight, salt,
water, curvature of your left thumb, tendency to visit dentists, everything is bad in
its extremes.
What then is the image of how this immune-like defensive system works? “We
need to be defended not defenseless or defensive and thus our minds naturally
look for the best view of things while simultaneously insisting that those views stick
reasonably closely to the facts” (Gilbert 2006).
“Reasonability” is the operative word here, undefined and as elastic as you could
want. We are seen as keeping ourselves happy in good part via self-deception:
denial, projection, disassociation, and so on. We cook the facts, we bias the logic,
we overlook the alternatives
in short, we lie to ourselves. Meanwhile, we
apparently have a “reasonability center” that, by unknown criteria, determines
just how far we will be permitted to protect our happiness via self-deception
(without, for example, looking ridiculous to others). Why was evolution unable to
produce a more sensible way of regulating such an important emotion as happiness?
Something more reasonable?
Contrast the real immune system. It deals with a major problem common to all of
life, that of parasites, organisms that eat us out from the inside. Parasites are often
the major selection pressure every generation on their hosts, much stronger than
that of predators, for example. The immune system uses a variety of direct realitybased molecular mechanisms to attack, disable, engulf, and kill a veritable zoo of
invading organisms thousands of species of viruses, bacteria, fungi, protozoa, and
worms themselves using techniques honed over hundreds of millions years of
intense natural selection. The immune system also stores away an accurate and
large library of previous attacks, with the appropriate counter-response now preprogrammed in advance.
The vertebrate immune system is at least 300 million years old and is extremely
sophisticated (similar evidence is now emerging for the insect immune system).
Several dozen cell types are produced and marshaled in a bewildering array of
patterns by a wide range of neurochemicals. The system is also very costly, perhaps
on the order of the brain itself. It generates several grapefruit volumes of new tissue
every 2 weeks. When unnecessarily aroused through false antigens (e.g., sheep red
blood cells), immune arousal compromises survival in nature, sometimes strongly.
Because it is so costly, it also acts as a great reservoir of energy against which the
larger system can borrow in times of need; for example, effects of stress and sex
steroids act to depress immune function, presumably the energy is spent on more
immediately pressing problems (whatever is causing the stress or sexual and
aggressive opportunities).
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What on earth does all of this have to do with the ego’s need to defend itself
against various “threats”? And what exactly are these a threat to? One’s selfopinion? Why is that a matter of pressing survival value? Parasites threaten your
life. And why adopt something as dubious as self-deception to solve this problem?
Put another way, where does our immune system improve its function by lying to
itself why is there no analogy on this key point? Granted the selection pressures
associated with social interactions have steadily increased in our lineage perhaps,
exploding with language but self-image, self-esteem, and ego strength are surely a
small part of all of this.
Is it possible that psychology has gotten all of this terribly wrong for so long by
simply taking seriously an inside outside approach to life introspection will show
us the way in which we, in effect, choose our self-deceptions as building blocks in
our theory? Social psychology has wedded itself to a thoroughly defensive view of
self-deception, one that is itself congenial to an inflated self-perception: I am not
lying to myself the better to deceive you but rather I lie to myself to defend myself
from attacks on my personal integrity, my very happiness.
18.8
Self-Deception Helps Fool Others, While Reducing
the Cognitive Cost of Doing So
Imagine two animals squaring off in a physical conflict. Each is assessing its
opponent’s self-confidence along with its own variables expected to predict the
outcome some of the time. It is easy to imagine that biased information flow within
the individual can facilitate false self-confidence, which some of the time will pay
for itself by fooling the opponent. Nonverbal self-deception can be selected in
aggressive and competitive situations, the better to fool antagonists. Much the same
could be said for male/female courtship relations. A male’s false self-confidence
may give him a boost some of the time. A biased mental representation can be
produced, by assumption, without language.
The above is meant to demonstrate that in at least two contexts aggressive
conflict and courtship selection for deception may easily have favored selfdeception; that is, biased information flow within an organism to its consciousness,
even when no language is involved. There are undoubtedly many other such
contexts, for example, parent/offspring. But the simple fact is that language must
have greatly expanded the opportunities for deceit and self-deception. If one great
virtue of language is its ability to make true statements about events distant in space
and time, then surely one of its social drawbacks is its ability to make similarly
distant false statements, so less easily contradicted than statements about the
immediate world. Once you have language, you have an explicit theory of self,
social relationships and the world, ready to communicate to others. Numbers of new
true assertions possible are matched by an infinitely greater number of false
assertions, and so on.
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A variety of ways in which self-deception helps deceive others has been described and many more await their day (reviewed briefly in Trivers 2000). What is
also probably true is that self-deception reduces the immediate cognitive costs of
deception. This alone should help hide the deception but it should also serve to
permit the ongoing cognitive activities that would otherwise be blocked by a fully
conscious effort to lie. The literature on cognitive dissonance suggests that conscious contradictions are costly and the mind acts to reduce dissonance in a variety
of ways (Tavris and Aronson 2007). Yet exploring this point further is very
difficult. We know that the brain consumes 20% of resting metabolic rate no matter
what (Clark and Sokoloff 1999). With the exception of a slight increase during
REM sleep, it does not increase under any known conditions, including hard mental
work, schizophrenia, or an LSD trip. It does not decrease under depression or under
any other state, except for a slight diminution in non-REM sleep. But the brain and
body can borrow from the immune system and do so under a variety of conditions
(e.g., stress, sex hormone increases, and thought suppression).
No one has been able to compare the cognitive costs of conscious deception with
the same act performed unconsciously. We know that when white Americans with a
strong implicit bias against black people (mostly unconscious, see below) are
forced to interact briefly and pleasantly with a black experimenter, their performance on a subsequent Stroop test is compromised apparently because of increased
cognitive load (Richeson and Shelton 2003). In the Stroop test, the subject must
give the color in which a word denoting a color is written. Thus, the word “red” may
be written in green ink, and the subject must say “green.” Since one’s first impulse
is to read the word, this must be suppressed to give the correct answer. While
viewing black faces, they also show activation of brain regions associated with
cognitive control as if already preparing a public face (Gehring et al. 2003;
Richeson et al. 2003). What we do not know, of course, is how conscious the
various people are of their own negative feelings and how differences in degree of
consciousness may be associated with variation in either cognitive control or
cognitive load.
18.9
Four Examples of Self-Deception
Although we have few examples of self-deception with the simple clarity and
power of Gur and Sackeim (1979), there is, in fact, an enormous literature on the
subject, crossing several disciplines including social psychology, behavioral
economics, animal behavior, neurophysiology, immunology, and the study of
everyday life. I have chosen four examples. First is self-inflation, often measured
verbally but now with new techniques at a deeper level. It appears to be very
general in human life. Second is the neurophysiology of thought suppression,
which combines spatial brain coordinates of ongoing mental activity (fMRI) with
measures of success at suppressing material from consciousness, an activity known
also to have immediate immune costs (Pennebaker 1997). Third is old-age positivity,
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R. Trivers
a series of cognitive biases that creep in by age 60 so as to give a rosier view of the
social world than is warranted. Does this produce immune benefits, especially
useful in later life? Finally, the study of monkey and ape behavior in the wild
suggests that deceptive behavior is more common per unit time in species with
relatively large neocortexes (social brains). If this is true within species, we might
expect brighter people to be, on average, more self-deceived, a possibility with
some serious social implications.
18.10
Self-Inflation is the Rule in Life
Animal self-inflation routinely occurs in aggressive situations (size, confidence,
color) as well as during courtship (same variables). Self-inflation is the dominant
style in human psychological life (Greenwald 1980; Gilbert 2006), adaptive selfdiminution appearing as an occasional variant (Hartung 1988). People routinely put
themselves in the top half of positive distributions and the lower half of negative
ones. Eighty percent of U.S. high school students place themselves in the top half of
students in leadership ability, but, of course, for extreme examples of self-deception
you can hardly beat academics: 94% of them in one survey place themselves in the
top half of their profession. I plead guilty. Even when tied to a bed in some back
mental ward of a hospital, I still believe I am performing better than half of my
colleagues and this is not only a comment on Rutgers University.
Subtler linguistic features of self-deception have been described. When describing a positive group effect, we adopt an active voice but when the effect is negative,
we unconsciously shift to a passive voice: this happened and then that happened and
then costs rained down on all of us (reviewed in Greenwald 1980). When in-group
members do something positive, we tend to make a general statement, “she is a good
person,” just as we do when out-group members do something negative “he is
wicked” but when an in-group member does something bad, we tend to describe it
precisely, “she stepped on my toes,” just as we do when an out-group member does
something good, “she gave me directions to the Bahnhof” (Maass 1999).
A recent methodology permits a very striking result (Epley and Whitchurch
2008). With the help of a computer, individual photos were morphed either 20%
toward attractive faces or 20% toward unattractive. Among other striking effects,
when the individual tries quickly to locate his or her real face, its 20% positive or its
20% negative in a background sea of 11 faces of other people, people were quickest
to spot the positive face (1.86 s), less so for the real face (2.08 s), and slowest for the
ugly one (2.16 s). On average, increasing degree of attractiveness improved the
speed of perception by 1/10th of a second out of 2 seconds. The beauty of this is that
there has not been the usual verbal filter what do you think of yourself? but only
a measure of speed of perception.
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18.11
385
The Neurophysiology of Thought Suppression
One particular kind of self-deception consciously mediated efforts at suppressing
true information from consciousness has been studied neurophysiologically in a
most revealing way (Anderson et al. 2004). Different sections of the brain may have
been coopted in evolution to suppress the activity of other sections in order to create
self-deceptive thinking.
Consider, for example, the active conscious suppression of memory. At times,
we actively attempt to suppress thoughts. In the laboratory, individuals are
instructed to forget an arbitrary set of symbols that they have just learned. The
effect of such efforts is highly variable, measured as the degree of forgetting
achieved a month later and this variation is associated with variation in the
underlying neurophysiology. The more highly the dorsolateral prefrontal area is
activated during directed forgetting, the more it suppresses the ongoing activity in
the hippocampus (where memories are typically stored) and the less is remembered
a month later. The dorsolateral prefrontal area is otherwise often involved in
overcoming cognitive obstacles and in prepotent motor activity, that is, preparing
for physical activity (muscle movement). It is tempting to speculate that this area of
the brain was coopted for the new function of suppressing memories because it was
often involved in affecting other brain areas, i.e., activating behavior. It may be
unrelated but whenever I experience an unwanted thought and act at once to
suppress it, I often experience an involuntary twitch in my arms, as if trying to
push something down (and out of sight).
18.12
Old-Age Positivity and Immune Function
There is a striking bias toward positive social memories and perceptions that sets in
by age 60 (and perhaps somewhat earlier). At ages 20 30, the human shows no
tendency to remember faces with positive expressions more often than those with
negative ones, or to spend more time examining such pictures. But by age 60, a bias
is apparent: positive faces are remembered more readily and they are attended to
more carefully (Mather and Carstensen 2005). When a dot is presented on the side
of a screen at which a positive face is presented, the dot is perceived more quickly if
it succeeds the positive face (and less slowly if it succeeds a negative one, compared
with neutral Mather and Carstensen 2003). This involves a measurable effect in the
amygdala, where positive faces evoke a stronger response than negative in older
people but not in younger people (Mather et al. 2004). Older people, compared to
younger, are more likely to respond to a musically induced negative mood by
preferentially looking at positive faces, as if actively inducing a positive mood
(Isaakowitz et al. 2008).
Why show such a positivity bias? Half the problem is trivial. Young people will
be wise to pay attention to reality, both positive and negative, the better to make the
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R. Trivers
appropriate responses later; by old age, it hardly matters what you learn and since
greater positive affect is associated with stronger immune response (Rosenkranz
et al. 2003), you may trade grasp of reality for a boost in dealing with your main
problem, that of parasites and cancer. A positivity bias sacrifices learning in the
future concerning negative outcomes the better to enjoy strong immune function
now. Grandchildren may admire gramps and grandma because nothing seems to
faze them, but gramps and grandma might be living in a positivity-enveloped world,
the better to deal with their internal enemies.
It is an interesting coincidence that although our implicit bias in favor of youth
over old age hardly changes with age from 20 to 70, we favor young over old by
our 40s, our explicit bias in favor of youth declines until at exactly 60 we start to
prefer older to younger like everyone else we implicitly associate youth with
positive features, but we start preaching the opposite at the same time at which we
ourselves display the old-age positivity bias (Nosek et al. 2002).
18.13
Are Intelligence and Self-Deception Correlated?
It is easy to imagine that intelligence and consciousness are two independent axes
of human behavior, perhaps equally important, but uncorrelated. Thus, one can be
bright and deluded or slow and honest with all combinations equally likely.
Likewise, it is easy to imagine that the two axes are positively correlated. The
smarter you are the less self-deception (greater consciousness). Your innate superiority in intellectual power can easily be turned back on your deceptive tendencies,
so that you see through your lies and adjust appropriately. But what does that
mean you lie less or lie more?
I do not believe that degree of self-deception and intelligence are uncorrelated or
that they are negatively correlated. I believe quite the opposite. Degree of consciousness and intelligence are positively correlated: brighter people are more
likely to act deceptively and to practice self-deception. This increases the chance
that the net effect of their actions will be negative instead of positive. This is, to put
it mildly, an underemphasized underbelly of high intelligence. Of course, there are
exceptions. It is not surprising that the academically less gifted are more likely to
cheat (and thus act deceitfully) as indeed they are.
One line of evidence comes from monkeys and apes. The size of the neocortex
or better still, its relative portion of total brain size is positively associated with
use of tactical deception in nature, where tactical deception includes any kind of
deception that can be seen to give an advantage. A large list of appropriate acts from
nature was assembled from the primary scientific literature and used to solicit a still
larger sample from active scientists. Study effort and group size were controlled as
were taxonomic effects. Conclusion: since neocortex size is correlated with intelligence including social intelligence across a broad range of monkeys and apes,
we know that deception occurs more often the smarter the species is. So, perhaps,
does self-deception.
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Another line of evidence comes from children. As children mature, they become
increasingly intelligent and increasingly deceptive. This is not an accident. The
very maturing capacity that gives them greater general intelligence also gives them
greater ability to suppress their behavior and create a novel behavior. There is also a
clear evidence that natural variation in intelligence, corrected for age, is positively
correlated with deception in children (Lewis unpubl. data) using the peek/no peek
lie/not lie paradigm that has been used to such good effect (e.g., Crossman and
Lewis 2006). Even health of the child at birth (as measured by a weighted sum of 32
perinatal factors) is positively correlated with lying.
Thus, if you wish to cherish a self-image that you are smarter than average or
even that your group is, you may also need to imagine that you (and the group) are
more prone to deceit and self-deception, with net effect on others uncertain.
18.14
Imposed Self-Deception
So far we have spoken of self-deception evolving in the service of the actor. This is
the natural first step, but we are also highly sensitive to others, to their opinions,
desires, actions, and so on. More to the point, we can be manipulated and dominated
by them. This can result in self-deception being imposed on us by others (with
varying degrees of force). Extreme examples are instructive. A captive may come
to identify with his captor, an abused wife may take on the world-view of her
abuser, molested children may blame themselves for the transgressions against
them and the resulting misery. These are cases of imposed self-deception and if
they are acting functionally from the standpoint of the victimized (by no means
certain), then they probably do so by reducing conflict with the dominant individual.
At least this is often the theory of the participants themselves: an abused wife may
be deeply frightened and rationalizes acquiescence as the path least likely to
provoke additional severe assaults, this is soon most effective if actually believed.
Let us consider another example of imposed self-deception, one with deeper
social implications. It is possible to measure something called a person’s “explicit”
self-preference as well as an “implicit” one. The explicit simply asks people to state
their preferences directly, e.g., for the so-called “black” people over “white” (to use
the degraded language of the United States) where the actor is one or the other. The
implicit measure is more subtle. It asks people to push a right-hand button for black
or “good” stimuli (e.g., positive words) and left for white or bad ones and then
reverses everything, black or bad, white or good. We now look at latencies how
long does it take an individual to respond when they must punch white or bad versus
white or good and assume that shorter latencies (quicker responses) means the
terms are, by implication, more strongly associated in the brain. Hence, the term
“implicit association test” (IAT), invented only in 1998 (Greenwald et al.), has now
generated an enormous literature, including (unusual for the social sciences) actual
improvements in methodology (Greenwald et al. 2003). There are several websites
that harvest enormous volumes of data over the internet (e.g., at Harvard, Yale, and
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R. Trivers
the University of Washington), and these studies have produced some striking
findings (Nosek et al. 2002).
For example, black and white people were similar in their explicit tendency to
value self over other, blacks if anything more strongly so, but when it came to the
implicit measures, whites were even more strongly in their own favor than they
were explicitly, while blacks on average preferred white over black, not by a
huge margin but, nevertheless, they preferred other to self (Nosek et al. 2002). This
is most unexpected according to evolutionary theory, where self is the beginning (if
not end) of self-interest.
This has the earmarks of an imposed self-deception valuing yourself less than
you do others and it may come with some negative consequences. For example,
priming black students for their ethnicity strongly impairs performance. This was
indeed one of the first of what are now hundreds of demonstrations of this
“priming” effect. Black and white undergraduates at Stanford arrived in a lab to
take relatively difficult aptitude tests. In one situation, the students were simply
given the exams; in the other, each was asked to give a few personal facts, one of
which was their own ethnicity. Black and white scored equally well with no prime.
With a prime, whites did slightly (but not significantly) better while blacks’ scores
plummeted by nearly 1/2. You can even manipulate one person’s performance in
opposite directions by giving opposing primes: Asian women perform better on
math tests when primed with “Asian” and worse when primed with “woman.” No
one knows how long the effect of such primes endures but nor does anyone know
how often a prime appears: how often is an African-American reminded that he or
she is such? Once a day? Every half hour? Once a month? I think the number is
somewhere between the second and the third.
The strong suggestions then is that it is possible for a historically degraded and/
or despised minority group, now subordinate, to have an implicit self-image that is
negative, to prefer other to self indeed, oppressor to self and to under-perform as
soon as made conscious of the subordinate identity. This suggests the power of
imposed or induced self-deception some or, indeed, many subordinate individuals
adopting the dominant stereotype regarding themselves. Not all of course, and the
latter presumably more likely to oppose their subjugation since they are conscious
of it. In any case, revolutionary moments often seem to occur in history when large
numbers of individuals have a change in consciousness regarding themselves and
their status. Whether there is an accompanying change in IAT is another matter.
One more form of induced self-deception is worth mentioning. It is surprisingly
easy to convince people to make false confessions to major crimes even though this
may and surprisingly often does result in incarceration for long periods of time.
All that is required is a susceptible victim and good old-fashioned police work
applied 24/7: isolation of the victim from others, sleep deprivation, coercive interrogation in which denial and refutation are not permitted, false facts provided and
hypothetical stories told (we have your blood on the murder weapon, perhaps you
woke in a state of semi-consciousness and killed your parents without intending or
being aware of it etc., with the implication that a confession will end their interrogation). People vary in in the susceptibility range they are to these pressures and
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in how much self-deception is eventually induced. Some go on to create false
memories to back up their false confessions (Kassin 2008).
There is also a kind of self-deception that could be called imposed self-deception,
but which could also be considered defensive self-deception. Consider an individual
being tortured. The pain can be so great that something called “disassociation” may
occur: the pain is separated off from other mental systems, presumably so as to
reduce its intensity. As if the psyche or nervous system protects itself from severe
pain by objectifying it, distancing it, and splitting it off from the rest of the system.
One can think of this as being imposed by the torturer but also as a defensive
reaction permitting immediate survival under most unfavorable circumstances. We
know from many personal accounts that this is but a temporary solution and that
the torture itself and utter helplessness against it endure as psychological and
biological costs. There are, of course, more modest forms of “disassociation”
from pain than that of torture, such as a mother distracting her child by tickling it.
18.15
Deceit and Self-Deception Seen as an Evolutionary
Game
To model the evolution of deceit and self-deception in humans more exactly, I
recommend pursuing the strategy that has proven so successful with reciprocal
altruism and cooperation. That is, model opposing strategies as simple rules with
specified costs and benefits in interaction with each other. This was first applied
successfully to the evolution of reciprocal altruism by Axelrod and Hamilton
(1981) modeled as a successive series of simultaneous moves with only two options
for each of the two players, cooperate or defect, each combination with specified
pay-offs (the so-called iterated Prisoner’s Dilemma).
It was shown that a very simple strategy tit-for-tat beat out all others in
computer tournaments and is a fairly robust strategy in evolutionary games: be
cooperative on the first move, and imitate your partner’s previous move on your
next one. In short, reward cooperation with cooperation and punish defection with
defection. An advantage of casting the problem in terms of games is that these can
be played both mathematically and for real. When their patterns are convergent as
they are here, we can have even greater confidence in the underlying logic.
Once the simple tit-for-tat strategy was described, certain problems were discovered that required modified strategies (briefly reviewed in Trivers 2005). For
example, occasional errors can lock two tit-for-tatters into a most unfortunate
situation, endless “vendettas” in which each reverses its move exactly out of
synchrony with the other, never achieving simultaneous cooperation, thereby
greatly reducing the success of the tit-for-tat strategy. “Generous tit-for-tat” solved
this problem by allowing a tit-for-tatter occasionally (say 1/3rd of the time) to
cooperate after a defection by the partner small cost, larger gain under a variety of
realistic conditions (Nowak and Sigmund 2004). Later, “win-stay, lose-shift” was
shown to be superior still against a background of tit-for-tatters, defectors, and
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R. Trivers
generous tit-for-tatters (Nowak and Sigmund 1993). Observer effects can also be
modeled (Nowak and Sigmund 1998a,b), leading to “indirect reciprocity.” In this
case, my strategy towards you depends not only on what you have done to me but
also on what you have been observed doing to others.
The simplest application of the above to deceit would be to treat it as a Prisoner’s
Dilemma. Two individuals can tell each other the truth (both cooperate) or lie (both
defect) or one of each. There are two problems with this. One is that a critical new
variable becomes important: who believes whom? If I believe you and you are
lying, I suffer. If you lie and I disbelieve you, you suffer. By contrast, in the
Prisoner’s Dilemma, each individual knows after each reciprocal play how the
other played (cooperate or defect) and tit-for-tat and its elaborations provide a
simple reciprocal mechanism that can operate under the humblest of conditions as
in bacteria. The second problem is that with deception, there is no obvious reciprocal logic. If you lie to me, this does not mean my best strategy is to lie back to you
it usually means that my best strategy is to distance myself from you or punish you.
Karl Sigmund (pers. comm.) has suggested that it might be useful to adapt the
Ultimatum Game to this problem. In the UG, a proposer offers a given split of (say)
$100: e.g., $80 to self, $20 to the responder. The responder, in turn, can accept the
split, in which case the money is split accordingly or the responder can reject the
offer, in which case neither receives anything. Often the game is played as a oneshot anonymous encounter, i.e., individuals play only once with others whom they
do not know and with whom they will not interact in the future.
Sigmund argues as follows. Imagine a modified UG in which there are two
possible pots (say $100 and $400) and both players know this. One pot is then
randomly assigned to the proposer. Now let us say the proposer offers you $40; this
could represent 40% of the pot (in which case you should accept) or 10% (most
people would reject). The proposer is permitted to lie and tell you that the pot is the
smaller of the two when, in fact, it is the larger. You can trust the proposer or not but
the key is that you are permitted to pay to find out the truth from a (disinterested)
third party. (This measures the value you place in reducing your uncertainty
regarding the proposer’s honesty). If you then discover that the proposer lied, you
should have a moral (or, at least, moralistic) motive to reject the offer, and the other
way around, for the truth (all compared to uncertainty, i.e., not paying to find out).
Note that from a purely economic point of view, there is no benefit in finding out the
truth, since it costs money after which it may lead to an (otherwise) unnecessary
loss of whatever is offered. In Sigmund’s words: “how much would a responder be
prepared to pay for reducing the uncertainty and go for a possibly inconvenient
truth?” Note that the game can be played in real life with varying degrees of
anonymity and also multiple times, as in the iterated Prisoner’s Dilemma. As ability
to discriminate develops, the other person will benefit more from your honesty
(quickly seen as such) and suffer less from deception (spotted and discarded). When
people are in greater need, they may be expected not to pay to find out the truth but
rather to accept the offer whatever its relative size is.
When we add self-deception, a possible game quickly becomes very complicated. One can imagine actors who are stone-cold honest (cost: information given
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391
away, naive regarding deception by others), consciously dishonest to a high degree
but with low self-deception (cost: higher cognitive cost and when detected),
dishonest with high self-deception (more superficially convincing at lower immediate cognitive cost but suffering later defects and acting more often in the service
of others, and so on). Without anything else to offer along these lines, I suggest that
those talented at the mathematics of simple games or studying them via computer
simulations might find it rewarding to define a set of characters along the lines just
mentioned, and then assign variable quantitative effects so as to explore their
combined evolutionary trajectory. Perhaps, results will be trivial and trajectories
will depend completely on the relative quantitative effects assigned but more likely
deeper connections will emerge, seen only when the coevolutionary struggle is
formulated explicitly.
18.16
The Cost of Deconstructing Lies
The cost of seeing through deception is not trivial, and in some cases, it is
substantial. Think of the daily drumbeat of propaganda emerging from the government in times of war or class warfare (2001 2006 in the U.S., for example). On a
personal level, I was first exposed to this when I was about 6 years old. Having
saved 6 dollars over the space of 2 months to buy a knife displayed in a window, I
showed up, only to be told that I was 1 dollar short. Nonsense, I said, the sign
outside says “$6.” The shopkeeper took me to the sign and showed me that written
after the 6 in very small letters was 98, i.e., almost 1 dollar. I was incredulous and
very angry how did it make sense, I wanted to know, for him to misrepresent the
true value of an item by subtracting two pennies so as to generate just the kind of
mistake I had, in fact, made? He assured me that the practice was widespread. I soon
confirmed that this was true. Almost all prices gas, food, furniture when spotted
at a distance, appeared to be one full unit below what they actually were. For a
couple of weeks I walked around in a daze, benumbed at the amount of unnecessary
arithmetical calculation this system required: always adding a unit or two to the
total in order to calculate the real value. How was it possible, I kept asking myself,
that this was the system of posting prices that had developed?
And, I think, there was my mistake. This was not a rational system agreed upon
by all actors or what Jesus might have told us to do this was the system that had
actually developed over time. In a nutshell, honesty is not evolutionarily stable. It is
easily displaced by deception which, in turn, forms a new equilibrium. Further,
deception may be counter-selected but so may be a return to honesty, since honest
valuations will often be devalued along with those that are hyped. That is, we will
frequently add a unit to honest prices, decline to buy them, and the honest shopkeeper suffers.
So also with self-deception. Over evolutionary time, we have been driven
downhill by selection such that a degree of self-deception is common to most or
all of us. It has formed a new equilibrium such that honest people may come off as
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R. Trivers
lacking, in part because others unconsciously compensate in their estimates for the
expected degree of inflation. No inflation so we make them smaller than they really
are. And, perhaps, the happiness thermostat in our body has been reset so as to
assume a certain degree of self-deception.
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Chapter 19
Human Universals and Primate
Symplesiomorphies: Establishing
the Lemur Baseline
Claudia Fichtel and Peter M. Kappeler
Lemur and Propithecus are both socially intelligent and socially dependent. They are,
however, hopelessly stupid towards unknown inanimate objects. In this branch of the
primates, the basic qualities of primate society have evolved without the formal inventive
intelligence of true monkeys.
Alison Jolly (1966a: 165 166)
Abstract Social behavior, culture, and cognition are domains where presumably
most human universals exist. Identification of these derived human traits depends
and relies on comparisons with other primates, notably chimpanzees. This approach
can also be used to reconstruct primate and human behavioral evolution. Accordingly, traits found in both Homo and Pan can be inferred to have existed in their last
common ancestor as well. By analogy, traits shared between humans and other
primates can be traced back even further down on our family tree. Here, we look at
the other side of human universals, i.e., behavioral and cognitive traits of the most
basal living primates, which ought to represent the common primate legacy upon
which later taxon-specific specializations were built. Specifically, we review studies investigating cognitive abilities and social behavior of the lemuriform primates
of Madagascar. The Malagasy lemurs are particularly important for this purpose
because they alone, among strepsirrhine primates, have evolved group-living,
which characterizes most living haplorrhines. Even though lemurs have relatively
smaller brains than New and Old World monkeys and great apes, their ability to
solve problems that require technical intelligence is qualitatively on par with that of
haplorrhines. In the domain of social intelligence, however, lemurs deviate from the
C. Fichtel (*)
Department of Behavioral Ecology & Sociobiology, German Primate Center, Göttingen, Germany
e mail: claudia.fichtel@gwdg.de
P.M. Kappeler
Department of Sociobiology/Anthropology, University of Göttingen, Göttingen, Germany
e mail: pkappel@gwdg.de
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 19, # Springer Verlag Berlin Heidelberg 2010
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C. Fichtel and P.M. Kappeler
better-known haplorrhine models (i.e., cercopithecines) in several respects. Most
importantly, their behavioral strategies reflect an emphasis on within-group
competition, rather than cooperation, which may represent lemur-specific adaptations to an ecologically unpredictable environment, rather than fundamental
deficits in social intelligence. In any event, a broad comparative perspective
including the best living models of the earliest gregarious primates can enrich
reconstructions and other evolutionary analyzes of primate social behavior,
including that of humans.
19.1
Introduction
Behavioral characteristics unique to Homo sapiens can only be identified as such by
reference to a meaningful out-group. Because humans are members of the order
Primates, this lineage provides the natural out-group for such comparisons. Primates, however, are a diverse group with hundreds of living species and 80 million
years of evolutionary history, so that specific deviations from our basal evolutionary legacy may not be that evident. Specifically, our biological continuity with
other animals is evident in those behavioral, morphological, and physiological traits
that have a genetic basis and define our affiliation to vertebrates, mammals,
primates, haplorrhines, catarrhines, and hominids. Our bodies and minds can
therefore be seen as a complex puzzle made up of pieces we share with these
various groups, interspersed with a few derived pieces. Traditionally, most comparative studies attempting to identify the derived human pieces of this puzzle have
relied on the contrasts between humans and chimpanzees (Pan troglodytes).
Sequencing of the chimpanzee genome revealed <1% difference with the human
genome (Mikkelsen et al. 2005), and molecular clock studies pinpointed the last
common ancestor of Homo and Pan at around 6 million years before the present
(Bradley 2008). Chimpanzees (and bonobos) therefore provide the most immediate
step back into our deep behavioral past and the most appropriate specific referents
for comparative studies of human behavior (e.g., Boesch 2007; Whiten, this volume). This approach can also be used to reconstruct primate and human behavioral
evolution. According to this logic, characteristics or traits found in both taxa today
are assumed to have been already present in their most recent common ancestor,
i.e., they represent plesiomorphies in the terminology of cladistics. In this terminology, human universals represent autapomorphies, i.e., derived traits that are
unique to a terminal group.
Extending comparisons of behavioral traits beyond the obvious Homo-Pan
contrasts has led to additional insights. For example, the demonstration of social
and technical traditions in orangutans (Pongo pygmaeus) has led to the novel
conclusion that such cultural abilities must have been shared by the last common
ancestor of all great apes (van Schaik et al. 2003), which lived about 14 million
years ago. Similarly, comparative studies of cognitive abilities of great apes and
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other haplorrhine primates revealed that New and Old World monkeys share many
common features, and that only the level of performance varied among species
(Amici et al. 2008). These examples highlight the importance of considering
behavioral plesiomorphies in the analysis of potential human (or hominid) universals. In other words, characteristics attributed to the last common ancestor of
chimpanzees and humans (or of the hominids) may, in fact, have an even longer
evolutionary history. Thus, comparative analyses of human behavior limited to
chimpanzees or other great apes risk drawing inaccurate conclusions by failing to
explicitly recognize primate symplesiomorphies, i.e., character states that originated in an earlier common ancestor.
Evidence for the existence of precursors of cognitive and social traits in the
behavioral domains, where human universals are most pronounced, has been
produced for various New and Old World primates, including great apes
(reviewed in Tomasello and Call 1997). Capuchin monkeys (Cebus spp.) exhibit
remarkable technical intelligence (Visalberghi 1993), all too familiar socio
emotional responses (Brosnan and de Waal 2003), and local variation in cultural
traditions among wild populations (Perry and Manson 2003). These observations
question the existence of a possible deep behavioral and cognitive gap between
hominins or hominids and all other primates. On the other hand, there are
pronounced grade shifts in relative brain size among primate lineages (Martin
and Harvey 1985; Dunbar and Shultz 2007), which may underlie qualitative
differences in their social behavior and cognitive abilities (Deaner et al. 2007;
Shultz and Dunbar 2007, Dunbar this volume). The cognitive and cultural capabilities of haplorrhine primates have been reviewed elsewhere (e.g., Tomasello
and Call 1997; Whiten and van Schaik 2007). Here, we extend these comparisons
to the most basal living primates: the strepsirrhine suborder. While recent strepsirrhines (lemurs and lorises) have their own distinctive evolutionary history and
adaptations, they have retained a number of primitive features that almost certainly characterized the earliest primates (Yoder 2007). These living species,
therefore, represent the legacy upon which all living primates have built their
specific derived adaptations.
Malagasy lemurs (Lemuriformes) are particularly interesting in this context,
because they are the only strepsirrhines that have evolved multi-male multi-female
groups, like those that are characteristic of most haplorrhines. Group-living lemurs
therefore represent the most appropriate models to establish the baseline for primate
social intelligence and complexity, whereas cognitive abilities related to technical
intelligence should be found independent of a particular social system. The specific
goal of this chapter is to summarize and evaluate studies of social and technical
cognitive abilities among lemurs. Even though Alison Jolly (1966b) established the
importance of comparative studies of lemur social intelligence in the early days of
primatology, subsequent research on lemur cognition and social communication
has not been conducted with the enthusiasm and rigor that has characterized similar
research on haplorrhines, and great apes in particular, in recent decades. Nevertheless, by bringing together some old, often overlooked studies and some more recent
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work, we can begin to sketch the outlines of lemur cognition and social behavior.
This endeavor will help to put comparative work on human universals, and on
hominid behavior more generally, into broader perspective, insofar as that a closer
look at the basal living primates will provide baseline information about shared
ancestral traits of all primates.
19.2
Lemurs
Based on genetic differences and several morphological features, primates can be
divided into two suborders: strepsirrhines (lemurs and lorises) and haplorrhines
(New and Old World monkeys including the great apes). The living lemurs represent the 100 or so endpoints of an adaptive radiation following a single successful
colonization event of Madagascar during the Eocene (Karanth et al. 2005; Tattersall
2007). They can be grouped into five families and 15 genera, which, together with
recently extinct taxa, exhibit almost the full range of diversity in social, ecological,
and life-history adaptations found among all other primates (Richard and Dewar
1991). The majority of living lemurs are nocturnal and solitary or pair-living, but,
according to recent genetic analyzes (Horvath et al. 2008), life in multi-male, multifemale groups has evolved independently in the Lemuridae (in the genera Lemur,
Eulemur, Hapalemur, and Varecia) and Indridae (in the genus Propithecus). After
controlling for body size and phylogenetic effects, lemur groups in both families
are, on average, smaller than those of haplorrhines (Kappeler and Heymann 1996),
and they are generally characterized by even adult sex ratios (Kappeler 2000). As in
many haplorrhines, group-living lemurs are characterized by predominant female
philopatry (Richard et al. 1993; Sussman 1992), diurnal activity (at least partially)
(Kappeler and Erkert 2003; Erkert and Kappeler 2004), and their vocal repertoires
sometimes include functionally referential calls (Fichtel and Kappeler 2002).
Relative brain size of lemurs tends to be smaller than that of haplorrhines (Armstrong
1985; Dunbar 1998) and olfactory communication is used in a variety of behavioral
contexts (Kappeler 1998; Pochron et al. 2005). Thus, the social systems (sensu
Kappeler and van Schaik 2002) of lemurs exhibit a mixture of idiosyncrasies as
well as convergences with those of other primates, but the basic pillars of sociality
appear to be comparable.
Below, we summarize the results of a literature review of studies of lemur
cognition and social behavior that bear relevance to the study of human behavioral
universals. We divide this review into two sections that deal with technical and
social intelligence, respectively. We do not attempt explicit and detailed comparisons with great apes or all other haplorrhines, and we refrain from extending
comparisons to other mammalian orders. Instead, we aim to provide a concise
summary of the socio-cognitive abilities of lemurs and other strepsirrhines that may
contribute interesting baseline information about primate sociality for comparisons
among other primates and mammals, including humans.
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19.3
399
Technical Intelligence
19.3.1 Space and Objects
The most critical challenge to survival is the ability to deal effectively with (threedimensional) space and objects, such as food, shelter, and predators. Physical cognition, i.e., the understanding of object features and their various spatial and causal
interrelations, is presumably most adaptive in the contexts of foraging and locomotion.
Independent of their social organization, most primates tend to remain within a
particular home range. Cognitive abilities that enable animals to identify their position, to remember what is located where, and to travel efficiently between these sites,
represent selective advantages (Anderson 1983; Gallistel 1989). Because successful
foraging and efficient locomotion are general ecological problems, the relevant
cognitive skills of lemurs are expected to be similar to those of other primates.
The cognitive abilities of lemurs in the context of spatial mapping and spatial
memory have been the focus of experimental studies in captivity and, more recently,
in observational and experimental studies in the wild. In Madagascar, gray mouse
lemurs (Microcebus murinus) inhabit dry deciduous forests with pronounced seasonal fluctuations in food availability. During a long dry season, when food availability is low, mouse lemurs mainly rely on resources that are sparsely distributed
but predictable in space, such as gum, secretions from colonial insects, and nectar
(Dammhahn and Kappeler 2008). Field observations revealed that solitary mouse
lemurs revisited stationary feeding sites more often than nonstationary feeding sites
(Joly and Zimmermann 2007). Using an experimental approach, Lührs et al. (2009)
set mouse lemurs a spatial memory task by confronting them with two different
patterns of baited and non-baited artificial feeding stations. Mouse lemurs used
spatial cues to relocate baited feeding stations and they were able to rapidly learn a
new spatial arrangement. In a release experiment, they also exhibited high travel
efficiency in directed movements, suggesting that their spatial memory is based on
some kind of mental representation that is more detailed than a route-based network
map (Lührs et al. 2009). The existence of a topological or route-based map has also
been proposed for group movements of two group-living lemurs, Milne Edwards’
sifakas (Propithecus edwardsi) and redfronted lemurs (Eulemur fulvus rufus, Erhart
and Overdorff 2008). Because route-based mental representation of spatial relationships, straight-line travel, and efficient goal-directed movements between distant
sites have been suggested for several haplorrhines (Boesch and Boesch 1984;
Gallistel 1989; Garber 1988; Menzel 1991; Janson 1998; Noser and Byrne 2007),
cognitive abilities in the context of spatial orientation do not appear to differ
fundamentally between lemurs and other primates.
Another set of cognitive spatial skills is required to search for hidden food, as in
object permanence experiments, or to trace invisible displacements of food hidden
by an experimenter. When various lorisids, ring-tailed (Lemur catta), and brown
lemurs (E. fulvus) were tested for their ability to find hidden food, only one
loris failed to master the task (Jolly 1964a). Object permanence was studied in
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redfronted lemurs, mongoose lemurs (E. mongoz), ring-tailed lemurs, and bamboo
lemurs (Hapalemur griseus) (Deppe et al. 2009). Lemurs performed well above
chance levels in tracking food that had been in clear view before being hidden
(visible displacements). However, when lemurs were not allowed to search for up to
25 s, performance declined with increasing time-delay. They did not outperform
chance levels in tracking food in invisible displacement tasks. Many haplorrhine
primates solve visible displacement tasks, whereas the ability to perform invisible
displacement has been demonstrated in apes, rhesus macaques (Macaca mulatta),
and cotton top tamarins (Saguinus oedipus) (de Blois et al. 1998; Call 2001; Hauser
2001; Neiworth et al. 2003; Mendes and Huber 2004).
Primates often face the problem of getting food that is out of reach, such as fruits
at the periphery of a branch. Thus, spatial understanding of objects that allows them
to determine to which branch a fruit is connected to pull it closer or to use another
branch as a tool to bring fruits into reach should be advantageous. Detour problems
test this kind of understanding. Lemurs quite successfully mastered detour problems, in which food was impaled on a bent wire and the subject had to move it to
the left or right and had to push or pull the food (Davis and Leary 1968). Although
Old World monkeys were best at performing these tasks, lemurs did not differ from
some New World monkeys, and squirrel monkeys failed the task entirely.
Maze experiments represent another type of detour problems, in which the
spatial memory of subjects is investigated. Picq (1993, 2007) conducted radial
maze experiments with captive mouse lemurs. In this experimental setup, subjects
learned to choose one out of eight possible arms to get access to a reward; in this
case, a nest-box. Mouse lemurs mastered this task quickly, and their learning curves
matched those of New and Old world primates, including chimpanzees.
Because of their fast life histories, mouse lemurs are also well suited to address
questions of aging in memory. Picq (2007) applied different visual and spatial
discrimination as well as generalization tasks in an eight-armed radial maze. Young
mouse lemurs were able to learn all tasks quickly; older mouse lemurs performed as
well as the young ones in some tasks, but showed impairment in several other tasks,
indicating that the acquisition of skills is not affected, but the shifts in attention
from visual to spatial cues and, thus, the flexible use of acquired memories in novel
situations as well as the generation of novel solution strategies were impaired.
Similar maze experiments with haplorrhines revealed that basic spatial memory
skills are comparable across taxa (Tomasello and Call 1997). Other tasks, in which
spatial understanding of objects is investigated, such as patterned-string problems,
in which subjects have to disentangle strings differently to get a reward, or mental
rotations skills, have not been conducted with lemurs so far.
19.3.2 Tools and Causality
Primate foods are either immediately ready to eat or require manipulation before
ingestion. Manipulation tasks vary from simple to complex: from just picking
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a fruit or leaf off the branch, through digging for roots, uncovering food items under
leaves or tree barks, capturing mobile insects, using tools to open nuts, and
modifying probes to fish for ants (van Schaik et al. 1999). Observations of wild
lemurs revealed little evidence for object manipulation in a foraging context, except
for the aye-aye (Daubentonia madagascariensis), which uses a thin, long,
tap-scanning and probing middle finger to locate and extract insects embedded in
trees or branches (Erickson 1995). Black (E. macaco) and brown lemurs sometimes
manipulate millipedes vigorously, presumably in an effort to annoint their bodies
with an insect repellent (Birkinshaw 1999; pers. observ.). No other manipulative
interactions with food items by lemurs have been reported so far.
Studies of manipulative interactions with novel objects in captivity are more
abundant. When various lorisids were confronted with different complex novel
objects, they either stared at them and struck them with their hand (Jolly 1964a, b)
or pushed, pulled, or even grasped them (Ehrlich 1970; Renner et al. 1992). Parker
(1973, 1974) compared manipulation behavior with hands and mouth towards novel
objects in ring-tailed and black lemurs with that of several haplorrhines, including
great apes. He found that manipulative behavior was most variable in great apes,
slightly less variable in macaques, and least variable in langurs, spider monkeys,
and gibbons. Both lemur species were intermediate between the great apes/macaques
and the other cluster of species. Because differences between groups could not be
explained by hand anatomy, but by habitat use, i.e., a distinction between feeding
specialists and generalists, Parker (1973) suggested that broad-niched opportunists
need to develop more explorative behavior than specialists to adapt to the wide
variety of circumstances in their habitat. A similar pattern of object manipulation
variability was found in another study comparing 74 species of primates. Lemurs,
marmosets, and leaf-eating monkeys showed less variable behavior than frugivorous and insectivorous Old World monkeys, but not folivores. Capuchin monkeys,
as well as great apes, showed the most variable behaviors (Torigoe 1985). Thus,
lemur manipulatory skills are roughly comparable with those of at least some New
and Old World monkeys.
In addition, more complex object manipulation skills appear to exist in gray
mouse lemurs. In a series of experiments, individuals first had to open a plastic box
in three different ways to get access to a reward (Schilling 2007). In the second task,
the reward was hidden in a cylindrical box sliding inside an opaque second box.
Subjects were required to manipulate a string to move the inner box closer in order
to reach and pull out the reward. All mouse lemurs learned the tasks rapidly and
improved over time. In the third task, mouse lemurs were tested with a vertical
mirror box presenting a mealworm hanging behind an opaque wall in such a way
that the reward could only be obtained by learning to use its reversed image. All but
one individual mastered this rather complex task, which may require some form of
mental rotation.
The aye-aye, which has the largest relative brain size among strepsirrhines
(Stephan et al. 1988), uses a unique form of percussive tap-foraging, during
which insect larvae are extracted from wood by a series of coordinated actions
with the elongated third digit (Miliken et al. 1991; Lhota et al. 2008). They are also
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able to open complex puzzle boxes and performed better than other lemurs on such
a test (Digby et al. 2008). Opening of simple boxes has also been demonstrated in
brown, black, and ring-tailed lemurs (Jolly 1964a, b; Kappeler 1987; Fornasieri
et al. 1990; Anderson et al. 1992), though Fornasieri et al. (1990) stated that they
showed “little comprehension” of the task.
Understanding of physical and causally relevant aspects of objects are prerequisites for using tools. Tool-use has been reported in several haplorrhine primates,
with chimpanzees and orangutans exhibiting the most complex skills (Whiten et al.
1999; van Schaik et al. 2003; Moura and Lee 2004). In contrast, tool-use has not
been reported from the wild for any strepsirrhine. However, there is one observation
of aye-ayes manipulating an object that required some sort of sensorimotor intelligence that is also required for tool-use: they grasped and moved a liana over a
branch under which they had been feeding to gain better access to a feeding site
(Sterling 1994). These sensorimotor skills were examined in more detail in an
experiment in which aye-ayes could use a simulated liana (rope) to get access to
feeding cups fixed to the wall (Sterling and Povinelli 1999). However, they failed to
move the rope horizontally close to the feeding cups, though they readily climbed
up and down the simulated liana. The authors concluded that aye-ayes do not
achieve comprehension to use tools, but rather may use trial-and-error learning to
develop tool-use behavior.
Hence, the question arises whether strepsirrhines simply do not posses the
underlying cognitive abilities to understand the functionality of objects for potential tool-use. In a recent study, Santos et al. (2005a) set up a series of experiments
with brown and ring-tailed lemurs to address this question. They used and
extended a design originally used with other haplorrhines (Hauser et al. 1999;
Povinelli 2000; Fujita et al. 2003; Santos et al. 2006). In these experiments,
lemurs were offered two cane-shaped tools to pull out-of-reach food items. In
the first series of experiments, tools were identical and differed only in the
orientation relative to the food reward, with one tool being more effective to
reach the food. Lemurs had to choose the more effective tool and did so just as
successfully as capuchins (Cummins-Sebree and Fragaszy 2001; Fujita et al.
2003). In the second experiment, lemurs were tested with novel tools differing
from the originals in one dimension, to test whether they spontaneously attend to
some of the features that are causally relevant for a successful pulling tool.
Lemurs attended more to the sizes than to the colors of tools, but made no
distinction between tools’ shapes and textures. The next two experimental designs
presented problems in which one of the tools had to be modified to access
the food. In these tests, the authors used familiar, already successfully used
tools, and unfamiliar tools. Lemurs did not prefer familiar over unfamiliar tools,
indicating that they chose tools on the basis of features that were functionally
relevant for the task. Thus, lemurs solved the can-pulling tasks like other toolusing haplorrhines, indicating that many primates share an ability to reason about
basic functional properties of different objects, even if they do not use tools
normally (see also Hauser et al. 2002; Spaulding and Hauser 2005). In contrast
to this basic understanding of features of tools, regular tool users have a more
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sophisticated understanding of causal relationships between features of the tool
and the problems they can solve with it (Martin-Ordas et al. 2008; Seed et al.
2009). Orangutans, for example, even spat water spontaneously into a transparent tube to get access to an out-of-reach peanut floating inside the tube (Mendes
et al. 2007).
19.3.3 Features and Categories
All primates locate and manipulate objects, which they identify on the basis of
certain observable features. However, in some cases, primates also identify objects
on the basis of conceptual categories that go beyond direct perception (Tomasello
and Call 1997). Basic discrimination learning of objects that vary either in shape,
color, pattern, brightness, location, or sound has been demonstrated in a variety of
lemurs and other strepsirrhines (reviewed in Ehrlich et al. 1976; Meador et al. 1987;
Tomasello and Call 1997).
Learning sets are tests in which individuals may become better at discrimination
when they solve different sets of similar problems over time and have learned to
deal with a particular type of problem in general. Numerous studies on learning set
phenomena have led to the consensus that successful problem-solving indicates the
use of some type of abstract rule (Harlow 1949, reviewed in Fobes and King 1982).
Tests of object discrimination learning sets have been conducted with bushbabies,
lorises, black and ring-tailed lemurs (Stevens 1965; Cooper 1974; Ohta 1983; Ohta
et al. 1984, 1987). A comparison across the primate order revealed that there are no
taxonomic differences with respect to success in object discrimination tasks: “after
200 problems, approximately 80% correct performance is achieved by species as
different from another as black lemurs, chimpanzees, rhesus macaques, and gorillas”
(Tomasello and Call 1997).
Reversal learning paradigms investigate the ability to reverse a previously
learned discrimination. Subjects first learn an object discrimination to get a reward
before a previously nonrewarded object becomes rewarded. This paradigm is
thought to reflect a subject’s ability to form and use abstract rules or hypotheses
(Rumbough 1970). When brown and ring-tailed, fork-marked (Phaner spp.), and
mouse lemurs were subjected to this test paradigm, their performance was inferior
to that of haplorrhines (Stevens 1965, reviewed in Rumbough 1997). However,
more recent studies of mouse lemurs revealed that their reversal skills are comparable to those of haplorrhines (Picq 1993, 2007). Cross-modal transfer of objects
from one perceptual domain to another also belongs to the kinds of tasks that go
beyond stimulus-response associations. Only bushbabies (Galago senegalensis)
among strepsirrhines were presented with this task, in which they were able to
transfer learned responses from vision to audition (Ward et al. 1976).
The delayed response paradigm investigates a subject’s memory or ability to
maintain a perception of an item when it is no longer available. Typically, the
subject sees a reward hidden in one of two locations, and after a certain delay, it
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may search for it. The few studies that applied this test paradigm to redfronted,
mongoose, bamboo, ring-tailed, and ruffed lemurs (Varecia variegata), as well as
to bushbabies, found that they were all inferior to haplorrhines in their performance (Harlow et al. 1932; Maslow and Harlow 1932; Jolly 1964a; Deppe
et al. 2009).
Discrimination learning of relational categories involves a concept that can be
learned only by comparing objects to one another and by inducing some relational difference (e.g., “larger than”). In oddity concept studies, subjects are
presented three stimuli, two of which are the same. Subjects are rewarded for
responding to the odd one; for example, in a “square square triangle” constellation. After that, some of the training subjects are confronted with three completely
new objects (line circle line). In this paradigm, the inference is that subjects
understand the concept of “odd.” This concept allows animals, for example, to
categorize environmental features such as different food items. Davis et al.
(1967) confronted several New World and Old World monkeys as well as ringtailed lemurs with oddity problems, and found that the performance of ring-tailed
lemurs was inferior to most haplorrhines, but better than guenons. In summary,
there are no qualitative differences in performance across major primate radiations in object discrimination learning set formation tasks. However, a few
studies on strepsirrhines suggest that they do not seem to be very skilled in
reversal learning and delayed response, but they do seem to have an understanding of oddity problems.
The ability to form categories of objects belonging to same or different classes
is another task that provides insights into cognitive abilities of animals. In such
tests, subjects are presented with many objects simultaneously and asked to sort
them into groups on the basis of their similarities and differences. This is a
demanding task because subjects are required to coordinate both the similarities
and differences of multiple objects simultaneously and then to manipulate the
objects in line with that understanding. The only study of such capacities in
strepsirrhine primates reported remarkable skills in serial ordering of objects in
ring-tailed lemurs (Merritt et al. 2007).
Many animals are also able to organize sequences in memory and retrieve
ordered sequences without language (Sands and Wright 1980; Straub and Terrace
1981). For example, capuchin monkeys and rhesus macaques were able to select a
series of photographs according to a consistent arbitrary order (D’Amato and
Colombo 1989; Terrace et al. 2003). In this simultaneous chaining paradigm, a
series of arbitrary stimuli (such as photographs) are presented simultaneously in
random spatial position on a touch-sensitive monitor. Subjects are rewarded when
they respond in a prespecified arbitrary order without error. This paradigm is
particularly useful for cognitive studies because it investigates the internal representation of the sequence. Merritt et al. (2007) tested ring-tailed lemurs with such a
paradigm. Ring-tailed lemurs were capable of learning three-, four-, and five-items
lists. Moreover, these lemurs showed a remarkable similarity in accuracy and
reaction time with that of capuchin and rhesus monkeys (D’Amato and Colombo
1989; Terrace et al. 2003).
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19.3.4
405
Quantities
Primates also need to have an understanding of quantities to estimate food availability at different feeding patches or the number of opponents in a potential fight.
There have been many studies demonstrating that monkeys and apes are able to
judge the absolute and relative numerousness of objects (Tomasello and Call 1997;
Beran and Beran 2004; Hanus and Call 2007; Evans et al. 2009). For example, it has
been shown that anthropoids possess numerical representation that is modulated by
Weber’s law, such that as the numerical magnitude increases, a larger disparity is
needed to obtain the same level of discrimination. By applying a search task in
which grapes were placed into a bucket, Lewis et al. (2005) studied mongoose
lemurs’ numerical ability. They were able to differentiate numerosities that differed
by 1:2, but not those that differed by 2:3 or 3:4. Thus, lemurs’ understanding of
numerosity also seems to be modulated by Weber’s law. Nevertheless, lemurs’
numerical discrimination seems to be inferior to that of New World and Old World
monkeys; tamarin monkeys (Saguinus spp.) were able to differentiate sequences of
syllables that differed by 1:2 and 2:3 but not the 3:4 ratios (Hauser et al. 2003), and
rhesus macaques even discriminated numerosities that differed by a 4:5 ratio
(Brannon and Terrace 2000).
Expectation about numerical events has been studied in ring-tailed, brown,
mongoose, and ruffed lemurs (Santos et al. 2005b). By using looking techniques,
they explored how lemurs represent small numbers of objects spontaneously in the
absence of explicit training (see Hauser 2000 for review). Santos and her colleagues
conducted experiments that were modeled after Wynn’s violation of expectancy
paradigm for human infants (Wynn 1992), and tested whether lemurs look longer
when the number of objects revealed behind a screen differs from the number that
should be there. They presented lemurs with two lemons that disappeared sequentially behind an occluder; lemurs looked longer at an unexpected outcome of only
one lemon than at an expected outcome of two lemons. Similarly, lemurs looked
longer at an unexpected outcome of three lemons than towards an expected
outcome of two lemons. In addition, lemurs attended to the size of objects; they
looked longer at an object twice the size of the original object than at an expected
outcome of two objects of the original size. Thus, these lemurs understand the
outcome of simple arithmetic operations of 1 + 1 events. These findings are in line
with those in human infants (Wynn 1992; Feigenson et al. 2002), rhesus macaques
(Hauser et al. 1996), and cotton-top tamarins (Uller et al. 2001). However, capuchin
monkeys (Cebus apella) have been shown to be able to judge the quantity of 1 5
items in a sequentially presented food choice experiment (Evans et al. 2009).
Moreover, great apes were able to differentiate quantities of up to ten items when
items were presented simultaneously. However, sequential presentation of food
items resulted in a correct judgment of only up to six items (Hanus and Call 2007).
Furthermore, some chimpanzees, which were trained in lexical language skills,
could judge of up to ten sequentially presented items correctly (Beran and Beran
2004). Thus, without training, haplorrhine primates are able to perform arithmetic
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operation of up to 6. Because newly hatched domestic chicks (Gallus gallus) are
able to add and subtract up to five sequentially presented items, mental number
representation might be present among many more vertebrates, however (Rugani
et al. 2009).
19.4
Social Intelligence
There is a wealth of studies of social cognition in haplorrhine primates (Tomasello
and Moll this volume, Cheney and Seyfarth this volume) from which the social
brain hypothesis has been developed (Dunbar this volume). Strepsirrhines have not
been well represented in this field of research
either because they are not
interesting in this context due to their relatively small brain size (cf. Deaner et al.
2006) or because Jolly’s (1966b) first impression of lemur intelligence has impeded
subsequent research endeavors. However, there are some lemur studies that are
relevant to assumptions and predictions of the social brain hypothesis, and show
that group-living lemurs exhibit some interesting differences in their social lives
from their haplorrhine cousins.
According to the social intelligence hypothesis, the challenges of living in social
groups have favored the expansion and reorganization of the primate brain (Whiten
and Byrne 1997; Dunbar and Shultz 2007; Silk 2007; Dunbar, this volume).
Comparative studies of brain size among primates revealed that relative brain
size correlates with several indices of social complexity, including group size
(Dunbar 1995), number of females in the group (Lindenfors 2005), the frequency
of coalitions (Dunbar and Shultz 2007), grooming clique size (Kudo and Dunbar
2001), the prevalence of social play (Lewis 2000), the frequency of tactical
deception (Byrne and Corp 2004), and the frequency of social learning (Reader
and Laland 2002). Below, we will summarize our current knowledge of lemur
social complexity, focusing on group size and composition, the structure of social
relationships (coalitions, cooperation, postconflict behavior, grooming networks),
deception, social learning, and innovations, as well as communication. The aim of
this review is not to be exhaustive, but rather to highlight the key differences and
similarities.
19.4.1
Social Complexity and the Structure of Social
Relationships
One way to test the social intelligence hypothesis experimentally is to examine
whether species with complex social environments show unusual intelligence in
nonsocial domains compared with closely related, less social species (Bond et al.
2003). Transitive inference (if A > B and B < C, then A > C) is a form of
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deductive reasoning that has been suggested as one cognitive mechanism with
which animals could learn the many relationships within their group’s dominance
hierarchy. This process, thus, bears relevance to the social intelligence hypothesis,
which posits evolutionary links between various forms of social and nonsocial
cognition. The relationship between social complexity and transitive reasoning
has been studied in ring-tailed and mongoose lemurs (MacLean et al. 2008). The
group-living ring-tailed lemurs and the pair-living mongoose lemurs showed similar transitive inference, indicating that both species possess similar fundamental
cognitive abilities in this respect, obscuring potential effects of group size and
complexity.
Because females are philopatric in many haplorrhine primates and form longterm social networks, the average number of females per group has an evolutionary
impact on the development of large brains (Lindenfors 2005), and, hence, social
intelligence. Lemur groups usually contain only one to five reproductive females,
which are also philopatric (Kappeler 2000). Social networks, such as matrilineal
dominance hierarchies, in which maternal kin occupy adjacent ranks and females
form close and stable relationships, have been described for many Old World
monkeys (Silk 2007). Similar bonds have not been observed among lemurid
females (reviewed in Kappeler 1999), even though transitive dominance relationships are established in most, but not all lemur species (Kappeler 1993b; Pereira
et al. 1990). In species with dominance relationships, all females dominate all males
(Jolly 1966a; Richard 1987; Pochron et al. 2003). Reproductive opportunities seem
to be more limited for lemurid females than for cercopithecine females, because, on
average, only one or two females give birth per year in groups of most lemurid
species (Overdorff et al. 1999; Kappeler 2000; Pochron et al. 2004). Targeted
aggression by female group members towards close relatives, often adolescent
females, resulting in severe injury or eviction, has been observed in captive and
field settings in representatives of both Lemuridae and Indriidae (Vick and Pereira
1989; Pereira 1993; Barthold et al. 2009; Kappeler unpubl. data). Furthermore,
infanticide by females has been observed in several lemur species (Andrews 1998;
Jolly et al. 2000). Coalitionary defense of home ranges against neighboring groups
indicates that competition between groups is also pronounced (Nunn and Deaner
2004; Benadi et al. 2008). Because lemurs live in a relatively harsh and unpredictable environment with pronounced seasonality (Wright 1999; Dewar and Richard
2007), ecological factors may have favored competitive, rather than cooperative
tendencies in group-living lemurid females.
Overt cooperative behavior, another hallmark of social complexity (Silk and
Boyd this volume), has only rarely been observed in lemurs. Coalitions of related
redfronted lemur males have been observed to take over other groups (Ostner and
Kappeler 2004), and ring-tailed lemur males sometimes migrate in pairs or trios
(Jones 1983; Sussman 1992). Only a tiny fraction of agonistic interactions among
females involve coalitionary support (Pereira and Kappeler 1997) even though joint
territorial defense is common (see above). Solitary species exhibit a spatio-genetic
structure characterized by spatial clustering of related females (Kappeler et al.
2002; Wimmer et al. 2002), which may facilitate cooperative behavior among
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relatives. For example, several gray mouse lemurs mobbed a snake that held a
conspecific until it could escape (Eberle and Kappeler 2008), and communal
breeding among closely related females with a high mortality risk may provide
each of them with a form of family insurance (Eberle and Kappeler 2006).
Postconflict reconciliation is another important mechanism with which many
haplorrhines deal with the disruptive social consequences of intragroup conflict on
group cohesion (Aureli and de Waal 2000). Although reconciliation has also been
described for other mammals (e.g., Cools et al. 2008), there is mixed evidence for
lemurs. Reconciliation has been demonstrated in redfronted lemurs, albeit at relatively low levels, but it could not be demonstrated in ring-tailed lemurs, despite a
clear dominance hierarchy and within-group kin structure (Kappeler 1993a). However, studies of other captive populations of ring-tailed lemurs found low levels or
seasonal occurrence of reconciliatory behavior (Rolland and Roeder 2000; Palagi
et al. 2005). Absence of reconciliatory behavior was reported for black lemurs
(Roeder et al. 2002), whereas sifakas (Propithecus verreauxi) reconciled during the
mating season (Palagi et al. 2008). Thus, in contrast to many haplorrhines, strategic
use of affiliative interactions to foster social relationships is not pronounced among
group-living lemurs.
Given their small group size, it is not surprising that lemur grooming networks
are relatively small (Kudo and Dunbar 2001). Grooming cliques have been considered to be synonymous with coalition size, on the grounds that primates use
grooming to reinforce the bonds on which coalitionary support is based (Seyfarth
and Cheney 1984). This potential function of grooming has been studied in redfronted lemurs (Port et al. 2009). Here, the exchange of grooming bouts is highly
reciprocal, but grooming is biased in favor of higher-ranking partners. In addition,
aggression occurred at higher frequencies between classes of individuals that were
characterized by nonreciprocal grooming, suggesting that grooming may serve as a
means to reduce aggression in dyads with a high potential for conflicts. Thus,
grooming might be exchanged for tolerance, suggesting that lemur grooming networks might form part of a biological market of the kind described for various Old
World monkeys (Barrett et al. 1999; Henzi and Barrett 1999).
19.4.2 Tactical Deception and Related Skills
Neocortex size also predicts deception rate in primates (Byrne and Corp 2004).
Deception of conspecifics is often thought to be evidence of considerable cognitive
sophistication (Mitchell and Thompson 1986), and reflects very efficient learning
ability and sensitivity to a wide range of social discriminations (Cheney and
Seyfarth 1990; Byrne and Corp 2004; see also Trivers, this volume). Deception in
lemurs seems to be rare. Deaner et al. (2006) tested ring-tailed lemurs with
the classical deception paradigm of Menzel (1973), in which a subordinate was
informed of the location of a hidden food item and was subsequently released into
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an enclosure simultaneously with an uniformed dominant female. Male ring-tailed
lemurs did not reliably deceive the dominant female, which is not too surprising
given the natural response of a male towards a female in a feeding context in this
species. However, recent field observations at artificial feeding platforms within the
home ranges of wild red-fronted lemurs suggested that some males and females of
two different groups behaved as if they deceived other group members. Several
times when a group passed the platforms at distances of about 150 m, some
individuals sneaked away silently (redfronted lemurs usually produce grunts
while locomoting!), ran quickly towards the platforms, and depleted them before
uttering their long distance contact calls to reestablish contact with their group
(Lennart Pyritz pers. comm.). Experiments in captive settings also indicated that
brown and black lemurs seem to learn to deceive a human competitor (Genty and
Roeder 2006; Genty et al. 2008).
The same authors also report on self-control behavior in brown and black lemurs
(Genty et al. 2004). Self-control has been operationally defined as the ability to
inhibit a natural tendency to reach for the greater of two amounts of foods
(Anderson 2001). Self-control behavior in preschool children, i.e., the ability to
delay gratification, has been related to later cognitive competencies (Mischel et al.
1989). Brown and black lemurs initially chose the larger array of food, but learned
after a correction procedure to choose the smaller array of food, indicating that
they show some form of self-control (Genty et al. 2004). Similarly, several New
World and Old World monkeys, chimpanzees, and even children over 4 years old
initially showed the tendency to select the larger array of a reward (Boysen and
Berntson 1995; Silberberg and Fujita 1996; Anderson et al. 2000; Kralik et al.
2002). Only orangutans showed the spontaneous ability to understand the task
(Schumaker et al. 2001).
Studies in a wide range of species, including apes, dogs, and goats (Tomasello
et al. 1998; Call et al. 2003; Kaminski et al. 2005) showed that individuals follow
the gaze of others. Such shared attention is thought to underlie a theory of mind
and language acquisition (Tomasello and Moll this volume). Earlier studies
reported that ring-tailed lemurs do not follow human gaze (Itakura 1996; Anderson
and Mitchell 1999). However, a recent study, in which ring-tailed lemurs were
equipped with a novel telemetric gaze-tracking system, showed that they preferentially gaze towards others, and follow other lemurs’ gaze while freely moving
and interacting in naturalistic social and ecological environments (Shepherd and
Platt 2008). Moreover, Ruiz et al. (2009) demonstrated that brown and black
lemurs use coorientation to find hidden food in an object-choice experiment.
Lemurs were more likely to choose correctly after having looked in the same
direction as the model, in this case a photograph of a conspecific, indicating that
the adaptive value of gaze following might be a way of reading the attentional
focus of others. Interestingly, other primates have been shown to coorient with
humans (Tomasello et al. 1998; Bräuer et al. 2005), but failed to reliably select the
correct location of the hidden food by using human cues (Call et al. 2000; Hare
and Tomasello 2004), which might be due to the fact that object-choice tasks and
coorientation have been tested separately. The integration of both tasks revealed
410
C. Fichtel and P.M. Kappeler
that there is a connection between visual coorientation and foraging choice.
These results, however, do not indicate that lemurs understand gaze as mental
perspective taking of others. Objects or locations may simply become more
salient for an observer, as a result of following another individual’s attention
to that object or location a process that has been defined as “gaze-priming”
(Ruiz et al. 2009). Thus, gaze-following or better gaze-priming ability is also
present in strepsirrhines.
19.4.3
Social Learning and Innovations
The social intelligence hypothesis also invokes behavioral flexibility as a key
advantage of enhanced brain size. Innovation and social learning allow animals to
exploit the environment in new ways, and brain size seems to correlate with
frequencies of innovation and social learning (Reader and Laland 2002). Feldman
and Klopfer (1972) suggested that social learning, i.e., stimulus enhancement, may
also play a role in object-choice performance in brown lemurs. Observations of
predatory behavior on insects, small mammals, and birds in captive brown, black
and ring-tailed lemurs also raised the question of whether such behavior may lead to
the development of local traditions (Glander et al. 1985; Jolly and Oliver 1985).
The first experimental study of the acquisition process of a novel behavior was
conducted with ring-tailed lemurs (Kappeler 1987). Adult females, but not males,
and juveniles acquired the novel behavior, and remembered it over several months.
Similar experiments with brown, black, and ring-tailed lemurs also showed that
novel behaviors, in this case opening a baited food-box, are learned socially
(Fornasieri et al. 1990; Anderson et al. 1992). Social influences on feeding decisions involving familiar and novel food have been shown in black lemurs. In these
experiments, the consumption of high-quality novel food was acquired individually, but the dominant female influenced the consumption rate of low-quality novel
food (Gosset and Roeder 2001). Social influences have also been shown in the
complex foraging behavior of aye-ayes. A comparison with ruffed lemurs revealed
that aye-aye mothers co-fed and shared food with their infants and engaged in
socially mediated learning more often than ruffed lemurs. As a consequence, ruffed
lemurs showed less neophobia towards novel food and relied earlier on their own
foraging decisions (Krakauer 2005).
Spontaneous innovation of a novel behavior has been reported for strepsirrhines
only once so far. Semi-free-ranging ring-tailed lemurs developed a new behavior,
that is, immersing the tail in water and then sucking on the wet tail. Almost all
group members acquired “drinking-from-tail” behavior, and individuals who did
not acquire it were allowed to drink from the wet tail of animals which did (Hosey
et al. 1997).
In a similar vein, variation in antipredator behavior between populations has
been documented in sifakas (Fichtel and van Schaik 2006, Fichtel and Kappeler
unpubl. data). In primates, the usage and comprehension of alarm calls, i.e., their
19
Lemur Cognition and Behavior
411
association with predator-specific escape strategies, appear to be socially learned
(Seyfarth and Cheney 1980; Fichtel 2008). Thus, alarm calls provide flexible
behavioral mechanisms that allow animals to develop appropriate responses to
local predators (Curio et al. 1978; Cook and Mineka 1989; Laland 2004). A
comparison of three types of alarm calls and antipredator strategies in a semifree-ranging and a wild population of Coquerels’ sifakas (Propithecus coquereli)
revealed that the captive and wild sifakas used their alarm calls in the same
contexts, but exhibited similar behavioral responses in response to only two of
the three calls. All members of the captive population, including a wild-caught
individual, apparently associated the third alarm call with the presence of a raptor,
whereas individuals of the wild population associated no specific threat with this
particular call.
Similarly, a comparison of two wild populations of Verreaux’s sifakas in
habitats with a low and a high density of carnivores also revealed a different
comprehension of the alarm calls given to these predators. Sifakas in the habitat
with a high density of carnivores associated a predator-specific escape response
with these alarm calls, whereas sifakas in the other habitat did not. This differential
comprehension of alarm calls is likely to reflect the operation of social learning
processes that caused changes in signal content due to changes in the set of
predators to which these two populations have been exposed (Fichtel and van
Schaik 2006). Thus, social learning appears to be present in lemurs, whereas
innovations and tool-use seem to be extremely rare, indicating that the innovative
and tool-using anthropoids show greater flexibility in developing new behavior to
exploit the environment.
19.4.4 Communication
The evolution of language is clearly one hallmark of humans. Vocal communication of nonhuman primates is very different from human language (Cheney and
Seyfarth, this volume). Nonhuman primates have a relatively small repertoire of
vocalizations, whose production is predominantly innate (Winter et al. 1973;
Hammerschmidt et al. 2001). Although their vocal repertoire is limited, it can
provide listeners with an open-ended, highly modifiable, and cognitively rich set
of meanings (Cheney and Seyfarth, this volume). In some cases, such as alarm calls,
the context eliciting a vocalization is narrowed down to the eliciting stimulus, in
this case the type of predator or danger. However, vocalizations given in other
contexts, such as during social interactions, depend on both the immediate social
context and the history of interactions between particular individuals.
Several haplorrhine primates have been reported to produce acoustically
distinct alarm calls for different types of predators, the so-called functionally
referential alarm calls (Seyfarth and Cheney 2003). Group-living lemurs have
developed different kinds of alarm call systems, from functionally referential
alarm calls in ring-tailed lemurs (Macedonia 1990; Pereira and Macedonia 1991),
412
C. Fichtel and P.M. Kappeler
to arousal-based alarm calls in ruffed lemurs (Macedonia 1990), and a mixed alarm
call system in sifakas and redfronted lemurs (Fichtel and Kappeler 2002). The
mixed alarm call system consists of functionally referential alarm calls for raptors
and general alarm calls that are given in response to predators and other threats.
Interestingly, the same sort of alarm call system has also been suggested to exist in
some New World monkeys, i.e., saddleback tamarins (Saguinus fuscicollis), as well
as white-faced and tufted capuchin monkeys (Fichtel et al. 2005; Kirchhof and
Hammerschmidt 2006; Wheeler 2008). Nocturnal strepsirrhines do not seem to rely
on early warning of predators, but produce general alarm calls that are primarily
directed to predators or conspecifics (reviewed in Fichtel 2007). These calls may be
the ancestral form of primate alarm calling.
As suggested by Cheney and Seyfarth (this volume), the vocalizations that
baboons give during social interactions depend on both the immediate social
context and the history of interactions between particular individuals. Although
several lemurs are group-living, the usage and potential function of vocalizations
during social interactions have not been studied yet. In the context of group
coordination, some haplorrhine species produce a particular travel call to initiate
group movements (reviewed in Boinski and Garber 2000). Sifakas converge in
several fundamental proximate aspects of group coordination, but they do not use a
particular call or other signals to initiate group movements (Trillmich et al. 2004).
Finally, several similarities seem to exist across primates, including humans, in
acoustic features of the expression of the caller’s arousal or emotional state.
Specifically, primates use common principles, such as an energy shift towards
higher frequencies, to encode basic emotions in vocalizations (Fichtel et al. 2001;
Fichtel and Hammerschmidt 2002, 2003; Scheiner et al. 2002, 2006; Hammerschmidt and Jürgens 2007). Most of the basic emotions of humans appear to have
deep phylogenetic roots, which extend back to the common ancestors of haplorrhine and steprshirrine primates (Fessler and Gervais, this volume).
However, in the domain of visual communication, i.e., facial expressions and
gestures, strepsirrhines clearly differ from haplorrhine primates. Only a few facial
expressions have been reported in aggressive contexts in ring-tailed lemurs (Pereira
and Kappeler 1997) and during play (play-face) in sifakas (pers. observ.). The use
of manual gestures seems to be almost absent, though several nocturnal lemurs
(red-tailed sportive lemurs (Lepilemus ruficaudatus), mouse lemurs) use a shaking
fist to threaten conspecifics or predators (pers. observ.). Strepsirrhine primates also
exhibit some body gestures constituting conspicuous displays. Just to list a few of
them, lorises, for example, raise their arms around their head while moving their
body in cobra-like fashion when threatened (Charles-Dominique 1977), male ringtailed lemurs exhibit conspicuous displays during their famous “stink-fights”: while
standing bipedally, they move their tail through the legs, “parfume” it with their
antebrachial glands, and wave it in a stereotyped fashion in front of opponents
(Jolly 1966a). Redfronted lemurs exhibit a friendly reciprocal arm-over display in
which they put the proximal arm over the partner’s back (Pereira and Kappeler
1997), and sifakas move their heads abruptly into the neck and back when aroused
(Jolly 1966a). Nevertheless, in haplorrhine primates, gestural signals seem to be
19
Lemur Cognition and Behavior
413
more flexible and subject to cultural transmission (Pollick and de Waal 2007;
Fogassi and Ferrari 2007). For example, gestures in apes vary not only between
social groups but also culturally between populations (Whiten et al. 1999; van
Schaik et al. 2003; Pollick and de Waal 2007), leading to the hypothesis that the
flexible use of gestures in combination with enhanced cognitive capacities played a
crucial role in the evolution of human language (Arbib et al. 2008, Cheney and
Seyfarth, this volume, Tomasello and Moll, this volume).
19.5
Discussion and Conclusions
This review indicates that primates are more heterogeneous with respect to aspects
of social complexity and social intelligence than in the realm of physical intelligence. Even though only a few lemur species have been tested on various tasks, and
the social structure of only a few species has been studied in detail in the wild,
preliminary conclusions about the presence or absence of certain abilities and traits
are beginning to emerge. More detailed comparisons of the level of cognitive
performance have to await more tests with the same experimental paradigms in a
larger number of lemur species, and more detailed comparisons of aspects of social
structure related to social intelligence require additional studies of lemurs, New
World monkeys, and colobines. On the basis of the available information, however,
it is possible to begin characterizing the cognitive abilities of lemurs and to outline
the cornerstones of their social complexity, but it remains difficult to separate traits
that may have been present in the earliest primates and or haplorrhines from specific
adaptations of lemurs that have evolved over the past 50 million years.
In most domains of technical cognition, in which experimental tests have been
conducted, strepsirrhines seem to have the same sort of basic cognitive abilities as
other primates, and the performance of lemurs was, in most cases, quantitatively not
different from that of other primates (summarized in Table 19.1). In the domain of
“space and objects,” lemurs have a route-based mental representation of spatial
relationships, show straight-line traveling, and efficient goal-directed movements
between distant sites. They also search for hidden food and are able to solve detour
problems. Lemurs manipulate objects less than most haplorrhines, but that might be
due to the dominance of their olfactory sense and the less dexterous use of their
hands. In discrimination learning tasks, they appear to be a bit slower and more
error-prone, but in learning-set tasks, they are as skilled as other primates. They also
seem to have some cross-modal skills, and sorting tasks are mastered just as well by
lemurs as by New World and Old World monkeys. Though their numerical discrimination skills seem to be inferior, they understand the outcome of simple
arithmetic operations. Tool-use and the associated abilities are a striking exception
from the lack of fundamental differences from haplorrhines in this cognitive
domain. Thus, if we simply consider whether strepsirrhines are able to perform a
certain task, their cognitive abilities in physical domains are, by and large, comparable to those of New and Old World monkeys.
414
C. Fichtel and P.M. Kappeler
Table 19.1 Summary of studies of technical and social intelligence of lemurs and other strepsir
rhines. The main categories in the left column are described in the text. The central columns
summarize the names of species, in which the corresponding abilities or traits have been demon
strated
Categories
Examples
References
1. Technical intelligence
1.1 Space and objects
l
Spatial memory
Mouse lemurs, redfronted lemurs,
Milne Edwards Sifakas
l
Hidden objects
l
Invisible
displacement
l
Detour problems,
Bent wire, Maze
experiments
Bushbabies, Bamboo lemurs, Brown
lemurs, Mongoose lemurs,
Redfronted lemurs, Ringtailed
lemurs, Pottos
Bamboo lemurs, Mongoose lemurs,
Redfronted lemurs, Ringtailed
lemurs
Ringtailed lemurs, Mouse lemurs
Joly and Zimmermann (2007),
Lührs et al. (2009), Erhart
and Overdorff (2008)
Jolly (1964a,b), Deppe
et al. (2009)
Deppe et al. (2009)
Davis and Leary (1968), Picq
(1993, 2007)
1.2 Tools and causality
l
Object manipulation
l
Simple box
l
Complex box
Bushbabies, Pottos, Mouse
lemurs, Brown lemurs
Jolly (1964a,b), Parker (1973,
1974), Ehrlich et al. (1976),
Torigoe (1985), Renner et al.
(1992)
Ringtailed lemurs, Mouse lemurs, Kappeler (1987), Fornasieri et al.
(1990), Anderson et al. (1992),
Black lemurs, Brown lemurs,
Schilling (2007)
Ringtailed lemurs
Mouse lemurs, Aye Ayes
Schilling (2007), Digby et al. (2008)
Tool use
l
l
Wild, captivity
Understanding of
tools
Ringtailed lemurs
Santos et al. (2005a)
1.3 Features and categories
l
Learning sets
l
Reversal learning
l
Cross modal transfer
Delayed response
l
l
l
l
l
l
Oddity
Serial ordering
Quantities
Estimating
numerosity
Simple arithmetic
operations of 1 + 1
Bushbabies, Lorises, Black
lemurs, Ringtailed lemurs
Brown lemurs, Ringtailed
lemurs, Fork marked
lemurs, Mouse lemurs
Bushabies
Bushabies, Ringtailed lemurs,
Ruffed lemurs
Ringtailed lemurs
Ringtailed lemurs
Stevens (1965), Cooper (1974), Ohta
(1983), Ohta et al. (1984, 1987)
Stevens (1965), Rumbough (1997),
Picq (1993, 2007)
Mongoose lemurs
Lewis et al. (2005)
Ringtailed lemurs, Brown
lemurs, Mongoose lemurs,
Ruffed lemurs
Santos et al. (2005b)
Ward et al. (1976)
Harlow et al. (1932), Maslow and
Harlow (1932), Jolly (1964a)
Davis et al. (1967)
Merritt et al. (2007)
(continued)
19
Lemur Cognition and Behavior
Table 19.1 (continued)
Categories
Examples
2. Social intelligence
415
References
2.1 Social complexity and structure of social relationships
l
l
l
l
l
Coalitions
Redfronted lemurs, Ringtailed
lemurs
Cooperation
Mouse lemurs
Post conflict behavior Redfronted lemurs, Ringtailed
lemurs, Black lemurs, Sifakas
Grooming networks
Dominance
relationships
Redfronted lemurs
Ringtailed lemurs, Sifakas
Ostner and Kappeler (2004),
Sussman (1992), Jones (1983)
Eberle and Kappeler (2006, 2008)
Kappeler (1993b), Rolland and
Roeder (2000), Palagi et al.
(2005), Roeder et al. (2002),
Palagi et al. (2008)
Port et al. (2009)
Kappeler (1993b), Pochron et al.
(2003)
2.2 Tactical deception and other related skills
l
Tactical deception
Redfronted lemurs, Ringtailed
lemurs, Brown lemurs
l
Learning to deceive
Self control
Gaze following
Black lemurs
Brown lemurs, Black lemurs
Ringtailed lemurs
l
l
L. Pyrritz pers. com., Deaner et al.
(2006), Genty and Roeder
(2006)
Genty et al. (2008)
Genty et al. (2004)
Anderson and Mitchell (1999),
Shepherd and Platt (2008), Ruiz
et al. (2009)
2.3 Social learning and innovations
l
Social learning
l
Innovations
Behavioral variation
l
Ringtailed lemurs, Brown lemurs, Feldman and Klopfer (1972),
Glander et al. (1985), Jolly and
Black lemurs, Ruffed lemurs,
Aye aye
Oliver (1985), Kappeler (1987),
Fornasieri et al. (1990),
Anderson et al. (1992), Gosset
and Roeder (2001), Krakauer
(2005)
Ringtailed lemurs
Hosey et al. (1997)
Sifakas
Fichtel and van Schaik (2006)
2.4 Vocal communication
l
l
l
Functionally
referential alarm
calls
Group coordination
Expressions of
emotions
Ringtailed lemurs, Redfronted
lemurs, Sifakas
Sifakas
Redfronted lemurs
Macedonia (1990), Pereira and
Macedonia (1991), Fichtel and
Kappeler (2002)
Trillmich et al. (2004)
Fichtel and Hammerschmidt (2002)
On the basis of a meta-analysis of global cognition variables (detour, patterned
string, invisible displacement, tool-use, reversal learning, oddity sorting, and
delayed response), in which they ranked the performance of species, Deaner et al.
(2006) concluded that strepsirrhines were inferior to most haplorrhines, but better
than marmosets and talapoin monkeys (Miopithecus talapopin). This data set,
however, included the performance of strepsirrhines in only four of the nine tasks
416
C. Fichtel and P.M. Kappeler
that were compared across species. Tomasello and Call (1997) identified 15 paradigms in the domain of physical cognition to which various primates were subjected. Our review revealed that strepsirrhines were able to perform successfully in
12 of these paradigms, but the level of performance in some tasks was not up to par
with haplorrhines. Nothing is yet known about strepsirrhines’ ability to understand
natural categories and the conservation of quantities. Thus, the grade shifts in brain
size are not reflected by fundamental gaps in performance in these spatial and
physical abilities among primates; but great apes, in particular, are superior on
several tasks.
In the realm of social intelligence, lemurs exhibit a number of traits that differ
from those described for the better-known haplorrhines, despite basic similarities in
several aspects of social organization, such as the multi-male, multi-female composition of groups, the existence of dominance relations, and female philopatry.
However, within-group coalitions, even between mothers and daughters, are
extremely rare or absent altogether, postconflict reconciliation is also rare, but
some basic exchange between grooming and other social commodities may exist.
Very limited preliminary evidence suggests that some basal aspects of tactical
deception exist and that lemurs can follow the gaze of conspecifics. Social learning
abilities are more widespread among lemurs, but true innovations of novel behaviors are apparently rare. As demonstrated by the study of behavioral variation in
the meaning of alarm calls among sifaka populations, however, more discoveries of
innovations and variation among populations are likely, once more than one
population is considered as the representative of its species. Finally, lemurs also
vocalize with functionally referential vocalizations, exhibit coordinated group
movements, and express their emotional status in structural features of their vocalizations. However, in the domain of visual communication, i.e., the use of gestures
and facial expressions, strepsirrhine primates clearly differ from haplorrhines and
use less variable signals.
With this information, the outlines of a proto-typical primate social structure and
social cognition begin to emerge. Many basic features of social complexity exist,
albeit often in rudimentary form, in lemurs, so that the observed variation among
major primate radiations is primarily one in quantity, rather than in quality. It is
striking that lemur social relationships differ most from the better-known haplorrhine models. Lemurs exhibit more similarities in this respect with New World
primates (e.g., small group size, female competition) (Wright 1997). More detailed
studies of additional New World monkeys, but also colobines, are required to
establish cercopithecine monkeys with maternal rank inheritance as the typical
haplorrhine reference for comparison with other primates (see also Strier 1994).
It is, therefore, difficult to evaluate the observed differences in social structure
between lemurs and haplorrhines. Because the traits where lemurs deviate most
obviously are functionally related to intense within-group competition, they may
represent lemur autapomorphies, rather than primate symplesiomorphies. These
lemur idiosyncrasies are thought to reflect either adaptations to unusually harsh
ecological conditions (Wright 1999) or an intermediate stage in a transition from
pair-living to group-living (van Schaik and Kappeler 1996).
19
Lemur Cognition and Behavior
417
In the quest to identify human behavioral universals, a broader comparative
perspective is useful. By acknowledging the biological continuity of some traits and
abilities across the primate lineage, more focused comparisons and reconstructions
among the various species of Homo, Pan, and their common ancestors are possible
(Chapais, this volume). Moreover, by mapping social and cognitive variation on the
full range of primate brain sizes, major grade shifts during primate evolution will be
easier to recognize; for this, a more fine-grained data set that includes more
stresirhhine species will be needed. Finally, lemurs should no longer be regarded
as our embarrassing relatives, because their cognitive abilities and social complexity are not as utterly primitive as previously thought by some.
Acknowledgments We thank Joan Silk and an anonymous referee for very helpful comments on
an earlier version of this manuscript.
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Part VII
Innovation & Culture
Chapter 20
Ape Behavior and the Origins of Human Culture
Andrew Whiten
Abstract Identification of behavioral characteristics shared by humans and our
closest primate relatives allow us to reconstruct the nature of our shared behavioral
ancestry. I use this approach to infer the core features of social learning, traditions,
and culture that characterized our ancestry before the evolutionary split that created
chimpanzees and hominins. An extensive corpus of field observations and behavioral experiments has, in recent years, provided a substantial empirical basis
through which to realize this approach to our cultural past. Features of culture
shared by ourselves and chimpanzees, and thus likely to have been shared also by
our common ancestor around 6 million years ago, include (1) the capacity to sustain
different local cultures composed of multiple and diverse traditions, both technical
and social; (2) related contents of such traditions, such as tool use; and (3) a
portfolio of different social learning mechanisms, extending to both emulation
and imitation, that are flexibly applied to acquire behavioral routines, with net
adaptive benefit. These would have constituted a crucial platform from which our
own unique and complex cultural nature evolved.
20.1
Introduction: The Culture Gap
Culture is apt to be high on anybody’s list of the key cognitive and behavioral
universals that separate us as humans from the rest of the animal kingdom. Some
might put culture at the top of the list, concurring with the adage that “culture
maketh man.” Only our species has left an archeological record of cultural progress
that stretches back over 2.5 million years to the earliest stone tools, and in the
meantime has generated such rich cultural practices, from our languages to our
A. Whiten
Centre for Social Learning & Cognitive Evolution and Scottish Primate Research Group, School
of Psychology, University of St. Andrews, St. Andrews, Scotland
e mail: aw2@st andrews.ac.uk
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 20, # Springer Verlag Berlin Heidelberg 2010
429
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technologies, that the culture into which a person is born will explain vast swathes
of their behavioral repertoire. Accordingly, human beings may behave in very
different ways according to the local cultural practices into which they are inducted.
Through these diverse cultures, humans have come to dominate almost every part
of the planet. The competitive edge this has given us is underlined by the fact that
our 6-billion-plus population is all too successfully exterminating our closest sister
species, the other apes, whose total world population has dwindled to the equivalent
of just one modest human township.
All this means that at first sight, the gap between the riches of human culture and
anything remotely resembling them in the rest of the animal kingdom may appear
unbridgeable. To the contrary, in this chapter, I will argue that a considerable
variety of aspects of the social learning and culture that characterize our own
species are at work in other primates, and some are yet more widely distributed
in other groups of animals; more particularly, in our closest relatives, the apes, we
find some features distinctively closer to human ones. The result is a substantial
literature of empirical findings that bear directly on questions about the evolutionary roots of culture. This presents a big contrast with that other distinctive human
universal, language, where there appears much less one can identify in other species
to directly bridge the evolutionary gap. In the case of culture, we are more lucky: in
recent decades, we have discovered an empirical goldmine in the present state of
the natural world that permits many inferences about the ancient origins of social
learning, traditions, and culture.
20.2
Some Definitions
In the animal behavior literature, there is a long-standing tendency to treat the terms
“culture” and “tradition” as equivalents (and of course, they are often used interchangeably in talking of humans too), but among authors who do distinguish the
two terms, “tradition” is the less contentious and so is a good place to start.
“A distinctive behavior pattern shared by two or more individuals in a social unit,
which persists over time and that new practitioners acquire in part through socially
aided learning,” the definition of a tradition offered by Fragaszy and Perry (2003:
xiii) would likely be acceptable to most researchers. “Social learning” is a crucial
ingredient and can be defined simply as learning from or through others, as distinct
from individual learning, in which the learner is dependent on only their own
resources. Numerous kinds of social learning have in turn been distinguished and
defined (Whiten et al. 2004) and several will be discussed further below.
Some writers, typically because they recognize that there is so much more to
human culture than is covered by the above definition of a tradition, insist on
additional criteria before they are willing to talk of “culture” in nonhuman animals.
Such criteria vary between writers (nicely illustrated in the range of approaches
offered in Laland and Galef 2009), but examples include (1) features taken to be
distinctively human forms of social transmission, notably imitation and teaching
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(Galef 1992); (2) signs that individuals perceive and react to transgressions of
cultural norms (Perry 2009); and (3) the existence of multiple traditions (Whiten
and van Schaik 2007). In this chapter, I continue with this last approach, because
when we talk of different human cultures, we are typically picking up on the
existence of differences across multiple traditions, including communication patterns (language/dialect, forms of greeting), artifacts, and cuisine.
20.3
How to Trace the Origins of Culture
We have no time-machine, so all approaches to tracing the origins of culture
are indirect and rely upon various kinds of inference. Nevertheless, a range of
different methods, used in conjunction, has allowed us to build up a compelling
picture of the past.
One such approach is to recover concrete remains, such as stone tools (Whiten
et al. 2009a). This has yielded artifacts that, due to their material form, remain
very much as they were a million years ago or more. However, we cannot witness
the behavior of those who made them. This, of course, is exactly what we can do
in the case of living primates. The question is, how can such present-day observations be used to reconstruct the past? Various approaches have been used, but
the most robust has been called the comparative method. If we study a related group
of species such as the great apes and find they all share a behavioral and/or
cognitive characteristic, then we infer that this is the result of descent from a
common ancestor that exhibited this character. This is the basis of the first of
three components of my own approach to the evolutionary origins of culture,
which I outline next.
20.3.1
The Comparative Method
In the example above, a characteristic shared by the great apes is attributed to the
common ape ancestor that lived approximately 14 million years ago. But by
zooming in to more closely related subsets of species, or alternatively zooming
out to bigger groups of more distantly related taxa and applying the same principle,
one can identify the characteristics of more recent and more distant common
ancestors respectively, and thence trace the evolutionary history of the characteristic
of interest in our case, some aspect of culture. So, for example, characteristics
shared by the two living species of chimpanzee (common chimpanzee and bonobo)
would be attributed to their recent common ancestor living just over 2 million years
ago, whereas characteristics shared by all primates will be attributed to their more
ancient common ancestor of around 65 million years ago.
It is also possible that any taxa will share a characteristic because it evolved
more than once, independently, but such convergent evolution is less likely than
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A. Whiten
shared ancestry when we are looking at taxa that are genetically most closely
related to each other. This and other considerations about using the comparative
method to reconstruct ancestral states are described in more depth by Harvey and
Pagel (1991) and Byrne (1995).
20.3.2 Dissecting Culture
Recent years have seen an enormous flowering of research on the subject of animal
culture (Whiten and van Schaik 2007; Laland and Galef 2009). Too often, however,
the over-simplistic question put in both academic and popular debates is whether
such-and-such a species “has culture.” The all-or-none assumption embodied in this
habit is not helpful to the goal of a serious scientific analysis of the subject.
Instead, my colleagues and I have advocated the dissection of cultural phenomena
into a set of elements, the distribution of which cannot be assumed to be correlated
across species, as it would be if some species had all the shared features and the
rest had none (Whiten et al. 2003; Whiten 2005, 2009). It will likely be more
productive to investigate the extent of such correlations between features as an
empirical question, acknowledging that the distribution of the various elements
may be somewhat mosaic-like across the animal kingdom.
There will be many ways to construct taxonomies of such elements but in the
papers cited above, I have begun to focus on three major aspects of culture: (1) the
population-level distribution of cultural variations; (2) the behavioral contents of
these; and (3) the underlying social transmission mechanisms. These, in turn, can be
dissected into subelements that may also vary independently among species. The
bulk of this chapter is concerned with these three major elements of culture as well
as significant subelements.
20.3.3 Eclectic Methodological Approaches
The best ethology often combines an eclectic mixture of methodological
approaches. Pure observation of natural populations is a crucial first step to map
out the natural forms of the phenomena of interest, and in the case of culture,
particularly to identify local variations in behavior likely to constitute traditions.
But it is difficult, through pure observation, to identify the crucial ingredient of
social learning, and even more difficult to discriminate particular forms, such as
imitation: experimental and control conditions (the first of these typically permitting social learning, the second preventing it) have much greater power to identify
the underlying learning processes and so offer a vital complement to observational
approaches. Conducting such experiments in the field represents another ethological ideal, but experiments performed under the more readily controlled conditions
of captivity may also play their part. This has proved very important in the case of
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social learning in primates, given the logistic and, sometimes, ethical constraints on
experimentation in the wild.
20.4
Dissecting Culture and Tracing its Origins
In the core empirical sections of this chapter, I apply the approaches described
above, discriminating multiple different aspects of culture through the eclectic mix
of methods advocated, and reconstructing ancestral evolutionary states via the
comparative method. However, to do so, with the thoroughness that the empirical
material now at hand would permit, is beyond the scope or scale of the current
chapter. In particular, to deal with each of the 14 elements of culture I distinguish,
for each of the multiple shared ancestral states one could trace in our past (ape
ancestor, primate ancestor, and so on: see Dawkins 2004, who dubs these common
ancestors “concestors”) is not feasible here. Given the topic of human universals, I
therefore choose to focus on the concestor of ourselves and our nearest living
relatives, the chimpanzees (Pan spp.). This concestor lived approximately 6 million
years ago. Reconstructing its cultural life makes very clear that our unique and
overblown capacity for culture, although vast when compared with the chimpanzee’s,
did not spring out of the blue; instead it evolved from an ancestral state that provided
many different foundations for the cultural phenomena that evolved in later
hominins. Of course, we do not simply take the chimpanzee as a model of our
concestor; instead we infer the cultural nature of this concestor from the features of
culture that the descendant taxa, Pan and Homo, can be shown to share.
Recent decades of research have identified “culture” in the sense of socially
transmitted traditions, in a variety of mammals, birds, and fish (Laland and Hoppitt
2003; Whiten and Mesoudi 2008). My approach here is to raise the threshold
criteria for cultural phenomena beyond this: to adopt as minimum threshold criteria
the existence of traditions and/or social learning, but then to additionally pick out
several key respects in which human culture goes beyond these starting points and
assess in what ways, if any, chimpanzees do so also, with the implications for the
concestral state outlined above. In principle, this analysis could be applied in any
comparative analysis of different taxa, and for reconstructing the cultural life of
corresponding concestors. Table 20.1 offers a simplified chart summarizing the
analyses set out below.
20.4.1 The Large-Scale, Population-Level Patterning
of Traditions
Multiplicity of traditions. Human cultures differ over time, and space, and these
differences can typically be mapped in terms of traditions of many different kinds.
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A. Whiten
Table 20.1 The scope of culture in chimpanzees, humans, and our “concestor.” The table lists
features of culture (1) shared by chimpanzees and humans; (2) distinctive in humans. Twelve
aspects of culture are discriminated within three broad categories. Each aspect represents one way
in which “culture” extends beyond the mere existence of a tradition: see text for a fuller
description. Each aspect is suggested by the scope of human culture, but there is some evidence
bearing on each in our closest relative, the chimpanzee; these features thus offer an outline of the
cultural nature of our common ancestor of approximately 6 million years ago
Hypothesis
Shared features
Distinctive in humans
1. Population level
patterning
Traditions have become so
1.1 Multiplicity of
Traditions very numerous
numerous as to be countless
traditions
compared with other species:
over 40 in both species
Local cultures are distinguished
Each local culture is defined by
1.2 Communities
by vast numbers of different
a unique suite of traditions,
differing culturally
traditions
in chimpanzees currently
in multiple ways
documented as between 9
and 24
Remains to be determined: some Occurs (e.g., LeVine 1984) but
1.3 Clustering of
clusterings may be consistent
traditions through
status is debated (Boyd et al.
1997)
with this
core ideas
1.4 Cumulative cultural
Minimal at best, and disputed
Extremely elaborate and
evolution
progressive (Tomasello 1999)
2. Content of cultures
Includes tool construction
2.1 Physical
Includes non tool foraging
methods, material culture for
(non social)
techniques as well as tools
hunting, trapping, clothing,
fashioned and used for
medicine, shelters, and more
foraging, comfort, hygiene
2.2 Social behavior
Includes social use of tools (e.g., Includes language and other
symbolic conventions, moral
leaf clip in courtship),
norms, ceremonies, and
grooming conventions,
institutions
possibly dialects
3. Social learning
processes
3.1 Copying
A portfolio of social learning
Higher fidelity copying of
mechanisms available
complex actions routine
including imitation and
emulation, capable of
recognizably faithful copying
of sequences of action across
repeated transmission
In some contexts, less selective
3.2 Selective
Selective copying modulated by
than apes, generating blanket
acquisition
sensitivity to causal and
copying (“over imitation”)
intentional structure of tasks
Strong conformity common,
3.3 Conformity
Limited evidence for tendency
extending to ready acquisition
to copy majority, even when
of arbitrary conventions, such
alternatives known of
as gestures
Able to upgrade sophistication of
3.4 Ratcheting
Cumulative social learning
repertoire by observational
constrained, in part by great
learning
conservatism
(continued)
20
Primate Behavior and the Origins of Culture
Table 20.1 (continued)
Hypothesis
3.5 Recognition of the
copying process
3.6 Teaching
Shared features
Chimpanzees and other apes have
been able to learn rule
“Do as I do”
Minimal “scaffolding” in limited
contexts at best; disputed
435
Distinctive in humans
Extended to intentional education
and propaganda
Now common in some contexts,
but apparently rare in hunter
gatherers so likely rare until
very recently everywhere
The culture of ancient Egypt, for example, differed from that of Egypt in more
recent times in a host of ways that include forms of communication, tools, weapons,
food, and religion, and just the same can be said of the contrast between Egyptian
culture and other, very different, but contemporaneous cultures, such as that of, say,
Scotland.
As noted above, traditions have been identified in many different animals too,
but typically, this is just one tradition per species or per study. Black rats in Israeli
pine forests have been shown to transmit special techniques for stripping seeds from
pine cones across generations, but that is all (Terkel 1995). Many birds have been
shown to pass on local dialects, but that is all (Catchpole and Slater 1995). In these
cases, we can talk of a tradition as defined above, but not cultures in the richer sense
defined by multiple traditions in our own species.
However, there are intermediate cases where multiple traditions have been
identified and these are the most diverse in the species with which we shared the
most recent common ancestor, the chimpanzee. The pooling of decades of recordings
from long-term field studies across Africa has revealed over 40 behavior patterns
that vary locally in ways that appear inconsistent with either genetic or straightforward environmental explanations, and so have been classed as putative traditions
(Whiten et al. 1999, 2001; Whiten 2005). In many cases, this inference is strengthened by data on the intense observation of the skills of their elders by the youngsters,
sometimes extending to correlations between generations in the style of technique
adopted (Lonsdorf 2005). A considerable variety of behavioral domains are implicated, including tool use, foraging techniques, social behavior, grooming methods,
and courtship gambits. This suggests that the richness of human culture, while
unmatched by any other species, did not spring from nowhere but instead evolved
from a state that already represented unusual cultural complexity compared with
other animals. The later finding that orangutans show very nearly as many different
traditions, encompassing social and material domains (van Schaik et al. 2003),
indicates that this state of affairs likely characterized the common ancestor of all
the great apes.
Further studies in recent years have indicated multiple traditions in other taxa,
although typically no more than a handful in each case. Killer whale communities,
for example, can be differentiated not only by their vocal repertoires but also by
their very different hunting targets (salmon versus seals) and social organization
(Rendell and Whitehead 2001). Bower birds in different localities exhibit bower
436
A. Whiten
styles that differ in multiple respects, such as overall architecture and the nature and
color of objects chosen for decoration (Madden 2008). Whether, as more such longterm studies accumulate, other taxa will be found to show the cultural complexity of
apes remains to be seen.
In all these studies, however, it is difficult through pure observation in the field to
establish that the variations are truly due to social transmission and not other
alternatives, such as genetics or environmental constraints. In recent years, the
chimpanzee field data have, therefore, been complemented by an extensive series of
social learning experiments of a particular kind “diffusion experiments” which
are explicitly designed to test for the spread of new behaviors through populations
(Whiten and Mesoudi 2008). In all but one of six such studies with chimpanzees, all
concerning different artificial foraging tasks, clear evidence of transmission
occurred in the most ambitious cases, including the spread of different behavioral
variants across a series of three groups with high fidelity, perhaps, in part, because
this employed the most complex foraging routines employed in these studies to date
(Whiten et al. 2007). However, the latter study relied on us allowing one group to
watch another in an adjacent compound, a scenario not possible in the wild, because
of intercommunity hostility (see Crofoot and Wrangham, this volume). Instead,
transmission is thought to occur in the wild though intercommunity transfer of
individuals, typically females. Simulating this experimentally would therefore be a
valuable next step. Nevertheless, the studies to date demonstrate that chimpanzees
have a capacity for cultural transmission consistent with the interpretation of
behavioral variations in the wild as representing extensive cultural variation.
Communities differing culturally in multiple ways. A related but different discovery has been that each chimpanzee community expresses a unique profile of
behavioral variants that define it culturally, implying a qualitative similarity to the
human case in this respect (Fig. 20.1). Of course, in quantitative terms, the
difference remains huge: on present evidence, the number of traditional behaviors
Fig. 20.1 The putative cultures of wild chimpanzees (after Whiten 2005). “Customary” acts are
those typical in a community, “habitual” are less frequent yet consistent with social learning. Each
community displays its own profile of such local behavioral variants, providing evidence of a
unique culture for each locality. Numbers identify behavior patterns in the catalog attached to
Whiten et al. (1999) and illustrated at http://culture.st and.ac.uk/chimp
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Primate Behavior and the Origins of Culture
437
Fig. 20.2 The results of diffusion experiments in captive chimpanzees. Each rectangle represents
a chimpanzee with two character ID codes. Tasks, named in the center, were available in both
local populations named on either side, but different techniques, color coded here, were seeded in
one individual, marked here as No. 1, in each population. The Doorian experiment was run as a
“transmission chain,” as indicated by the arrows; all other experiments involved “open diffusion,”
with no predetermination of potential order of transmission. At Bastrop, transmission extended
from groups B1 to B2 and B3, and from B4 to B5 and B6. Handclasp grooming spread spontane
ously in the FS1 population. Numbers represent order of acquisition for each task. For further
explanation, see text and Whiten et al. (2007). These studies demonstrate the capacity of chim
panzees to sustain multiple traditions cultures, consistent with the interpretation of regional
variations among wild chimpanzees summarized in Fig. 20.1
of any one chimpanzee community may typically lie in the teen numbers, whereas
for humans, such variations can be numbered in the thousands indeed, considering
all the differences between any two different languages alone, it may be more apt to
say the variations are “countless.” The captive diffusion experiments with chimpanzees have essentially replicated in controlled conditions, the multiple-tradition
cultures inferred to exist in the wild (Fig. 20.2), although on a smaller scale (Whiten
et al. 2007).
Whether the great apes are truly as different from other species, as this picture
currently suggests, or whether studies of other species will eventually identify
similar complexity, remains to be seen. This is a young science. One recent study
that underlines this caution concerns the peculiar behavior of stone-handling in
Japanese macaques, where the case for cultural transmission is strong because the
behavior appears so manifestly functionless (and thus unlikely to be shaped by
438
A. Whiten
environmental conditioning), and careful documentation has traced its emergence
and steady spread from its earliest manifestations (Huffman 1996). The most recent
studies have charted the variations between groups in numerous aspects of the way
the stones are handled, generating a chart of behavioral variations approaching the
complexity of that drawn for the great apes (Leca et al. 2007). One difference that
remains, however, is that in the macaques these are all just minor variations on the
same behavior, stone-handling, whereas those of chimpanzees span all major
modes of behavior.
Core cultural clustering. The analyses of the two sections above view culture as
made up of relatively particulate items traditions that can be enumerated. This is
consistent with the controversial idea that culture can be analyzed in terms of
cultural inheritance units analogous to genes, dubbed “memes” by Dawkins (1976).
Early cultural anthropology largely embraced this approach, but later in the
twentieth century anthropologists stressed the interconnectedness of elements of
culture “an organization of ideas rather than aggregate of independent traits”
(LeVine 1984). Precursors of this in a nonverbal creature might be sought in the
existence of correlated clusters of behavioral variants that would implicate core
cognitive orientations influencing a whole suite of different behaviors. Possible
examples of this in chimpanzees are discussed by Whiten et al. (2003) and include
the tendency for some communities to tackle varied problems in their environment
through the application of tools, whereas in other communities this orientation
appears markedly lacking. Possibly the tool-rich cultures may be underlain by a
global cognitive orientation to solve new problems technologically, a hypothesis
that could perhaps be tested by field experiments presenting such problems to toolrich and tool-poor communities. Compared to the data-rich comparisons that can be
made in the two sections above, what can be said on this topic in nonhuman species
remains presently only exploratory and will require more systematic investigations
by field primatologists, in future. That said, just how interconnected are the
components of humans culture remains a controversial issue by itself (Boyd et al.
1997); many elements do behave more like memes, spreading independently of
others, a familiar contemporary example being the spread of mobile telephone use
through very varied existing cultures worldwide.
Cumulative cultural evolution. By common consensus, the principal feature
that distinguishes human culture from what we see in any nonhuman is cumulation. Generation by generation, cultures build on what went before so that in the
later generations, achievements go massively beyond the earlier ones. We see this
very clearly in the technological sphere, where we can trace the process from the
earliest crude stone tools to the sophisticated technologies of today. For particular
cultural items like the gun or the computer, we can trace cultural phylogenetic
trees that chart diversification and progressive complexification over time and
space, in many ways analogous to biological evolution (Mesoudi et al. 2006).
Tomasello (1999) has described the progressive nature of this aspect of culture as
a “ratchet effect.”
At best, there are only very small signs of such ratcheting in apes, although we are
often hampered in examining this because the earliest cultural phases are lost to us.
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For example, chimpanzees in the Goualougo region in central Africa use a tool set
to harvest subterranean termites, first using a stout stick to puncture a deep hole
down into the ground, then withdrawing this and inserting a more delicate stem, the
end of which they fray, such that it then collects a better harvest of termites (Sanz
et al. 2004). This is unlikely to have emerged as a unitary brilliant invention, more
likely developing step-wise from the use of a single probing tool, as with chimpanzees elsewhere, but we have no direct data on this.
However, most of the many traditions documented for chimpanzees show little
evidence of cumulative culture, and cumulation is what has created the enormous
gulf between present-day human and nonhuman cultures, from technology to
language. We do not know why this difference arose. One explanation may
simply be that chimpanzees could benefit little from more advanced cultures in
the niche they have so successfully exploited for millions of years; by contrast,
cumulative culture was a crucial ingredient in our ancestors’ creation of the
socio-cognitive niche of hunting and gathering, that allowed them to survive
and thrive in the drastically changed African habitats that they engaged with
(Whiten 1999a).
A different (although not necessarily mutually exclusive) possibility is suggested by the results of the first systematic experimental investigations of the
question. Marshall-Pescini and Whiten (2008a) first exposed young, wild-born,
sanctuary-living chimpanzees to an artificial foraging task that required sliding
aside a small flap and dipping a stick-tool in the device to obtain small amounts of
honey. In the first phase of the experiment, youngsters were shown to learn this
socially, through watching a familiar human model the skill. In a second phase,
the youngsters saw the same model progress to a more complex action, poking
their stick in a hole to release a catch, so that the existing dipping action could
now be used to lever the whole top of the device open and obtain a rich mixture of
honey and nuts inside. If the chimpanzees would learn this, we would have a
modest case of cumulative cultural evolution. However, they did not: they stuck
to the existing, less productive skill they had mastered. This was not because the
more complex routine was too difficult, for some control subjects who had not
done the first simple dipping action did perform the more complex one. A similar
conservatism was demonstrated in another recent study, in which chimpanzees
who had learned to employ a tool to obtain out-of-reach food failed to copy a more
productive technique some of their own group invented (Hrubesch et al. 2009).
Together, these studies suggest that chimpanzees may exhibit a degree of conservatism that inhibits them from observationally learning to adopt approaches
that are better than their existing habitual ones, an essential requirement for
cumulative culture (Whiten et al. 2009b).
In sum, significant similarities and differences between chimpanzees and
humans have been established dealing with the large-scale patterning of socially
learned behavioral variations in time and space, described in the four sections
above. These suggest that the cultural nature of our concestor would have allowed
them to sustain cultures constituted by multiple behavioral variations, but with very
limited capacity for cumulative change compared with the human case.
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20.4.2
A. Whiten
The Contents of Cultures
Whatever the similarities and differences in the population-level patterning described in the prior section, the actual behavioral content may vary, and this
provides a second source of human-ape comparisons. For example, the content of
human but not ape cultures includes language and religion, whereas chimpanzee
courtship gambits that include various routines of noisy vegetation manipulation
are not used by humans (Whiten et al. 2001). Such comparisons remain to be
completed in any detail and so, by comparison with the foregoing (20.4.1) and
following (20.4.3) sections, the treatment here remains relatively cursory. However,
this is not to minimize its potential significance: as suggested by the examples
above, major differences between human and nonhuman culture exist at the level of
content and there are important shared features too.
Physical (nonsocial) content. This includes both material culture and other
forms of nonsocial behavior. Under the heading of material culture, a major feature
shared by humans and chimpanzees is the use and limited fashioning of tools,
together with the functional contexts of their use, which range across the procurement of dietary items (including water and both vegetable and animal sources
gained through dipping, probing, fishing, pounding, and stabbing techniques),
“hygiene” such as dabbing and wiping feces, blood, and semen, and protection
from elements, as in hat and seat-making (McGrew 1992; Whiten et al. 2001;
Pruetz and Bertolani 2007). Not all of these should necessarily be attributed to our
concestor some specific instances will surely have evolved in the interim period in
chimpanzees, as they so clearly have for humans but the general capacity for
learning a diversity of forms of tool use was likely there in our concestor, and
indeed in the concestor of all the great apes, judging by the technological repertoire
of orangutans (van Schaik et al. 2003).
Where did the differences begin? One important step taken by our hominin
ancestors but not by chimpanzees is the construction of tools, by fitting together
subcomponents. However, archeological evidence for this becomes available only
much later than another that appears beyond chimpanzees but is still in the
destructive mode: the fashioning of symmetrical stone tools (Acheulian blades:
Mithen 1999; Whiten et al. 2003), discussed in detail later in this chapter.
Other shared aspects of content include complex, non-tool foraging techniques,
and, perhaps, even medicinal plant use, which can require crucial discrimination
between medicinal and poisonous elements, such as pith versus other parts of the
plant (Whiten 2006).
Social behavioral content. On the basis of current published evidence, forms of
social behavior that are socially learned are less common than the nonsocial ones
outlined above. They include the categories of courtship gambits and physical
social interaction, although in both of these the precise forms of behavior differ
between chimpanzees and humans. Examples in chimpanzees include the A-frame
“grooming hand-clasp” in which each of two grooming chimpanzees grasps one
hand of the other above their heads (McGrew and Tutin 1978) and the more recently
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441
described “social scratch” that takes different forms in different communities
(Nakamura et al. 2000). Of course, an important category of social behavior is
communication, but here there is relatively little evidence for a role of social
learning in chimpanzees, with early signs of dialect differences giving way to
skepticism (Mitani et al. 1999). Indeed, apart from the courtship gestures noted
above, the most obvious ways in which humans and chimpanzees differ in the social
realm concern the socially learned vocal and gestural repertoire that became so
enormously extensive in humans. The social content of our concestral culture may,
thus, have involved only a relatively restricted set of physical routines and gestures,
compared with the technical content described in the last section.
In humans, additionally, social institutions represent communally agreed forms
of behavior, that community members are expected to abide by (Runciman 2001).
They include such phenomena as marriage, moral norms, and ceremonies. Although
such practices could be learnable to some extent just by observation, it is
difficult to see how they can be established in the first place without linguistic
negotiation, which would put them outside the range of nonhuman apes. The most
likely candidates for shared features here, or at least potential precursors of them,
may be “policing behavior,” in which powerful individuals “keep the peace” in
groups by, for example, breaking up fights by lower-ranking individuals as if they
are contravening some unwritten, preferred, peaceable norms (de Waal 1989).
However, we do not know if such policing is itself socially acquired; the closest
evidence bearing on this is that the tendency towards peacemaking in the sense
of reconciling after fights has been shown through cross-fostering experiments
with monkeys to be influenced significantly by social learning (de Waal and
Johanowicz 1993).
20.4.3 Social Learning Processes
The same category of behavioral content and the same spatio-temporal distribution
of the behavior in the population could be generated by a variety of different social
learning processes, so these provide the third principal basis on which grades of
culture may be compared. In the human case, learning may be in a verbal mode,
including listening to narratives, taking instruction, or engaging in dialog, and
comparative primate research appears likely to cast relatively little light on the
origins of these linguistic processes. Decades of research have focused instead on
identifying and discriminating numerous forms of nonverbal, observational
learning as well as different forms of teaching.
Copying. “To ape” means to copy or imitate. Several observational and experimental studies through the last century offered results apparently consistent with
this image. However, in the 1980s and 1990s, a series of experimental results that
did not support this picture combined with a series of powerful methodological
critiques of the field to yield a much more skeptical view; that imitation in apes
442
A. Whiten
(as well as culture in the wild) remained to be convincingly demonstrated (Tomasello
and Call 1997). Instead, it was suggested that what sometimes looked superficially
like imitation was actually the ape learning about the affordances of some aspect of
the physical world such as, that sticks can help rake in food a process labeled as
“emulation.”
In more recent studies, the pendulum has swung back somewhat, insofar as the
fidelity of transmission of alternative behavior patterns has remained high in what
might be thought of as the more demanding context of diffusion experiments that
involve multiple transmission steps (Whiten et al. 2007). Numerous factors might
potentially explain the differences in experimental results that litter the literature in
this area, together with the different conclusions that have been drawn from them.
For example, the relatively negative results have generally been obtained in the
context of individual testing, in which isolated (yet highly socially oriented)
chimpanzees may not demonstrate their true potential, which they do in the context
of diffusion studies conducted in the group context. At present, we cannot be sure.
But we are faced with the classic asymmetry of positive and negative results if the
results are positive in showing observers matching the actions of a model they
watched, then the animals have demonstrated they have a capacity to copy; if the
results are negative, with no copying, then this might be due to many possible
factors, some of them merely artifactual and misleading. Of course, negative results
are as important as positive ones they are crucial to any comparative science but
they need to be replicated and tested repeatedly before we can acquiesce in
concluding that the capacity at stake is lacking.
The roles of imitative and emulative processes have been further tackled
recently through “ghost” experiments, in which the model is removed from the
scene and the objects normally manipulated by the model are made to go through
their motions by a “ghostly” subterfuge of the experimenter pulling them with
fishing line. Applying this procedure to a tool-based foraging task that had
earlier been shown to diffuse through a group with significant fidelity, Hopper
et al. (2007) obtained the surprising result that chimpanzees could learn nothing
of this by just watching the tool do its job as if moved by a ghostly hand
effectively just what an individual learning by emulation is supposed to be
focusing on. In a follow-up experiment with a much simpler task that involved
merely pushing a door to left or right to obtain the food hidden behind it,
chimpanzees did show evidence of emulation on their first attempt, matching
the direction they saw the door move in, but they quickly abandoned this in later
trials (Hopper et al. 2008). By contrast, they were highly faithful to the direction
in which they saw another chimpanzee push the door, persisting in this over
repeated trials. Being able to actually copy another ape thus seems particularly
important to them.
A very different source of evidence consistent with this came from a study that
recently provided some of the first robust experimental evidence that chimpanzees
will learn how to use a stone hammer to crack nuts (one of the putative local
traditions of wild chimpanzees) through observing a proficient conspecific. In this
study, the observer chimpanzee was several times seen to move its arm in sympathy
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Primate Behavior and the Origins of Culture
443
Fig. 20.3 Matching the
actions of an expert. Two
frames from a video record
of an experiment assessing
the role of social learning in
the acquisition of nut
cracking using a stone
hammer. Here, a naı̈ve young
chimpanzee (top) watches the
performance of an already
proficient nut cracker. The
two frames illustrate the way
in which the observer
spontaneously mirrored the
actions of the nutcracker,
moving its arm up (first
frame) and down (second
frame) repeatedly in
synchrony with tool user. The
video clip, in supplementary
information to Marshall
Pescini and Whiten (2008b)
can be viewed at http://dx.doi.
org/10.1037/0735
7036.122.2.186.supp
with that of the model wielding the hammer (Fig. 20.3), going through the motions
witnessed before ever having grasped the hammer stone (Marshall-Pescini and
Whiten 2008b: video viewable in the electronic supplementary information to
that paper). Similar sympathetic movements in humans (as in watching sports on
the TV) have been interpreted as the effects of “mirror neurons” (Dijksterhuis 2005;
Gallese 2007) that have the special property of firing both when performing and
watching the same behavior, and on these occasions escape the inhibition to which
they must normally be subject while watching others (otherwise, the observer
would helplessly mimic all the other individual is doing). There is evidence that
such neurons are involved in imitation in humans (Iacoboni et al. 2005), so the
serendipitous finding in our nutcracking study may have profound implications for
similarities in the processes underlying how chimpanzees and humans match
conspecifics’ actions.
Imitation has now been demonstrated experimentally in a wide range of species,
including birds (e.g., pigeons and budgerigars) (Zentall 2004). Chimpanzees, like
humans, have additionally been shown, experimentally, to copy such complexities
as tool use and the sequencing of constituent subcomponents actions (Whiten et al.
2004). Something approximating these levels of imitative competence would
accordingly be expected of our concestor.
444
A. Whiten
Selective acquisition. Imitation by infants less than 2 years old has been shown
to be already “rational” insofar as they will copy an adult performing a bizarre act
like bending to switch on a light with their forehead, but only if the adult has their
hands free: if the hands are enmeshed in clothing so that butting the light is simply
the best thing to do, infants who then approach the task with their own hands free
will eschew imitation and instead use their hands to press the switch (Gergely et al.
2002). A similar effect has recently been shown in chimpanzees (Buttelmann et al.
2007). In other experiments, chimpanzees who witnessed a component of a complex task that could be seen to be causally irrelevant eschewed copying that
component, although they copied it in a version of the task where the materials
were all opaque, so causally irrelevant events could not be directly perceived
(Horner and Whiten 2005).
Together these studies indicate that chimpanzees do not blindly or mindlessly
“ape” what they witness; their observational learning incorporates appraisal of both
relevant and irrelevant components. Intriguingly, we found this was less true of
young children tested in the experiment contrasting visible versus invisible causally
irrelevant elements (Horner and Whiten 2005): children copied with high fidelity in
both conditions, a tendency we interpreted as consistent with a more thoroughgoing assimilation of their cultural environments. Under just what conditions
children are discriminating (as in the Gergely study cited above) or instead imitating in “mindless” blanket fashion is the subject of current studies. Developmental
psychologists have begun to describe the latter tendency as “over-imitation” and
have found that in certain contexts, children appear unable to inhibit it even when
strongly encouraged to do so (Lyons et al. 2007). More generally, although our
work has demonstrated a considerable capacity for moderately faithful copying by
chimpanzees, it nevertheless remains the case that a substantial corpus of direct,
comparative studies have shown children to copy with consistently greater fidelity
(Whiten et al. 2004; Call et al. 2005).
Conformity. One mark of humans’ cultural proclivity is conformity. Social
psychologists have provided ample studies showing effects of the kind in which
an experimental subject is exposed to a group who make judgments that fly in the
face of what the subject can perceive with their own eyes (such as that a line A is
longer than B, when it must be obvious to the subject that line B is significantly
longer), yet, the subject goes along with the majority view: they conform (abjectly,
we might say) (Asch 1956).
In a recent social diffusion experiment with chimpanzees, in which alternative
foraging techniques were seeded in each of two separate groups, we found that
some chimpanzees discovered the alternative technique for themselves; nevertheless, when tested again 2 months later, there was a significant tendency for these
innovators to “return to the fold,” conforming to what a majority in their group were
doing (Whiten et al. 2005). We can describe this as conformity to a local “norm” in
the statistical sense, without requiring that chimpanzees conceive of a norm, as
such. Thus, the conformity we recorded may indicate the importance of cultural
transmission to chimpanzees, as it does in the human case. However, Galef and
Whiskin (2008) have followed up our study by showing a similar case of
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445
conformity in rats. Although this concerns only the rats’ choice amongst two foods,
rather than acquiring the complex, tool-based foraging technique that our study was
concerned with, this suggests that the basic phenomenon of conformity may be
more widespread in the animal kingdom. Conformity may take many forms and
occur in different grades, so a systematic program of research on this topic is now
warranted (Laland 2004).
In humans, conformity may be further shaped by active sanctions imposed by a
group on any who do not conform (Hill 2009). Even without such sanctions, there
may be a recognition that such-and-such is a proper or correct way to behave. In
studies in which we have exposed young children to members of another group
seeded with an alternative technique, we repeatedly hear the refrain “but that’s not
how you do it!.” There is an early sense of how things “ought” to be done (see
Tomasello et al. 2005 for an extended analysis of this and related aspects of social
cognition). This sense of “ought” is difficult to test for in nonverbal apes, but in any
case, clear signs of sanctions against nonconformers have not been documented.
Indeed, it is not easy to say why chimpanzees should care if one of their group
members does things a different way which in turn raises the intriguing question
of just why this has become so significant for humans, and when and why it may
have done so.
Ratcheting. As noted in Sect. 20.4.1, cumulative cultural evolution is a distinctive characteristic of human culture, notably absent or minimal in nonhuman
species. The recent experiments we described in that section, showing that chimpanzees offered an opportunity to ratchet up from their existing foraging technique
to a superior one performed by a group mate, failed to do so (Hrubesch et al. 2009;
Marshall-Pescini and Whiten 2008a), should be cross-referenced here because this
really comes into the category of social learning mechanisms. This appears such a
crux in what makes human culture different to that of other apes, that it is to be
hoped that this will be a growth area in future research.
Recognition of the copying process. Preschool-age children become aware of
when they are and are not copying others, such that the process of cultural acquisition may become to this extent, self-reflective. We see a sign of this “metacognition” in the ability of apes to learn the rule to attempt to copy another
individual in the “Do-as-I-do” paradigm of imitation research and it contrasts
with the failure of several attempts to train monkeys to do this (Whiten et al.
2004). In other words, apes appear to grasp the “concept” of imitation in a special
way, as do children, which suggests that in some way, their cultural acquisition
mechanisms are operating at this higher cognitive level.
Teaching. There are few signs of teaching in apes (Whiten 1999b). This may
appear a striking contrast with the human case, but, in fact, anthropologists have
remarked on the lack of teaching also in hunter-gatherer societies (Draper 1976;
Hewlett and Cavalli-Sforza 1986): a serious question-mark thus remains about the
significance of teaching for human culture in all but very recent historical times.
This is discussed further by Whiten et al. (2003). It may be that among animals,
behavior that can be called teaching, or perhaps more accurately “scaffolding”
supporting the acquisition of difficult skills (Whiten 1999a), is to be seen in
446
A. Whiten
predatory species in which the young must make a big leap in skill to become
independent foragers, such as in meerkats (Thornton and Raihani 2008).
20.5
Conclusions
What kind of being, then, was the human/chimp concestor? The comparative
evidence reviewed above suggests that it was a significantly cultural creature.
Although culture was to become inordinately more complex as our own line later
evolved, this analysis, nevertheless, suggests that the repertoire of our concestor
provided a significant cultural “platform,” that makes the origins of our unique
cultural nature much less mysterious than could any analysis blind to the evolutionary past.
Summarizing the key cultural features of our concestor based on the foregoing
analyses, we would infer that while there would be little or no significant teaching
occurring, observational learning would have been important in their lifestyle. This
would have included a portfolio of social learning processes, including both
emulative and imitative components permitting the acquisition from existing cultures of a whole repertoire of useful behaviors like foraging techniques. These
would have extended to complex, sequentially structured actions of the kinds
illustrated by the use of tool sets in wild chimpanzees, and incorporated into our
diffusion experiments that have so robustly demonstrated the capacity for multiple
cases of such behavior patterns to be spread by social learning. Social learning
would have been sufficiently strong to create a degree of conformity to local norms
of behavior, sophisticated enough for the mind to hold some concept of what it is
to copy others, and capable of sustaining different local cultures defined by multiple
and diverse traditions. Whiten and van Schaik (2007) have suggested that this
adaptive complex may help explain the relatively large brain size of great
apes, with culture requiring cognitive sophistication and culture, in turn, making
tradition-acquirers smarter (the “cultural intelligence hypothesis”: Whiten and van
Schaik 2007).
Note that although the above analysis might superficially appear to amount to
saying our ancestor was culturally like a chimpanzee, this is not the nature of our
methodological enterprise. To take the chimpanzee as a straightforward representative of our ancestor what has been called a “referential model” for our past
would be wrong. We did not evolve from a chimpanzee, any more than did
chimpanzees evolve from humans. The crucial approach is instead to identify
shared features of chimpanzees and humans and attribute them to the common
ancestor. Derived features unique to either chimpanzee or humans do not figure in
this, so the ancestor we are reconstructing was not simply like a chimp. Having said
that, however, it was likely more like a chimpanzee than a human, one important
reason for this being that its brain was still chimpanzee-sized, and thus only about a
third of that of the cultural creature we have become.
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When we turn to the other side of the coin and ask where the cultural differences
between the concestor and our later ancestors began, the principal feature of interest
must be the capacity for cumulative culture. This becomes evident in the archeological record as Oldwan tools gave way to the symmetry and sophistication of
Acheulian blades and yet more sophisticated technology that was built step-wise on
what had been achieved before (Mithen 1999). The common explanation for the
emergence of cumulative cultural evolution, among both leading comparative
psychologists (Tomasello 1999) and leading anthropologists (Boyd and Richerson
1996), has been the emergence of a capacity for imitation, which during the 1990s
was argued to be lacking in nonhuman species such as the chimpanzee. As will have
been evident from the foregoing review, more recent research with chimpanzees
has demonstrated considerable powers for rather high-fidelity transmission of
behavior, which suggests it was not a lack of imitative capacity that constrained
cumulation. It is true that children are more imitative than chimpanzees, but the
point is that chimpanzees, nevertheless, have ample copying ability to sustain
cultures constituted of many complex behaviors. My colleagues and I have, therefore, suggested that the key to the rise of cumulation was instead the evolution of a
greater degree of intelligence, associated with encephalization, that permitted more
sophisticated innovations such as the Acheulian industry (Whiten et al. 2003). This
is consistent with the analyses of archeologists such as Mithen (1999), who points
out that Acheulian artifacts are sufficiently complex that they must have been
acquired through considerable imitative capacity, yet there was minimal cumulation at this stage, with no change over millennia: ergo, imitation does not beget
cumulation. The conservatism of social learning that we have identified in recent
experiments with chimpanzees suggests an additional factor that may limit cumulation: chimpanzees and our concestor, we infer, could acquire much culture
through observational learning, but the crystallization of well-worn habits soon
constrained the ability to assimilate any new innovations that emerged. With our
triple-sized ape brains, that is what we humans became supremely able to do.
Acknowledgments The author was supported by a Royal Society Leverhulme Trust Senior
Research Fellowship during the preparation of this work.
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Zentall TR (2004) Action imitation in birds. Learn Behav 32:15 23
Part VIII
Conclusions
Chapter 21
The Coevolution of Genes, Innovation,
and Culture in Human Evolution
Richard McElreath
Abstract Much adaptive human behavior is much too complex to be invented
by any single individual in his lifetime. Such complex behavior can be learned
and maintained in human populations, however, because our species possesses
evolved psychological abilities for acquiring and modifying complex behavior
and knowledge. One puzzle surrounding the origin of human behavior, with its
strong reliance on socially transmitted knowledge, is how natural selection can
favor costly abilities for complex social learning before the existence of complex
behavior to be learned. The finding of special-purpose social learning abilities in
other apes has only sharpened this puzzle if other apes are good at imitation, is the
key difference between ourselves and chimpanzees instead rates of innovation? In
this chapter, I explore this puzzle by considering the simultaneous coevolution of
both social learning ability and individual innovation. When one allows both
innovation and the accuracy of social learning to evolve independently of one
another, natural selection can favor increased investment in social learning, but
only if it first favors increased innovation. However, once social learning evolves to
high accuracy, high innovation rates are no longer needed, and natural selection
favors reduced investment in innovation. Thus, the debate about whether innovation or imitation defines the gap between humans and other apes may be misstated.
Instead, the emergence of human culture may have required the coevolution of both
kinds of learning.
R. McElreath
Department of Anthropology and Graduate Groups in Ecology, Animal Behavior and Population
Biology, University of California, Davis, CA 95616
e mail: mcelreath@ucdavis.edu
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 21, # Springer Verlag Berlin Heidelberg 2010
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Introduction
Wheat is one of humanity’s great inventions. Coming in great variety, locally
adapted to microclimates, it converts energy into a form people can use to make
more people. It is also nearly wholly dependent upon people for its survival like
other grains, wheat seed does not easily break from the grass, but instead stays firm,
stuck to a “tough” rachis, waiting for the farmer or machine to remove them all at
once. A sensible wild grain instead disperses seed to the wind. Other domestic
species are similar; they have partly outsourced their reproduction to humans, in
exchange for security. Domestic cattle whatever you think of their intelligence
have done quite well, compared with their extinct wild ancestors.
All of the human species’ domesticated inventions represent a transfer of
information. Information about the environment how to extract energy from it,
how to resist drought, how to make harvest easier for a human farmer makes its
way into wheat’s genes, during artificial selection. Generations of human farmers
have shuttled information about the world, the world’s pests, and their own preferences into the plant’s genome, creating a technology that “knows” about the
world we live in. Of course the farmer must have some extra information, in order
to profit from wheat. But a tremendous amount of information must be contained in
the plant’s genome, and it accumulated there over many generations.
When I say “information” here, I mean that if we knew how to interpret it, the
sequence of bases in wheat DNA would tell us new things about the environment
and how to adapt to it. However, there are more formal definitions of information
that suggest that natural selection accomplishes a similar feat, building information
about the environment into our own genome (Frank 2009). The fit the Darwin
observed between organisms and their environments reflects this flow of information. Each organism embodies a recent step in a long chain of information commerce, between the environment and the population of genes. This commerce is
not without friction, of course. Sexual reproduction and recombination interfere
directly with selections ability to describe the environment. But over time, natural
selection manages to adapt organisms to the environment, by differentially favoring
alleles.
Of course real environments fluctuate and vary. The planet we live in now is
quite different than that of the Pleistocene, and spatial variation from pole to pole is
at last as great as temporal change. As a result, some of the information that
organisms accumulate about the environment is meta-information, information
about information in the environment. Whenever a seed assays available moisture
and postpones germination as a result, the plant is employing this kind of accumulated meta-information. In less philosophical language, it learns.
Learning is a kind of phenotypic plasticity, a condition in which information in
the genome teaches the organism to respond to information in the environment.
Instead of natural selection building in a direct description of the environment,
there is instead meta-information about variation in the environment. This
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meta-information might be an array of phenotypes that are triggered as the organism receives information from the environment, during its development. This is
what the seed does, when it “decides” whether or not to germinate. The metainformation can also be an exploration strategy, however, so that instead of the
organism’s genome containing information about, say, the location of a water
source, the genome contains information about how to find a water source. This
information is still relevant to only some environments, and is therefore information
about these environments, because different exploration strategies different ways
of learning are better or worse under different conditions.
These are two kinds of information about the environment: information built into
the genome by generations of natural selection and information we acquire during
our individual lifetimes, interpreted in light of information in the genome. But there
is also an important third sort of information that some species make use of. Much
of the knowledge that most farmers employ to manage wheat accumulated over
many generations, but it is not contained in anyone’s genome, at least not in any
simple sense. Instead, farmers inherit each generation the accumulated culture of
farming. This information is fit to the environment, just as other human traditions
can exhibit amazing adaptation. But no individual in the course of his or her lifetime
could accumulate it. Instead, it has taken many generations to develop, in a way
similar to how information over many generations accumulated in wheat’s genome.
In the case of wheat, humans built the information directly into the plant. In the case
of other elements of culture, humans built the information into human brains, and
later books and other forms of storage that human brains can access. This information is also often meta-information, providing strategies for solving specific problems as well as strategies for learning in itself.
In this chapter, my aim is to provide an introduction to, and an example of, theory
development within the evolution of this kind of accumulated cultural information
and the genetic information that makes its accumulation possible. Unlike other apes,
humans rely upon accumulated bodies of adaptive information culture that do
not reside in the genome, but nevertheless do depend upon information in the
genome for their continuity and pattern of evolution. In order to understand why
humans are the only ape to have crossed this “gap” and become so committed to and
dependent upon socially transmitted complex adaptations, we must understand both
the genetical origins of the psychology that makes cultural evolution possible the
information-about-information that resides in our genomes and the behavioral
origins of accumulated socially transmitted information-about-information culture.
How did cultural evolution evolve? Our goal is to ultimately understand why the
human genome, contrasted with those of other apes, has learned to learn about
the environment in a way that generates complex behavioral adaptations that rival
the complexity of those produced directly by natural selection. Addressing this
question brings up a number of puzzles. I focus on two closely related concepts.
Accumulated culture is a poor guide to the origins of accumulation. First,
complex cultural adaptations like boats and agriculture appear obviously worth learning. However, when cultural abilities were first evolving, these fancy, accumulated
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bodies of information must not have yet existed. Explaining the origins of domesticated wheat cannot reference contemporary highly domesticated varieties, but
must instead reference wild varieties with their inconvenient wild characteristics.
Similarly, we cannot explain the origin of human culture with reference to contemporary cultural adaptations. The information that has to be built into the genome, in
order for a child or adult to acquire complex accumulated culture, would seem to
pre-require the complex culture. How can culture get started, if there is little culture
yet to acquire (Boyd and Richerson 1996)?
Culture makes humans evolvable, but that is not why culture evolved. Second,
while culture makes human societies highly evolvable they can quickly (in
genetic time) generate complex local adaptations to a large number of novel
circumstances this evolvability is not easy to understand as the original reason
for the evolution of cultural learning. Quite simply, the first cultural learners would
not have benefitted from the evolvability. The long-term population-level benefits
of complex accumulated culture seem obvious. Once fancy human culture existed,
it allowed us to adapt to every terrestrial environment (except Antarctica) and
accumulate powerful bodies of information like quantum mechanics. The fitness
of any contemporary human has been buoyed by many past generations of accumulated extraction of information from the environment and through transmission
and curation of this information. In this way, cumulative culture generates important and powerful group advantages. But these group benefits are not easily
understood as the reasons for humans’ evolving cultural abilities. Instead, evolutionary ecologists would rather attempt to explain the origins of cultural capacities
with individual benefits accruing to individual learners. Thus, instead of marveling
at the adaptedness of accumulated human culture, our task is to understand how
psychological abilities driven by individual selective advantage can build group
benefits as a by-product (Boyd and Richerson 1985).
The approach I take in this chapter is to first review the gene culture or dualinheritance approach to human evolution. I present relevant preexisting theory on
the problem of getting cumulative culture started. I use this theory then to
introduce a new model that considers the simultaneous coevolution of all three
kinds of information transfer that I have outlined here: (1) the incorporation of
meta-information information about how to use information adaptively into the
genome through natural selection, (2) the adaptive use of direct environmental
information through individual learning, and (3) the accumulation and transmission of environmental information across generations, outside of the genome. I
will show that allowing for all three of these dynamics simultaneously illuminates
one potential path across the cultural “gap,” whereby cumulative culture can get
started, despite our first puzzle above. In the process, however, the evolutionary
dynamics seem to cover their own tracks, hiding the initial changes that make the
crossing possible. Finally, by allowing social learning to enhance innovation in
ways other than merely allowing one to start where others left off, the model can
produce levels of behavioral adaptation much greater than are possible if innovation and cumulative social learning are considered orthogonal psychological
abilities.
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The Gene–Culture Framework
One way to view the development of the body of theory known as gene culture or
dual-inheritance theory (Cavalli-Sforza and Feldman 1981; Boyd and Richerson
1985; Durham 1991) is to think of it as a revolutionary idea that suggests that
human evolution cannot be understood in the same purely genetical way that works
for other organisms. One could view it this way, but I do not.
In evolutionary ecology, there is a long and successful tradition of modeling
organic evolution as an interaction among information contained in genes, the
structure of the population, and the state of the environment. In this framework,
the only heritable variation is contained in genes, and so evolution is described as a
change in this variation over time.
Or is it? In the simplest models, it is true that the only evolving aspects of the
population are genes. But in slightly more complex models that consider population
structure, gene frequencies cease to be the only evolving information. Now the
distribution of genes, age and sex structure, and local population densities can all
evolve and exert very strong influences on the future changes in gene frequencies.
These systems cannot be reduced to gene-only descriptions additional information about the population and environment is needed to understand and predict
change. Applied mathematicians are keenly aware of this fact, because we must
define “state variables” for each of these evolving bits of information.
Routinely, genes are not the only state variables, even in culture-free models.
Gene culture models, therefore, dwell well within this successful tradition in
population biology. There is not necessarily anything unusual about theorizing
and modeling the idea that extra-genetic information is required to adequately
describe a population, even when our focus is on genetical evolution.
If gene culture models are special in any way, it is that the extra-genetic
information directly influences individual phenotype in a way very similar to the
action of genes. The basic issue is to identify the minimal requirements for
representing evolution of phenotype in a species. For example, we could construct
a very simple genetic model in which the change (D) in the frequency of an allele, p,
is a function of environmental state, E. This system would have a single recursion:
Dp ¼ Fðp; EÞ;
where the function F(p, E) is to be specified depending upon what model of
adaptation to the environment we might choose. It might be that E has little effect
on individuals with different alleles, or it might be that E favors one over the others.
It might be that E is fluctuating, so that selection favors different alleles at different
times. The change might depend upon p itself, as it does in the example of sicklecell anemia and other cases of overdominance. But nowhere do we allow in such a
system for E itself to evolve in response to p.
The scientific question is whether such models are sufficient to model the evolution of a given organism’s phenotype. For example, some moths imprint on the plants
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they graze upon as larvae. When they are adults, they will seek out these same plants
to lay their eggs. The information about which plant to seek is not coded in the
genome, even though the strategy for imprinting is. In this case, if we only know
genotypes and the state of the environment, we could not predict the behavior of
organisms in the next time period. We also need to know the distribution of imprinted
memories among moths. In such a case, we need at least one more equation:
Dp ¼ Fðp; q; EÞ;
Dq ¼ Gðp; q; EÞ;
where q is the frequency of some learned variant (an imprinted plant, say), and
G(p, q, E) a function telling us how learning responds to environment, E, and its
own previous state, q, and the frequency of an allele, p.
This all sounds rather complex. And it can be. However, when important parts of
a phenotype are acquired during development and depend upon previous phenotypes, some system like this is useful for understanding how the organism evolves.
Unless we think the existing behavior could be predicted solely from knowing the
environment and the distribution of genes, at some point evolutionary models may
incorporate the dynamics of behavioral inheritance. No heroic assumptions are
required for behavioral inheritance to exist: if portions of phenotype depend upon
the phenotypes of other individuals, then weak or strong inheritance of behavior can
exist. In the long run, in a given model, it might turn out that behavioral dynamics
have little effect on the outcome. In others, it will make a huge difference.
Cultural evolutionary models (as well as niche construction models, see OdlingSmee et al. 2003) can model just the nongenetic behavioral dynamics, as if q above
did not depend upon p, as well as joint dynamics of a coupled gene culture system
(Cavalli-Sforza and Feldman 1981; Boyd and Richerson 1985; Durham 1991). In
each case, however, the structure of the model is decided by the question of interest.
There are no global models that encompass all questions about evolution. This is
why we call them “models.” While a few axiomatic mathematical theories do exist
in biology the Price equation being the most famous for the most part, formal
evolutionary models are attempts to understand the consequences of assumptions
and to explore the sufficient or necessary conditions for a given outcome. With such
models, we can study which kinds of strategies and population structures can
possibly produce a phenomenon these are possible sufficient conditions. We
can also study which assumptions can be omitted, still producing a phenomenon
these are then not necessary conditions.
The demonstrable success of the modeling strategy across the sciences recommends it well. While most of the work in evolutionary anthropology and evolutionary psychology is concerned with more proximate phenomena than the population
dynamics of joint gene culture systems, understanding both the details of the
psychological differences between humans and other apes and the different population dynamics of human and ape societies will be necessary, before we have a
satisfactory set of answers for how humans evolved, both in relation to other
primates and broader trends in animal societies.
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457
What Does It Mean to Say that Culture Is an Inheritance
System?
While evolutionary principles are equally applicable to almost any dynamical
system, many researchers approach models of cultural transmission and evolution
via an analogy with genetical evolution. Analogies are often useful, but can disguise
important differences. This has rightly led some to be concerned about the strength
of the gene culture analogy (Sperber 2000). If cultural variants are not discrete, are
prone to “mutation,” and are strongly affected by learning biases, then is it useful to
speak of “transmission” of culture at all?
While I have no particular attachment to the term “transmission,” the answer is
definitively “yes.” Even if all the above is true, culture can still be an evolving
system that leads to cumulative adaptation. This does not mean that evolved
psychology has no role to play in how culture evolves (on the contrary, psychology
has a huge role to play in understanding culture), but it does mean that dismissing
cultural evolution on the basis of imperfection of the genetic analogy is unwarranted.
Many people enthusiasts of the “meme” approach and critics alike seem to
have been persuaded by Richard Dawkins’ abstract statements on what is required
for adaptive evolution to occur. In The Extended Phenotype (1982), he argued that
any successfully replicating entity must exhibit (1) longevity, (2) fecundity, and (3)
fidelity. The entity must last long enough (longevity) to make copies of itself
(fecundity) that are reasonably similar to it (fidelity). Some have interpreted this
to mean that anything with high mutation rates cannot be a successful replicator.
Thus, if cultural ideas change in the process of social learning, the conclusion is that
they do not constitute an evolving system at all (see citations in Henrich and Boyd
2002). Similarly, if cultural variants are continuous and blended entities, then they
never exactly replicate, and again cannot produce adaptive evolution.
These conclusions are unfounded. Read very generally, Dawkins’ conditions are
necessary and sufficient there must be some heritability for adaptive evolution to
occur. However, there are many ways to produce heritable variation. So in the strict
sense many people have read them, while Dawkins’ conditions are sufficient, they
are definitely not necessary. Reverse-engineering DNA may tell us how inheritance
can work, but it does not tell us how it must work. Henrich and Boyd (2002)
examine the problems with this reverse-engineering in greater depth (see also
Henrich et al. 2008).
It is understandable that there is confusion about what is needed for adaptive
evolution even textbooks are confused. Before the union of genetics and Darwinism,
most biologists, including Darwin, thought that inheritance was a blending process:
offspring were a mix of parental phenotypes. Darwin was troubled by Jenkin’s
(1867) argument that natural selection could not produce adaptations, because
inheritance would quickly deplete the variation that natural selection relies upon.
Fisher’s (1918) argument reconciling genetics with continuous phenotypic variation
purportedly rescued Darwin. Many textbooks repeat this version of the history,
reinforcing the notion that low-error discrete entities like genes is a necessary
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condition for adaptive evolution. But in reality both Jenkin’s argument and those
who think Fisher saved Darwin are simply wrong: blending inheritance can preserve
variation, and particulate inheritance is neither necessary nor sufficient to preserve
variation. Maynard Smith (1998) has a chapter that examines this problem, still not
entirely resolved within modern population genetics (see also Barton and Keightley
2002).
21.2.2
Is Extra-Genetic “Inheritance” Common?
I think this history lesson teaches us that our verbal models of even genetical
evolution are sorely lacking. If so, then, our informal appreciation of nongenetic
influences on behavior is also a suspect. In many baboons, females inherit dominance rank from their mothers and sisters (Silk and Boyd 1983). In these species,
fitness is strongly affected by this extra-genetic inheritance: any female adopted at
birth into a high-ranking matriline would be better off than if she were adopted into
a low-ranking matriline. And this female will have her dominance rank before she
fights a single member of her social group. Dominance is heritable, has important
effects on fitness, and yet the mechanism of inheritance is at least partly nongenetic.
The rules of how this inheritance works are complicated and very unlike genes. It
probably depends upon the composition of one’s own matriline, the composition of
the entire social group, and local resource density and feeding competition. And yet
no primatologist could completely understand baboon biology without taking this
complicated extra-genetic pedigree into account. Its existence may lead females to
strive for rank because of its downstream consequences, in addition to its immediate resource access effects (Boyd 1982; Leimar 1996, Watts, this volume).
Extra- or “epigenetic” (Maynard Smith 1990) systems like this are increasingly
recognized: everywhere biologists look, they find hints of inheritance systems
either built on top of genes or built from entirely different mechanisms. If the key
question is what mechanisms account for heritable phenotypic differences among
organisms, then the answer appears to be “many.” Jablonka and Lamb’s Evolution
in Four Dimensions (2005) mounts the empirically rich argument that heritable
differences in many species are due to the action of several inheritance systems
(genetic, epigenetic, behavioral, and symbolic), sometimes interacting, sometimes
acting in parallel.
If one thinks about cell division for a moment, it is obvious that processes other
than the replication of DNA are needed to explain how it works. Organelles need to
be copied (Sheahan et al. 2004), and the genetic code itself needs to be copied (and
this is not contained in the DNA, nor could it be). Beyond cell division, adult
phenotypes depend upon imprinting and other forms of learning that may channel
the environments offspring are exposed to (a kind of niche construction OdlingSmee et al. 2003). And finally, most biologists believe that DNA was certainly not
the first form of hereditary biological material (Szathmáry and Maynard Smith
1995). Thus, some inheritance systems must be able to sometimes create complementary and even usurping inheritance systems.
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In light of these plausible “inheritance systems,” it appears that human culture
may not be so special or surprising at all, in the sense of being a nongenetic system
of inheritance. Organisms as diverse as Arabidopsis (a small plant related to
mustard that is a favorite of geneticists), common fruit flies, and single-celled
microscopic animals, such as paramecia, exhibit heritable differences due at least
in part to mechanisms other than the sequence of nucleotides in their DNA. The
existence of social learning as a system of inheritance and adaptation that functions
in complement to DNA may turn out to be unremarkable.
There will always be aspects of human behavior and evolution that can be
usefully modeled as culture-free. These are, after all, just models: all of them are
wrong, but some are more useful than others. To someone who makes formal
models of evolutionary systems, the question that we must answer is what qualitatively different phenomena we miss when we represent human (or any other
organism’s) evolution with just state variables for its alleles. If we sometimes
require state variables for early childhood experience, imprinting, or behaviors
acquired via social learning, to make useful models of our own evolution, then
attempts to construct culture-free models are simply scientifically inadequate. As
with each of the possible systems above (e.g., Maynard Smith 1990; Jablonka and
Lamb 1991; Pál and Miklós 1999), the specific dynamics and consequences of
cultural learning may be rather unique and very important for understanding both
micro- and macroevolution.
21.3
Simultaneous Evolution of Innovation and Social
Learning
The literature on gene culture evolution often presents individual and social
learning as alternatives. At some point in its lifetime, an organism is forced to
choose between relying upon individual experience or socially acquired information. This dichotomy obviously does not imply that social learning is free of
inference or unguided by individually learned theory. But evolutionary models
almost always engage the strategic and population levels, not the psychological.
At the level of abstraction of our models, these psychological platitudes are granted,
and different strategic uses of information are the focus.
So while evolutionary models do treat individual learning and social learning as
strategic alternatives, I know of no theory that treats them as orthogonal influences
on behavior. Even in the simplest sort of model, individual and social learning
interact at the population level and across generations (Rogers 1988). These interactions are antagonistic, in some cases. Social learning parasitizes innovation, in
the same way that students who cheat on tests parasitize (or attempt to parasitize)
the students who study. To understand the pattern of behavior that emerges from
these models, we have to appreciate that individuals are relying differentially upon
both kinds of information.
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Consider the genetic analogy again. Evolutionary theorists routinely speak of
mutation and selection as separate “forces.” Each is governed in part by unique
parameters, within an evolutionary model. And yet no one charges theorists with
assuming that mutation and selection do not interact. Indeed, there is still active
theory development concerning the interaction of mutation and selection in determining patterns of genetic variation (Barton and Keightley 2002).
Similarly, gene culture models have considered how individual and social
learning interact, over an organism’s lifetime, as well as over evolutionary time.
Boyd and Richerson (1985) devoted considerable space to the adaptive interaction
of individually and socially acquired information, guided variation. There are
other, more subtle kinds of interactions. An adaptive synergy between individual
innovation and social learning can arise from conditional effort in innovation (Boyd
and Richerson 1996). Suppose that an organism can assess the efficacy of the
information it acquires via social learning. The assessment can function through
an initial trial or from analysis. If the organism judges that the behavior it acquired
socially is below some threshold of efficacy, the organism has the option of then
spending additional time innovating.
A second kind of adaptive synergy is when information gathered through
individual learning can be build onto information gathered from social learning.
When this is possible, the two kinds of learning interact to produce accumulations
of information across generations (Boyd and Richerson 1996). There has been
much less theory developed in the cumulative culture case. Previous models
demonstrate that there may be a fitness valley between simple, inaccurate social
learning that cannot accumulate complex behavior and more accurate social
learning that can. Boyd and Richerson (1996) developed both a discrete behavior
and truly continuous behavior versions of their model. In both cases, there were
values of the parameters for which cumulative cultural learning could not invade
when rare but was stable when common. However, neither of these models allowed
individual learning to co-evolve with social learning. Innovation rate was a fixed
parameter, and this lack of feedback hid some interesting dynamics.
The model in the remainder of this chapter treats the simultaneous dynamics of
innovation an organism’s investment in acquiring new adaptive information
directly from the relevant environment and cumulative social learning and
organism’s investment in acquiring complex information from other individuals.
21.3.1
What Is Learning For?
Levins (1968) produced what is probably the first broad formal analysis of the
evolution of simple phenotypic plasticity. He asked us to imagine a large number of
different environmental states which may vary through time and across space.
Suppose there is a unique allele that is optimal in each state. Now imagine an
alternative strategy that, instead of providing a fixed phenotype, assesses the
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environment and attempts to change phenotype to match it. Levins extracted
several general conclusions from analyzing models of this type.
1. Both spatial and temporal variation reduce fitness, compared to a uniform, static
environment. Call this fitness cost the “naive cost.” It is what an unlearning,
unresponsive organism pays, in the presence of environmental change.
2. An optimal strategy that either bet-hedges against change or learns can be found
for any pattern of spatial/temporal change, but this optimal strategy cannot
remove the entire naive cost.
3. A learning strategy itself imposes a cost on the organism, either because of
sampling effort or processing costs. Call this cost the “learning penalty.” Therefore in order for learning to evolve, it must reduce the naive cost by an amount
greater than the learning penalty.
These generalizations have remaining quite robust to the specific forms of
models.
Spatial and temporal variation do generate different results, however. Germane
to the arguments in this chapter, when the environment varies through time, natural
selection will favor a bet-hedging strategy that maximizes the geometric mean
fitness of the organism, over the environmental states it experiences. Temporal
variation does not maintain polymorphic fixed phenotypes, in the absence of
learning. Spatial variation, on the other hand, may favor polymorphism in a species,
depending upon the details.
The rest of this chapter builds a model that allows for both investments in
individual exploration and, later, the ability to copy behavior and strategy from
other individuals. In this section, I’ll construct the individual learning and innovation core of the model. I’ll analyze this core, before laying on social learning
and analyzing the simultaneous dynamics of both cumulative social learning and
innovation. By presenting the model in this way, I hope to lose fewer readers and
better explain how cultural learning alters the outcomes.
Consider the evolution of simple learning, a form of phenotypic plasticity that
uses information in the environment during an individual’s development to alter
phenotype, whether it is morphology or behavior. The evolution of the strategy of
learning incorporates information into the genome that biases learning to be
adaptive, rather than self-destructive.
Let’s represent an individual’s “genotype” with d > 0. The notion is that a
number of regulatory and other genes combine to produce this continuous genotype
that influences the amount of exploration and innovation. Individuals search and
gather and process information about the environment so that each gains q ¼ d
units of adaptive knowledge, in the current environment. These units translate into
fitness, p (q), with diminishing returns. I have explored a number of specific
diminishing returns functions, but found all of them to produce the same qualitatively behavior. The easiest to analyze is:
pðqÞ ¼
bq
bd
¼
;
bþq bþd
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where b > 0 determines the strength of selection, and b > 0 is a parameter that
determines the rate at which marginal fitness benefits decline. When b is large, p(qi)
is approximately linear, and there are no diminishing returns to knowledge. When
instead b is small, the fitness benefits of increasing knowledge diminish rapidly.
Because of the specific form of the function p, b turns out to be exactly the value of
qi that produces half of the maximum value of p, b/2. That is, p(b) ¼ b/2.
Suppose there are a very large number of environmental states. In each state the
environment could take, different phenotypes are favored. Each generation, there is
a chance u that the environment changes to another random state. Since the
variation here is stochastic, in the absence of phenotypic plasticity, a bet-hedging
strategy will evolve that pays Levins’ naive cost of variability. Let fitness after
paying this naive cost be w0.
Individuals can do better than this baseline, by attempting to learn the current
state of the environment and use information from it to reduce the naive cost of
variation. An individual who invests d in learning pays a cost cd, the learning
penalty. Since investment in learning is continuous, this cost scales with it. As the
fitness benefits of environmental knowledge, q, have diminishing returns, eventually the marginal benefits and costs of learning equal. At this point, selection will
favor no further investments in plasticity.
These assumptions give us the following fitness, for an individual with genotype d:
wðdÞ ¼ w0 þ pðdÞ
dc:
^ by solving
We find the evolutionarily stable investment in individual learning, d,
∂w/∂d ¼ 0 for d. This yields:
d^ ¼
p
bb=c
b:
(21.1)
This is greater than zero, provided b/c > b, in which case selection favors
learning. If this condition is not met, however, selection favors instead the bethedging fixed strategy that suffers the full naive cost of variation.
Are we ready yet to answer the question: what is learning for? According to this
model, learning allows an organism to recoup fitness lost to temporal environmental
variation. Note that I have assumed so far that this is an entirely asocial process.
Fitness is not frequency dependent and there is no learning from conspecifics. In the
next section, however, I add the possibility of social learning to the model. Then
learning can be for building complex adaptations that fit the environment beyond
the amount q.
21.3.2
Adding Cumulative Social Learning
Many organisms are capable of phenotype plasticity. All primates and indeed all
mammals are capable of individual learning of the kind modeled above. In novel
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circumstances, animals employ search strategies that may allow them to adaptively
exploit new environments. One of the best understood of these is foraging in rats
rats explore trash eagerly, but sample in small amounts and remember and avoid
foods that make them ill (Galef 1996). This strategy allows rats to exploit varied
urban environments, but it is possible because information about how to search for
and use relevant information has been built into their genome by natural selection.
Animals also sometimes exhibit specialized adaptations for using conspecifics as
cues of adaptive behavior. When Norway rats smell food on the muzzle of another
rat, they are more likely to eat that same food (Galef 1996). Information in the rat’s
genome makes this possible, by directing attention to odors on conspecifics and
enhancing memory of socially-encountered foods.
In humans, the motivations and psychological adaptations that we might call
“social learning” involve symbolic communication, abstraction, and substantial
individual practice. Speech is a good model while substantial social input is
necessary for any human to learn the speech patterns of his or her community, a lot
of individual practice with sounds is needed, because the inputs (sounds) are quite
different than the information that an individual eventually needs to encode in order
to produce them (motor memory). Every individual has a differently shaped vocal
tract, and so in order to “imitate” another speaker, all of us had to experiment with
sound production. Likewise, acquiring a complex skill like hunting or agriculture
may require years of instruction and practice. Readers who have learned to play a
musical instrument may find it to be a rich source of intuitions about the assumptions of this model. Playing the cello takes many years of individual practice, but
this practice is much more effective when guided by a master cellist. A lone cellist
may eventually attain the skill of a master, after many years of individual effort, but
it is much easier to match or surpass the master, if the master provides instruction or
simply allows observation. The purely “social” component of social transmission
may be quite small, in terms of the time it occupies. But very little transmission, if
any, is possible without the social component.
Begin with the model of individual innovation presented above. Assume now
that there is another set of loci that influence an individual’s ability and motivation
to learn socially. The “genotype” at those loci is represented by s, and an individual
with s > 0 can successfully copy a fraction s of the adaptive behavior displayed by
an adult from the previous generation. In order to separate innovation and social
learning, I restrict s < 1, such that social learning will never accidently generate
behavior that is more adaptive that what was observed. Investments that increase
s may be attentional improvements in studying and representing the behavior of
other individuals or motivational increases in the extent to which goals and ways
of achieving goals are open to social input. In both cases, greater investments in
time or ability to acquire complex behavior from others results in the eventual
acquisition of a larger portion of previously innovated behavior. If s is large
enough, innovations generated over several generations may accumulate, generating behavior more complex than any individual innovation could in a single
lifetime. If s remains low, however, then no amount of innovation will result in
these complex behaviors, because each generation has to re-invent too much.
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Increasing the accuracy of social learning is, however, costly. It costs the learner
energy for upkeep and use of the psychology that makes social learning possible,
and it costs the learner time in observation, practice, and missed opportunities to
enhance fitness in other activities. I represent the total cost of social learning of this
kind by ks. The more an individual invests in accurate copying of information, the
more the individual pays.
With these assumptions, we can define a new fitness expression, now for a rare
(mutant) individual investing d in innovation and s in social learning, in a population in which the common type invests d* and s* in each, respectively.
wðd; s; d ; s Þ ¼ w0 þ pðqt Þ
dc
ks;
where qt is the individual’s behavioral phenotype, after both social transmission and
innovation. Because learned information can be maintained across generations
now, q will depend upon the amount of accumulated adaptive behavior in the
population. This in turn depends upon the common phenotype, d*, s*, and the
rate at which the environment changes and renders previously innovated behavior
non-adaptive. The correct expression for qt is:
qt ¼ ð1
ut Þsq0 þ d;
where ut is a random variable taking the value 1 or 0, depending upon whether the
environment changed last generation (with probability u) or not (probability 1 u),
respectively. The symbol q0 defines a recursion for the dynamics of behavior that is
transmitted across generations. The behavior available to learn socially depends
upon the common genotype, not that of the individual whose fitness we are modeling. The dynamics of behavior from one generation to the next are defined by:
q0 ¼ ð1
ut Þs q þ d ;
where q above is the average behavioral phenotype in the previous generation.
Because ut and ut 1 are random variables, there is no equilibrium amount of
adaptive behavior in the population. Instead, q is reset to zero after each change
in the environment and then begin climbing until the next change. One could
assume instead that a proportion of adaptive information is retained across changes
in the environment, but all this does is reset q to some minimum, rather than zero.
There still will never be a stable value of q across generations. To cope with this
kind of stochastic system, we solve for the mean of the stationary distribution of q.
While there is no equilibrium, in a linear system like this one, the distribution of q
across generations will eventually settle down. This is the system’s stationary
distribution. We can compute the mean of the stationary distribution, by taking
expectations across generations and solving for q^, the mean of the stationary
distribution. Doing this yields:
q^ ¼
1
d
s ð1
uÞ
:
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465
This expression tells us that the mean level of adaptive behavior increases with
increases in d* and s*, but decreases as u increases. It is helpful to consider some
limiting cases. Suppose for example that the population has yet to evolve any
effective social learning, s* ¼ 0. Then the average level of adaptive behavior
will be q^ ¼ d*. No adaptive behavior accumulates beyond what individuals can
learn for themselves. Now suppose instead that s* ¼ 1. Now q^ ¼ d/u if u is small
enough, substantial adaptive behavior will accumulate, because social learning is
very (unrealistically) accurate.
Of course d* and s* are evolving genotypes. In order to analyze the simultaneous
dynamics of innovation and social learning, we need to substitute q^ into the fitness
expression:
^
q þ dÞ dc
wðd;
s; d ; s Þ ¼ w0 þ pðð1 uÞs^
ð1 uÞsd
¼ w0 þ p
þd
1 s ð1 uÞ
ks;
dc
ks:
Note that the adaptive behavior available for the mutant individual to acquire
depends upon the population genotypes d* and s*, while the accuracy of her own
social learning and power of her own innovation depend upon the individual
genotypes d and s. In this way, the invading genotype plays against the population
in game theoretic fashion. Our goal is to find the values of d* and s* that cannot be
invaded by any other values d and s, respectively.
21.3.3
Joint Dynamics of Innovation and Cumulative
Social Learning
Before deriving the un-invadable values of innovation and social learning, it is
useful to summarize the combined, two-dimensional, dynamics of this model. This
system can evolve to two qualitatively different outcomes. First, social learning
may increase when rare and evolve until its theoretical maximum. Second, social
learning may be unable to invade when rare. Which of these two outcomes is
realized depends upon the amount of innovation favored, when social learning is
rare. If innovation is cheap, for example, then enough of it might be favored when
social learning is absent. Social learning will then increase from s* ¼ 0, because
there is complex information in the population worth copying. Once social learning
begins to increase, however, selection favors less innovation, because of the
diminishing fitness returns on knowledge. Eventually innovation may fall to the
same level it was at, before social learning invaded. However, social learning
remains high in the population. Once social learning can get a start from initially
high innovation levels, it can invade.
Any potential evolutionarily stable values of d* and s* are found where
∂w/∂d|d,s d*,s* ¼ 0 and ∂w/∂s|d,s d*,s* ¼ 0. Call the evolutionarily stable values
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R. McElreach
^ s* ¼ s^ and solving these equations for d^ and s^ yields one
d^ and s^. Setting d* ¼ d,
possible equilibrium for d*:
d^ ¼
p
bb=c
b ð1
s^ð1
uÞÞ:
(21.2)
Again note that innovation does not always evolve. If b > b/c, the expression
above is negative and no innovation is favored by natural selection. The possibility
of social learning, however, affects the stable amount of innovation. As s^ increases,
d^ decreases. If the environmental rate of change u is large, however, then the effect
of social learning on reducing d^ is reduced.
Instead of having an equilibrium value, s* can either decrease or increase until it
reaches zero or one (or another theoretical upper limit). That is, s^ ¼ 1 or s^ ¼ 0,
depending upon the parameters. The condition for social learning to increase from
zero, and so invade a population, is given by @w=@sjd¼d;s¼s
0. This reduces to:
^
u<1
p
k
bbc
bc
:
If the environment changes too quickly, social learning is never favored. But if
the marginal cost, k, of social learning is low enough and fitness benefits of behavior
do not diminish too rapidly, then social learning will invade and increase until its
theoretical limit.
These expressions do not immediately reveal what is happening, however. It is
easier to understand the behavior of this model, by visualizing the joint evolution
of innovation and cumulative social learning. Figure 21.1 shows the phase diagram
of this model, for two different sets of parameter values. In each plot, position
along the horizontal axis represents the value of d*, from zero to one. Position
along the vertical axis represents s*, also from zero to one. Arrows represent the
direction and magnitude of change for the system, at each point. The point in each
plot is the eventually evolutionarily stable combination of innovation and social
learning, in each case. On the left, the cost of innovation is set high, but not so
high as to prevent individual learning from evolving at all. The high cost,
however, does prevent d* from ever evolving to high enough values to provide
enough adaptive behavior to be worth investing in accurate social learning.
Therefore, where ever the system begins, selection will eventually reduce social
learning to its minimum. Cumulative culture does not evolve in this case, although
rather fancy behavior is invented each generation, because of the non-zero equilibrium value of d*.
On the right, the cost of innovation is slightly reduced. Suppose the system
begins in the lower-left corner, at d*, s* ¼ 0. Now innovation can increase to a
higher level than on the left, before the arrows turn the other way and selection no
longer favors any increases in innovation. Innovation can reach a high enough level,
in fact, that the behavior that is invented each generation is now worth copying
through investments in accurate social learning. Therefore the system evolves
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Innovation, Genes and Culture
467
Fig. 21.1 Evolutionary dynamics of innovation and cumulative social learning. Arrows show the
direction and magnitude of evolutionary change at each point in the possible state space of the
population, defined by the average investment in innovation, d*, and the average investment in
social learning, s*. Black dots in each panel show the only stable equilibrium in each case. In both
panels, b 5, u 0.05, b 1, k 1. Left panel: c 3. Right panel: c 2. On the left, higher
costs of innovation prevent individually learned information from reaching high enough levels for
natural selection to favor cumulative social learning. Without much behavior worth copying, the
system remains at a high level of innovation, but no imitation evolves. On the right, a slightly
reduced cost of innovation leads initially to a higher investment in individual learning, a higher
level of individually acquired behavior, and eventually to the invasion of social learning. As social
learning increases, however, natural selection favors reduced investments in innovation, because
of the diminishing fitness returns to knowledge. This system comes to rest where innovation is
lower than the panel on the left, but social learning is highly accurate
towards the interior, favoring increasing amounts of social learning as it heads for
the top of the figure. As selection favors social learning, however, it also favors less
innovation (Expression 2). Thus the eventual equilibrium has highly accurate social
learning (^
s is near one), but lower levels of innovation than the plot on the left. The
behavior invented each generation is modest in comparison to the population in
which social learning did not evolve. However, the mean level of adaptive behavior
is twice as large. On the left, q^ ¼ d^ ¼ 0.29. On the right, q^ ¼ 20, d^ ¼ 20
(0.029) ¼ 0.58.
21.3.4
How Much Cumulative Culture?
There is an irony lurking within the solution above, however. While the evolution
of social learning appears to have resulted in higher levels of adaptive behavior,
“culture,” q^, in reality social learning has only provided a cheaper way to attain the
same amount of adaptive behavior the population would have enjoyed, if it had
relied entirely upon high levels of innovation. This is obvious, once we inspect the
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R. McElreach
expression for stationary mean q^, at d^ s^. Let ^
q^ be the value of q^, evaluated at d* ¼ d^
*
s ¼ s^. Then the expression for this expected average level of adaptive behavior is:
q^ ¼
p
bbc
c
bc
¼
p
bb=c
b:
Note that this expression does not contain s^. Therefore, it does not depend upon
social learning at all. Furthermore, it is the same amount of adaptive behavior we
would expect from the a-cultural model presented earlier (Expression 1)! Cumulative social learning has evolved, but it has failed in this model to produce long-term
information gains beyond what would already have been possible using (highly
advanced) innovation.
What is happening in the evolutionary economics is that lower costs of innovation allow behavior to reach a threshold that then allows social learning to invade.
Once social learning invades, selection favors less innovation. Because behavior
has diminishing returns, individuals do better by investing in an optimal mix of
innovation and social learning. This optimal mix trades off the costs of innovation
against the potentially unreliable benefits of socially learned behavior. Because
individual benefit is driving the evolution of both innovation and cumulative culture
in this model, selection does not necessarily maximize the group benefits of
cumulative culture.
One way out of this unsatisfactory result is to note that we have only modeled a
single domain of behavior. Social learning ability will be applied potentially to
other domains with much lower relevant rates of environmental change. Consider
that bows and arrows continue to function, even when climate changes substantially.
Therefore different technologies and strategies experience different rates of change
(u in the model). If fitness gains from more-slowly changing domains are
important enough, then social learning will be pulled up to a higher level of
accuracy, even in fast-changing domains, than would be optimal, if we consider
those domains alone.
But this is a hand-waving argument. Are there other theoretical solutions that do
not invoke large numbers of parameters that are poorly understood and potentially
unmeasurable?
21.3.5
When Social Learning Enhances Innovation
How can we get selection to increase adaptive behavior beyond this selfish optimum? One way is by allowing social learning to improve the efficiency of innovation. This hypothesis is reasonable, if you believe that the psychological abilities
that make cultural transmission possible also enhance an individual’s ability to
represent, remember, and explore new solutions. For example, language is a
symbolic capacity that allows us to represent abstract systems, much like the
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469
model in this chapter. While language makes it possible to acquire complex
behavior from other people, it also makes it possible to organize and transform
information within one’s own head. The dialog scientists carry on with themselves
sometimes out loud suggests that at least some abilities that are possibly selected
for enhancing social learning can simultaneously enhance imagination and innovation. If one of the things social learning does to human cognition is provide a
quite open motivational and association system, so that we can remember arbitrary
scripts and develop novel goals through communication, then any energies given to
innovation may be able to tap these same abilities.
When we allow new synergy between social learning and innovation (so
that social learning actually makes innovation cheaper), we have a new fitness
expression:
wðd; s; d ; s Þ ¼ w0 þ pðð1
uÞs^
q þ dÞ
ð1
s=zÞcd
ks:
The parameter z > 1 determines the amount of synergy. When z ¼ 1, the
above reduces to the previous fitness expression and the result is unchanged.
When z is small, however, there may be substantial cost reductions to innovation
as social learning abilities increase. When z ¼ s, innovation is effectively free (note
that this is impossible, by the constraint that z > 1). The new steady state accumulated culture becomes:
q^ ¼ p
bz
bczðz
s^Þ
b:
Figure 21.2 plots this expression over all possible values of s^, for two values of z.
As z ! s^ from above, this quantity increases rapidly. In biological terms, as social
Fig. 21.2 The expected
amount of adaptive behavior,
Expression 3, as a function of
the amount of cumulative
social learning in the
population, s^. Horizontal line:
b 5, c 3, b 1, z 1.
Sloped line: z 3. When
social learning reduces the
costs of innovation, the
evolution of social learning
leads to increases in adaptive
behavior, beyond what
innovation alone could
provide. Otherwise, selection
adjusts the amount of
innovation so that the amount
of adaptive behavior remains
the same, whether social
learning invades or not
0.55
0.50
b
0.45
0.40
0.35
0.30
0.0
0.2
0.4
0.6
s
0.8
1.0
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R. McElreach
learning increasingly makes innovation cheaper and more efficient, the steady state
amount of adaptive behavior increases. p
In contrast, as z ! 1, the above
approaches the previous expression for ^
q^, bb=c b, resulting in no change in
the amount of adaptive behavior as social learning increases.
21.4
Where Did Culture Come From?
Natural selection builds adaptive information into the genome. Learning gathers
information about the environment, to be used by information in the gnome.
Cumulative social learning takes information from behavior whatever its source
and allows it to be stored and accumulated in human brains. The obvious adaptive
utility of the products of this process technologies and strategies too complex for
any individual to invent in his or her own lifetime make puzzling the gap between
humans and other apes in this regard. If “culture” is such a great adaptive trick for
genes to acquire, then why are not other apes similarly cultural? This question is
additionally puzzling, given the evidence of at least proto-cultural social learning
abilities in chimpanzees (see Whiten this volume).
The theory I have reviewed and developed in this chapter addresses the question
of the origins of human cultural abilities. The first goal of the theory is to understand
how natural selection on genes can fail to favor cumulative social learning and under
what conditions it will lead to cultural evolution and accumulation. The second goal
is to understand how the population-level adaptive benefits of this accumulation can
appear, without these being the selective reasons for investments in learning.
21.4.1
Evolving Cultural Evolution
The first goal is addressed by the combined dynamics of innovation and social
learning. When social learning allows accumulation and costs more and more as the
complexity of what is copied or the accuracy with which it is copied increases, then
a fitness value can appear between an a-cultural population and a cultural population
(Boyd and Richerson 1996). If individual learning is effective enough, however, the
model in this chapter suggests that it can provide a way around this valley. If
selection favors improvements in innovation, independent of cumulative social
learning, eventually there is complex behavior that while not accumulated across
generations is nevertheless worth copying, because the costs of social learning are
lower than those of innovation itself. Proximately, the lower costs of social learning
may arise because innovation is an inherently harder activity. Many good ideas are
hard to stumble upon, and much individually learned behavior takes a lifetime to
assemble, despite not being a product of social learning. Once complex behavior is
available, selection might favor acquiring it before any individual effort is made in
innovation. Once this happens, selection trades off innovation against social
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471
learning, reducing the once-large innovation rate that was needed to cross the
cultural valley. This may or may not be the right idea, to explain the gap between
humans and other apes. But it points in the same direction as others have suggested
(see for example Whiten this volume). Innovation and social learning are potentially
co-adapted in humans, and explaining one may ultimately require an equally deep
understanding of the evolution of the other.
In the end, the theory here required investments in social learning to make
innovation more effective or cheaper. A number of other alternations could be
made to the model, to modify the relationship between innovation and cumulative
social learning. An obvious one is to allow the amount of adaptive behavior that is
socially learned, rather than the efficiency of social learning itself, to enhance
innovation. The idea is that previously evolved information may make future
innovation easier, because it defines the relevant parts of the problem and provides
tools to finding solutions. Much of how modern science works involves the development and dissemination of tools, not products. In this way, science is as much
about building intellectual and technological solutions for discovery as it is about
discovery itself.
Similarly, many of the social institutions and cooperative arrangements in modern
societies enhance innovation. Governments actively structure patent law, so that
more innovation is encouraged than would be individually optimal for firms. While
patent law does not necessarily become more effective as knowledge accumulates,
further enhancing innovation, there are other institutions which might. Division of
labor and the exchange institutions that make it possible also enhance innovation, in
two ways. First, division of labor carries with it the benefits of specialization.
Economies of scale make innovation easier in each domain of behavior, and new
information can be traded among specialists more easily than it can be independently
discovered by all of them. Second, as culture accumulates, eventually the sum of
what the population knows exceeds what any individual can learn, even with
advanced social learning. The readers of this chapter are probably among the most
educated people on the planet, and yet each is unlikely to be expert in more than one
or two areas of science. Your author spent a decade learning to understand the
intersection of anthropology and evolutionary ecology, and yet he still has little
deep understanding of some branches of both anthropology and ecology. Like most
scientists, he relies upon experts in other areas combined with active skepticism
and habits of thought to keep track of relevant advances in neighboring fields. This
division of labor allows knowledge in any particular domain hunting large animals
versus gathering palm fiber or processing medicinal plants versus childcare to
grow beyond the limits of individuals to learn and practice all domains.
21.4.2
Evolvability as a Side Effect
The second goal of this chapter has been to highlight the kind of theory that is
required to understand the accumulation of socially-transmitted adaptive behavior,
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R. McElreach
without the eventual highly-adaptive accumulations being the initial reason for the
evolution of the psychology needed to make cumulative culture possible. Human
societies are no doubt more adaptable than those of other apes
we have
conquered (and according to many, ruined) nearly every environment on the
planet, while other apes shrink in tropical refuges. Part of the explanation for
our world dominance is the ability to generate complex, locally adapted behavior
over generations (Richerson and Boyd 2005). Foragers in different parts of the
world need quite different knowledge and strategy. The combination of innovation
and social transmission makes local specialization and regional adaptability both
possible.
And yet, selection does not favor costly social learning abilities, unless there is
an immediate benefit to the organism. Our ancestors did not lug around brains
capable of cumulative culture, because it would turn out to allow our species to
dominate the planet. Instead, we have to seek short-term, individual fitness benefits
in order to explain why an organism would cross the cultural gap. An acorn detects
moisture when it decides whether or not to germinate, because acorns that were
initially slightly sensitive to a moisture gradient produced more descendants. These
descendants them had mutations that favored more sensitivity, until some rough
optimum was reached. Selection favored every step, even though the eventual level
of adaption was higher than the initial. Similarly, the theory in this chapter
hypothesizes that cumulative social learning began as a way to avoid the costs of
innovation. Especially as learned behavior becomes more complex, social learning
allows an individual to rapidly acquire sensible locally-adapted behavior, saving
time and energy for other activities. As each individual continues to add some
continued improvement to what is learned socially, the average adaptiveness of
behavior may increase over generations. However, selection favored each step
along the way because of the benefits and costs at each step, not because of the
population-level benefits that would eventually arise.
The specific model developed in this chapter suggests that one path to evolving cultural evolution lies in first getting selection to favor increases in innovation, as summarized just above. However, any successful theory of the evolution
of evolvability must contend with this same challenge. Students of the evolution
of development (“evo-devo”) are fond of noting how animal body plans can make
life very evolvable, over macro-evolutionary time. Developmental genes are
organized in such a way as to make compartmentalized changes possible the
genome can make one set of limbs longer or even replace them with the genetic
information for another specialized set (see Kirschner and Gerhart 1998). But
while this source of evolutionary novelty may turn out to explain the very
long term success of some groups of organisms (like bilaterally symmetric
animals), it cannot be the reason the body plan arose in the first place. Higherlevel selection, at the population or species level, can indeed explain the maintenance of such adaptations. A popular theory of the maintenance of sexual
reproduction suggests that sex indeed makes populations more evolvable (Maynard
Smith 1978).
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Silk JB, Boyd R (1983) Female cooperation, competition, and mate choice in matrilineal macaque
groups. In: Wasser SK (ed) Social behavior of female vertebrates. Academic, New York, NY,
pp 315 347
Sperber D (2000) An objection to the memetic approach to culture. In: Aunger R (ed) Darwinizing
culture: the status of memetics as a science. Oxford University Press, Oxford, pp 163 173
Szathmáry E, Maynard Smith J (1995) The major evolutionary transitions. Nature 374:227 232
Chapter 22
Mind the Gap: Cooperative Breeding and the
Evolution of Our Unique Features
Carel P. van Schaik and Judith M. Burkart
Abstract Humans are strikingly different from our close relatives, the great apes,
in mind, behavior, and life history. We propose that the evolution of these derived
features was a consequence of the adoption of cooperative breeding by early Homo.
Among the species that adopted it, cooperative breeding generally produced
changes in psychology toward greater prosociality and greater cognitive abilities.
We propose that in our ancestors, the major energetic inputs to breeding females
due to cooperative breeding explain the derived features of human life history and
lifted energetic constraints on brain enlargement. Moreover, in combination with
great-ape -level cognitive abilities, the cooperative-breeding psychology led to the
evolution of many of the unusual socio-cognitive traits that we now celebrate as
uniquely human: pedagogy, extensive cumulative culture, and cultural norms;
intensive and nearly indiscriminate within-group cooperation and morality; a
cooperative declarative communication system known as language; and fullblown theory of mind.
22.1
Introduction
Related species tend to share many features. Our species, Homo sapiens, is an
African great ape. Our ancestors separated from the other apes a mere 6 8 million
years ago (Glazko and Nei 2003). Hence, it would not be surprising if we shared
many features with chimpanzees, bonobos and other great apes. Indeed, the similarities between humans and great apes generated by the research of primatologists
are numerous, and their presence in humans does not require any other explanation
C.P. van Schaik (*) and J.M. Burkart
Anthropological Institute and Museum, University of Zürich, Zürich, Switzerland
e mail: vschaik@aim.uzh.ch, judith.burkart@access.uzh.ch
P.M. Kappeler and J.B. Silk (eds.), Mind the Gap,
DOI 10.1007/978 3 642 02725 3 22, # Springer Verlag Berlin Heidelberg 2010
477
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C.P. van Schaik and J.M. Burkart
than that they have been present for a long time and apparently are not patently
maladaptive in our species, allowing them to persist.
Every species also has derived features, not shared with its closest relatives, or
else it would not be a separate species. Perhaps the most remarkable thing about
humans from a comparative perspective is how many of those unique features
arose. Humans are different enough from great apes that one could easily be misled
into thinking that this biological relationship is irrelevant to understanding human
nature the path chosen by the human sciences for millennia. This dramatic
departure is all the more striking since it is becoming ever more clear that it was
not soon after the split with the other apes, around 6 million years ago, but only with
the emergence of the genus Homo, roughly 2 million years ago, that many of these
traits arose.
Here is a brief summary of the non-morphological features that are derived in
humans relative to the great apes and that seem particularly relevant to us when it
comes to understanding the processes that produced them (see also Flinn et al.
2005; Richerson and Boyd 2005; Burkart et al. in press). First, there are pronounced
life-history differences with the other great apes. Humans have slower development
(later age at sexual maturity) and a longer life span than our great ape relatives. At
the same time, women also show higher birth rates, produce relatively larger
neonates, which are nonetheless weaned much earlier and experience much earlier
than expected cessation of reproduction, known as midlife menopause (Robson
et al. 2006).
Second, our subsistence ecology became radically different from that of any
other anthropoid. Hunting large game and gathering a limited set of plant resources
requires learned, skill-intensive techniques and delayed processing (Kaplan et al.
2000), systematic sharing, especially of meat (Ridley 1996; Gurven 2004), and
some degree of specialization, mainly by sex. This life style is based on extremely
intense cooperation: high social tolerance and prosocial helping within social units,
targeted largely toward kin, toward bonded non-kin, which we, nonetheless, surprisingly call relatives too, and affiliated non-relatives. This intense cooperation
also finds expression in occasional, systematic violent between-group conflict (Gat,
this volume). Cooperation extends to our social organization, which is based on
long-term (monogamous or polygynous) pair bonds, which serve in part as economic units, and in which there is discreet sexual activity not limited to short
periods of sexual attractivity, as in most other primate species.
Third, humans have far more elaborate and cumulative material culture than the
great apes, involving complex artifacts and knowledge, but we also uniquely use
symbols and build institutions based on them, and maintain cultural norms based on
religiously informed normative values. Culture, therefore, plays a decisive role in
both our ecological niche and our “groupishness” (group-serving behaviors).
Fourth, human cognition is distinguished by unusual physical and spatial intelligence, involving causal understanding, episodic memory, and long-term planning.
Even more striking is our social understanding, involving mental perspective
taking, and understanding and sharing of intentions. Humans uniquely use language
to coordinate and plan activities, discuss reputations, and intentionally teach the
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young. Many of these cognitive abilities are ultimately based on the presence of
another uniquely derived motivational feature, shared intentionality, i.e., the ability
to participate with others in collaborative activities with shared goals and intentions
(Tomasello and Rakoczy 2003; Tomasello et al. 2005). It is based on the desire to
share emotional states and knowledge, which, in turn, is due to a prosocial (sharing
and helping) motivation.
What could explain this drastic and rapid divergence? We believe these differences are linked, and are caused by the fact that our ancestors adopted cooperative
breeding. Here, we will present the relevant comparative evidence to establish the
basic credibility of this cooperative breeding hypothesis. Basically, the idea is that
our ancestors were the first Old World primates to engage in extensive allomaternal
care (cooperative breeding). Comparative data suggests that cooperative breeding
installs a more prosocial psychology, which functions to support the more intensive
cooperation in such species, and has immediate consequences for cognitive performance, and in some cases leads to larger brain size. In our ancestors, who had apelike cognitive abilities, this fundamental change in attitudes led to a cascade of
cognitive changes. This idea was first broached by Hrdy (1999), and then developed
by Hrdy (2009) and independently by Burkart and van Schaik (in press; see also
Burkart et al. 2007, in press; Burkart 2009).
22.2
Cooperative Breeding and Human Nature
22.2.1 Cooperative Breeding
Cooperative breeding is defined in rather different ways, depending on the taxonomic focus of the biologists using the term, and as a result there has been quite a
lot of confusion in the literature (Hrdy 2009). For the present purpose, the presence
of extensive allomaternal care, i.e., routine care by other individuals than the
mother, suffices. Thus, fathers, grandmothers, older immature siblings, and aunts
and uncles can all be allomothers. The energetic significance of cooperative
breeding is that it directly or indirectly provides energy inputs to the mother,
allowing her to reproduce more successfully than would otherwise be possible.
Obviously, there is large variation in how many others contribute to rearing the
young, how much they are involved, and what forms their caretaking takes.
There is no doubt that, in sharp contrast to any of the extant great apes, humans
are cooperative breeders (Hrdy 2005, 2009; Mace and Sear 2005). The evidence is
clear among foragers, but remains visible in most derived societies. First, among
foragers, men bring in two-thirds of the calories on average (Marlowe 2003), even if
provisioning is not always exclusive to the nuclear family, perhaps depending on
their opportunities for costly signaling (Hawkes 1993). Second, grandmothers
spend more time foraging, especially for difficult-to-process foods, such as tubers
(Hawkes et al. 1989, 1998), and in many settled societies, grandmothers support
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Fig. 22.1 Summary of positive effects of allomaternal care in humans (data from Sear and Mace
2008). Positive effects on child survival of different categories of care givers in 45 studies of
natural fertility populations. g’mos grandmothers, g’fas grandfathers
mothers in other ways. Third, among foragers and non-foragers, older siblings also
play a major role in childcare and babysitting (Hawkes et al. 1995; Kramer 2005).
We now have quantitative estimates of the effect of this help on birth rates and
infant performance in terms of growth and survival. Grandmothers have been shown
to improve infant survival and maternal reproductive rates, not only among settled,
natural fertility populations (Lahdenperä et al. 2004), but also among foragers
(Blurton Jones et al. 2005). Other helpers also have a positive impact. Figure 22.1
summarizes the results of the review by Sear and Mace (2008).
Extensive allomaternal care can account for the derived features of human life
history listed above, in particular the larger relative size of neonates, the much
earlier weaning, and higher birth rates of humans relative to the other great apes
(Robson et al. 2006), when the opposite would be expected given that brain size sets
these developmental and reproductive parameters in primates (Barrickman et al.
2008) and human brains are about three times as large as those of the other great
apes. Midlife menopause can also be regarded as an expression of cooperative
breeding (Hawkes et al. 1998).
22.2.2
The Cooperative Breeding Hypothesis
Which features of a species are affected by the adoption of cooperative breeding?
First and foremost, cooperative breeding creates a different social system, in which
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group members show extreme social tolerance and intensive cooperation, ranging
from food sharing to collective action, in the form of communal predator mobbing
or territory defense, and even to incipient forms of division of labor, such as sentinel
duties (Burkart and van Schaik in press). This unusual social system requires an
unusual psychology. Among cooperatively breeding primates and canids, we see a
motivational predisposition (prosociality) that produces spontaneous assistance to
others and empathic responses to signs of need, reflected in a concern for others and
eagerness to share food and information with others and cooperation in a wide array
of contexts, in some cases extending to active care for injured or ill group members
(Burkart and van Schaik in press; Burkart et al. in press). Among callitrichid
monkeys, cooperatively breeding primates, prosociality also extends to others in
the group beyond infants, probably as a secondary development, or as a necessary
corollary to cooperative breeding in Callithrix jacchus (Burkart et al. 2007),
although the evidence is mixed for Saguinus geoffroyi (Cronin et al. 2005, in
press; Cronin and Snowdon 2008). Capuchin monkeys (Cebus spp.) also show
elements of cooperative breeding (Fragaszy et al. 2004); accordingly, they too
show elements of spontaneous help toward others, although they were less indiscriminate (de Waal et al. 2008; Lakshminarayanan and Santos 2008). Other primates
tested, including chimpanzees, do not show such prosociality (Silk 2007).1
The most relevant consequence of cooperative breeding for this chapter is that
the cooperative-breeding psychology affects cognition, both directly and indirectly
(Burkart 2009; Burkart et al. in press). Prosociality is expected to produce an
immediate improvement in cognitive performance in two ways. First, it should
improve the conditions for social learning, because it leads to higher social tolerance, increased attention to others, and active involvement of role models (teaching
is above all a form of prosociality: Hoppitt et al. 2008). Second, prosociality should
improve the coordination of activities, such as cooperation, in part through higher
social tolerance, greater attentional biases toward others, and practice in the coordination of infant care. A review of the cognitive performance of cooperatively
breeding callitrichids confirms this prediction: they outperform their independently
breeding sister taxa, especially with respect to these aspects of social cognition, and
less formal comparisons suggest a very similar pattern among carnivores (Burkart
and van Schaik in press).
Indirectly, the great increase in opportunities for social learning improves the
efficiency of use of brain tissue. The Cultural Intelligence hypothesis (Whiten and
van Schaik 2007; van Schaik and Burkart in press) is based on the fact that the high
energetic costs of growing and maintaining brain tissue imposes high obstacles to
the evolution of larger brain size. Thus, any process that lowers these costs will
lower the obstacles to selective benefits favoring increased brain size (Isler and
1
There is large variation in reproductive skew in societies with extensive allomaternal care, from
very high, where one breeding pair monopolizes mating (as in meerkats and callitrichid monkeys),
to rather low, where all adult group members potentially breed (as in capuchin monkeys and
humans). Likewise, species vary in which classes of helpers are the most important (siblings,
males, grandmothers). How this variation affects prosociality remains to be examined.
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van Schaik in press). Reliance on social learning rather than on individual exploration is perhaps the most obvious of such processes. Because the amount and quality
of environmental inputs have been shown to affect the cognitive performance of
adults (van Schaik and Burkart in press), most dramatically so in humans (Tomasello 1999), animals that can rely on social learning can achieve more with the same
brain size. This leads to the prediction that the more species can rely on social
learning to acquire their various skills, the better their cognitive performance
(controlling for brain size) should be. As noted above, cooperative breeders can
rely more on social learning than others. Over time, therefore, cooperative breeders
may attain a larger equilibrium brain size than their independently breeding sister
taxa, provided that improved cognitive abilities sufficiently enhance survival or
reproduction. The latter prediction is, indeed, confirmed in preliminary analyzes of
birds and mammals (K. Isler et al. unpubl. data).
22.2.3 Cooperative Breeding and Human Evolution
The cooperative breeding hypothesis argues that this breeding system installs a
prosocial psychology in a species, which not only affects the nature of cooperation,
but also cognition directly. Indirectly, it may improve cognitive abilities over
evolutionary time. The influences of cooperative breeding on the derived aspects
of human life history, behavior, and cognition can be arranged in three fundamental
classes: (1) direct expressions or consequences of cooperative breeding, such as
midlife menopause; (2) evolutionary consequences of cooperative breeding in the
social or cognitive domain; and (3) side effects of the increase in brain size.
Some distinctive human features are merely an expression of cooperative breeding.
Thus, facultative paternal care for infants and long lifespans following midlife
menopause are simply a reflection of cooperative breeding (Hrdy 2009). It is also
possible that pair-bonding, at least originally, was a direct expression of cooperative breeding. Grandmothering is most plausibly considered as an adaptation
through which aging females achieve better fitness returns than when they were
to continue to breed (Hawkes et al. 1998). This may have arisen in part because
maternal mortality rises rapidly with age in humans (Temmerman et al. 2004;
Pavard et al. 2008), probably more so than in other taxa, in part because human
children rely for much longer after weaning on care (Williams 1957), making
continued survival of the mother important. Nonetheless, the importance of grandmothers for the survival of grandchildren makes grandmothering a special case of
allomaternal care. Cooperative breeding may also have affected our longevity
because care for the sick and injured may reduce unavoidable mortality to lower
levels than in independent breeders, such as great apes, thus enabling selection for
the physiology underlying longer life (Kirkwood and Austad 2000).
The most obvious immediate evolutionary consequence of the adoption of
cooperative breeding is the intense cooperation in humans, which is so different
from that of other apes. Human cooperation must be underlain by a different
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psychological regulation system than found in great apes: prosociality makes active
sharing and collective action possible, especially if aimed at all members of the core
group (creating indirect reciprocity, if combined with concern for reputation). In
humans, this prosocial attitude has been called strong reciprocity (Gintis 2000), and
also encompasses Alexander’s (1987) moral altruism. Although punishment of noncooperators is rare in cooperative breeders (probably because non-cooperation is
rare: Snowdon and Cronin 2007), strong reciprocity in humans also involves
punishment of non-cooperators. However, the recent finding that altruistic (thirdparty) punishment is virtually absent among foragers (Marlowe et al. 2008; but see
Wiessner 2009) suggests that it is a more recent development, necessitated by living
in larger-scale societies. Thus, the psychology of altruism in humans living in
small-scale foraging societies shows a strong similarity, perhaps reflecting convergence, to that found in cooperative breeders.
Cooperative breeding has also produced cognitive changes in humans, as it did
in non-human cooperative breeders, by improving social learning and coordination
of joint cooperative activities. Shared intentionality, i.e., the formation of shared
goals and coordination of their actions in pursuit of these shared goals, forms the
basis of the human ability for cooperative or joint problem solving, the origin of
language, the presence of a full-fledged theory of mind (not just in competitive
contexts), and our tendency to abide by social norms (Tomasello and Carpenter
2007). It also makes teaching intentional, and thus more effective, and thereby
contributes to the presence of cumulative culture (Table 22.1). While its critical role
in human cognition is now recognized, the evolution of shared intentionality
Table 22.1 The transitions from ape like ancestral states to the human derived states that can be
explained by the adoption of cooperative breeding, and its psychological underpinnings
(in particular, prosociality)
Ancestral state
Human state
Social learning and culture
Observational social learning in
! Joint attention and teaching (pedagogy)
apprenticeship
Simple material culture
! Cumulative material culture
Individual innovation
! Cooperative problem solving
Communication
Vocal displays, suppressing information ! Language (information donation), including much
harmful to ego; imperatively used
declarative use
Visual displays, suppressing
! Cooperative eyes (signaling of gaze direction),
information harmful to ego
blushing, crying, and disgust face
Cooperation
Direct, relationship dependent
reciprocity
Dyadic obligations
Unspecialized cooperation
Cognition
Theory of Mind abilities, applied
especially in competitive contexts
! Reputation based indirect reciprocity
( indiscriminate within group altruism, if all
cooperate)
! Morality, religion
! Division of labor
! Shared intentionality and full blown Theory of
Mind, applied to coordinate cooperative
activities
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remained obscure. We suggest that prosociality gave rise to shared intentionality,
because it allowed the nascent theory-of-mind abilities in our ancestors to be
deployed to prosocial ends, and thus gradually enhanced.
Cooperative breeding can, therefore, be seen as underlying many of the cognitive differences with the great apes (see extensive discussion in Burkart et al. in
press). Basically, while chimpanzees, and perhaps all great apes, meet many of the
relevant cognitive preconditions for the evolution of human cognitive potential,
they lack the motivational preconditions. In humans alone, these two components
have come together, the cognitive component due to common descent, and the
motivational component, resulting from the selection pressures associated with
cooperative breeding (Table 22.1). Thus, the high social tolerance of mothers and
eager apprentice attitude of the infants in great apes became the active teaching by
parents and the full-blown system of pedagogy (Gergely et al. 2007). This change
strongly facilitated cultural evolution. The prosocial attitude resulting from cooperative breeding also led to a fundamental change in communication, toward
declarative use of communicative signals, which largely honestly reflected intentions and attitudes, thus enabling language evolution. By extending prosociality
toward all in-group members, dyadic, relationship-dependent cooperation could
become group-wide, indirect reciprocity and more intense collective action, later
backed up my morality and religion.
Cooperative breeding can explain why these cognitive benefits could actually be
expressed in increased brain size in the hominin lineage. Larger-brained organisms
show strong reductions in maximum reproductive potential, rmax (Isler and van
Schaik 2009), which is an estimate of a species’ ability to recover from population
crashes or to evolve new adaptations in the face of rapidly changing environmental
conditions. This negative relationship indicates that each lineage has a maximum
sustainable relative brain size, beyond which reproduction is so low that population
extinction, and thus species extinction, becomes likely. Evolving organisms may
therefore bump into a gray ceiling. In particular great apes are so large-brained that
their demographic viability is severely compromised: they have the lowest rmax on
record. Thus, further brain enlargement in great apes would be prohibited by its
negative demographic consequences.
Given that this strong negative relationship is set by the energetic constraints of
growth and reproduction imposed by larger brains, it is to be expected that it is
relaxed among cooperative breeders (Hrdy 1999). Indeed, cooperative breeders
have higher rmax than similar-sized independent breeders (Isler and van Schaik
2009), and tend to have larger brains than independent breeders, suggesting that the
constraint on brain size has been lifted. Thus, if cooperative breeders experience
clear fitness benefits from enhanced cognitive abilities, their brain size can increase
beyond that possible for independent breeders. This process can account for the
spectacular increase in brain size in the hominin lineage.
The third category of derived features in humans can be regarded as a by product
of the large increase in brain size during hominin evolution. Brains are very costly
tissues metabolically (Mink et al. 1981). According to the expensive brain
hypothesis (Isler and van Schaik in press), brain size increases must be paid for
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either by increases in energy throughput (as indexed by basal metabolic rate) or by
reduced energy allocation to production (i.e., growth and development as well as
reproductive rates). There is good comparative evidence for these predictions
among both mammals and birds (Isler et al. 2008; Isler and van Schaik in press).
The reduced allocation to production, at least in precocial organisms, implies slower
rate of development, controlling for body size, and reproduction once adult, unless
the increased brain size can counteract these costs (more likely for reproduction than
for development, when the brains are not yet mature). Again, these predictions hold
up well in a large comparative sample of precocial mammals and birds. However,
for selection to favor increases in brain size, the larger brains must compensate for
the trend toward delayed and slower reproduction by making the organisms outlive
(or, less plausibly, outreproduce) the smaller-brained variants. Increased longevity
of larger-brained organisms is indeed observed (Ross and Jones 1999; Deaner
et al. 2003; Isler and van Schaik in press).
The increased brain size can, therefore, account for the slower development of
humans (Barrickman et al. 2008), and thus suggests that other, less parsimonious
explanations are less likely to hold. Indeed, the developmental pace of humans
corresponds to the value predicted for a non-human primate of our brain size (Isler
and van Schaik in press). These effects can also explain the strong increase in
human lifespan.
22.3
Cooperative Breeding in Hominins: When Did it Arise?
It may be next to impossible to develop a fully reliable estimate of the timing of
the origin of cooperative breeding in the hominin lineage. It is, nonetheless,
important to do this because we have argued that many other derived human
features depend on the presence of prosociality, which therefore requires that
cooperative breeding arose relatively early, i.e., before these other derived features
arose. As a first step, we can bracket the timing of origin by examining the
endpoint and starting point of the hominin lineage. Extant humans all descend
from cooperative breeders, but it is also likely that Neanderthals, given their strong
reliance on hunting and food sharing, were cooperative breeders, pushing the
origins back at least as far as the ancestor of modern humans and Neanderthals.
On the other hand, it is almost certain that the common ancestor of Pan and the
hominins were not cooperative breeders, given the remarkable absence of any
tendencies in that direction among all extant apes. Thus, we can surmise with some
confidence that cooperative breeding arose somewhere between the earliest australopithecines and mid-Pleistocene Homo.
Several lines of argument suggest that the origin coincided with the emergence
of Homo erectus (or H. ergaster) in East Africa around 1.8 Ma. First, there is
evidence that these were the first hominins to acquire meat from large mammals
(Foley 2001; Pobiner et al. 2008), which necessarily indicates the presence of
cooperative hunting or at least cooperative defense of large carcasses. The large
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carcass size strongly suggests extensive and systematic food sharing, perhaps
serving, as among extant hunter-gatherers, to even out the highly variable yields.
Both these traits are essentially modern, suggesting the operation of similar psychological tendencies toward cooperation and food sharing, which, we suggest, are
based on cooperative breeding.
Second, survival in the newly colonized savanna habitat presumably relied on
skills acquired gradually during development, largely through social learning,
involving some combination of tool-assisted hunting and processing of mammals
and tool-based extraction of underground storage organs (O’Connell et al. 1999;
Kaplan et al. 2000). However, resources harvested as efficiently by juveniles as
adults, such as soft fruits, are much scarcer in savannas (Hawkes et al. 1995). As a
result, immatures would have found it increasingly difficult to achieve a positive
energy balance through their own foraging, unlike in all other primates, which
would have produced increasingly long birth intervals, unless, of course, these
offspring were provisioned by adults. This condition fits with those favoring
cooperative breeding: helping is favored where successful dispersal is difficult,
for whatever reason, and helping has a large positive impact on the immatures
receiving it. The unpredictability of food supply in savanna habitats also may be a
fairly common pacemaker for the evolution of cooperative breeding in birds
(Rubenstein and Lovette 2007) and mammals (Clutton-Brock 2006) generally.
Third, H. erectus was the first hominid to colonize habitats outside Africa, which
were different yet again from the habitats occupied earlier. Hrdy (2005, 2009) has
argued convincingly that colonizing new habitats is facilitated by cooperative
breeding, given the periods of scarcity encountered in a new habitat before novel
solutions have been invented to deal with them.
Fourth, because female H. erectus were much larger in both body size and brain
size than the females of the taxa that preceded them, an increased reproductive
burden must have ensued. As a result, there must have been “a revolution in the way
in which females obtained and utilized energy to support their increased energetic
requirements” (Aiello and Key 2002). We suggest that this revolution included the
emergence of shared care and provisioning, which therefore must have started
around that time.
Finally, as we argued above, external energetic inputs (in particular through
allomaternal care and provisioning, i.e., cooperative breeding) allow a taxon to
break through its taxon-specific maximum viable brain size. This argument would
put the beginning of cooperative breeding in the period directly preceding the rise
of H. erectus, when hominin brain sizes clearly exceeded the great ape range for the
first time (Schoenemann 2006). It is supported by reconstructions of the dental
development of east African H. erectus, which suggest that this was the first taxon
among the hominins to develop more slowly than the extant great apes (Dean et al.
2001). Among primates and other precocial mammals, longer maturation time is a
direct consequence of increased brain size (Barrickman et al. 2008; Isler and van
Schaik in press).
If this circumstantial evidence is accepted, it is consistent with the critical
assumption that cooperative breeding preceded the gradual accumulation of the
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uniquely human features. Many of them would be expected to have arrived later, as
brain size expanded further.
22.4
Discussion
Here, we address some obvious immediate possible objections. First, how does this
idea relate to the other popular scenarios for the evolution of our unique features?
Second, given that prosociality is now recognized as a major derived feature of
humans, how does the cooperative breeding hypothesis relate to the various prominent hypotheses recently proposed to account for our unusual “groupishness?”
Finally, what is known about the selective processes that favored the origin and
guarantee the maintenance of cooperative breeding in general?
22.4.1 Relationship with Other Scenarios for Human Evolution
Producing reconstructions of human evolution, and the development of scenarios to
explain the reconstructed course of events, has been a major pastime of paleoanthropologists from the very beginning. This is understandable, because otherwise
there would be little point to attempting to find fossils of our ancestors. However,
the reconstructions are often limited by the sparseness of the record and the limited
reliability of paleontological “facts” (see the debates over age at death or aging and
tooth wear: Hawkes and O’Connell 2005). Moreover, especially the earlier scenarios have been little more than fanciful teleological stories, explicitly or implicitly
relying on notions such as progress and improvement (Cartmill 1993). Many were
also simplistic silver bullet theories that attributed all of human evolution to one
factor, such as bipedality or hunting. Subsequent ideas or reworkings of the old ones
have been much more informed by evolutionary biology and comparative primatology, and generally focused on specific abilities or taxa, usually the origin of the
genus Homo (e.g., Foley 2001).
Admittedly, the cooperative breeding hypothesis presented here (Hrdy 2009;
Burkart et al. in press) is somewhat of a silver bullet theory for explaining Homo.
What makes us so bold? The main reason is that this hypothesis does not postulate a
single, exclusive force, but rather serves to provide the context that enables many
other, previously identified mechanisms to operate, thus in turn raising the plausibility of these ideas, and sometimes even removing some of their weaknesses.
The predominant explanation for the evolution of human uniqueness has long
been the hunting hypothesis (discussed in Cartmill 1993; Hawkes 2006). This
hypothesis still has much to offer in a modified form, which puts the origin of
hunting much later and no longer as a direct response to bipedality and the secondary
altriciality of newborns as a result of the remodeling of the pelvis it induced. Life on
the savanna opened up the hunting niche, which is skill-intensive, and thus requires
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an unusually large brain as well as long learning periods and hence delayed maturity.
This system is only feasible if immatures are subsidized until reaching adulthood,
and thus if there is parental provisioning (“embodied capital” hypothesis: Kaplan
et al. 2000; Kaplan and Robson 2002). This proposal is therefore very similar to the
cooperative breeding hypothesis, which explains why men, in particular, became
hunters who brought back meat to share it with others. Men may engage in costly
signaling to advertise their value as mates or allies (Hawkes 1993), but they
generally also provision their families (Gurven 2004), and without prosociality
and shared goals, the whole foraging ecology of gathering, hunting, processing
(including cooking), and then systematic sharing with the family or the whole
camp would quickly break down. Nonetheless, there are some differences. For
instance, the embodied capital hypothesis does not consider the help provided by
older siblings, other kin, or grandmothers. Most critically, it assumes that the need
for skill learning is the limiting factor for the age at maturity, whereas the cooperative breeding hypothesis assumes that the slow development of humans can easily be
accounted for by the large size of our brain, and not time needed to learn skills. This
issue is subject to lively debate (but note that in species with altricial young,
maturation time is not linked to brain size: Isler and van Schaik in press).
The grandmothering hypothesis aims to explain our derived life history features,
in particular the presence of the long post-fertility lifespan shown by women
(Hawkes et al. 1998). It argues that extended lifespan beyond the reproductive
years were favored by selection because older women were more effective at
helping rearing grandchildren than producing and rearing their own. It is, of course,
fully compatible with the cooperative breeding hypothesis. However, neither the
embodied capital nor the grandmother hypothesis haves systematically explored the
consequences for the evolution of human psychology and cognition.
The controlled use of fire and the cooking of food it enabled have also been held
responsible for the origin of many of our unique features (Wrangham et al. 1999).
The hypothesis puts its origin at 1.9 Ma, around the origin of H. erectus, although
many experts insist on far more recent dates. Regardless, cooking food makes the
cooks extremely vulnerable to theft, and can only realistically have emerged in
groups with social tolerance that went far beyond the level shown among great apes
and which almost certainly systematically shared food. Moreover, managing fire is
generally based on collective care and sharing. Thus, only hominins that bred
cooperatively could have managed fire and cooked.
Unprecedented short-term variability in climate during the Plio-Pleistocene has
been held responsible for the evolution of human cognitive and, especially, technological abilities (Potts 1998). It did so, not by selecting for particular genetically
based adaptation but instead by selecting for enhanced phenotypic plasticity that
generated adaptation to the new conditions through cognitive solutions. Although
this idea can explain several observations (Potts 1998), one problem with it is why
only hominins should have responded in this way. However, once cooperative
breeding removed the obstacles to increased cognitive abilities and removed
energetic constraints on brain size increases, the enhanced cognitive abilities,
social learning capacity, and cooperative problem-solving skills envisaged by the
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489
cooperative breeding hypothesis allowed hominins to achieve adaptation through
plasticity (cf. Hrdy 2005). Thus, the cooperative breeding hypothesis explains why
variability-imposed selection could weigh in so heavily with the hominins without
similarly affecting other synchronous and sympatric lineages.
On its own, human-like lethal between-group conflict (Alexander 1987; Flinn
et al. 2005) has some difficulty accounting for the whole package of derived human
traits. First, the kind of warfare seen among mobile foragers, in the form of raids, is
rather similar to that seen among chimpanzees (“war below the military horizon”:
Crofoot and Wrangham this volume), which suggests that it cannot explain the
radical reorganization of altruistic psychology that accompanied human evolution.
Second, there is no good evidence for either saturated habitats or systemic humanlike warfare until the last 15,000 years or so of human existence (Hrdy 2009), an
observation consistent with estimates of Pleistocene population sizes (Pennington
2001). On the other hand, the cooperative breeding hypothesis can provide exactly
the group-wide within-group prosociality required to sustain systematic warfare in
humans, well beyond the level seen in chimpanzees, and difficult to achieve by
independently breeding non-human primates (van Schaik et al. unpubl. data). Thus,
cooperative breeding has ultimately made it possible for warfare to evolve to the
level seen in humans, matching that seen among eusocial insects, which are, of
course, obligate cooperative breeders.
The cooperative breeding hypothesis is less consistent with hypotheses that
consider human behavior and cognition as driven by the need to deal with social
complexity, including the various versions of the Machiavellian Intelligence hypothesis (Dunbar 2003). However, these hypotheses do not explain why different
primate lineages differ so much in intelligence (van Schaik and Deaner 2003), or
more specifically why humans became so different from the other apes, whose
social complexity was comparable to that of early hominins (Rendall et al. 2007).
Dunbar (1998) suggested that humans required larger brains and language, because
their groups became too large and new mechanisms of group cohesion were needed.
In the absence of data on hominin group sizes, this idea is nearly impossible to test
(see Rendall et al. 2007).
22.4.2 Alternative Hypotheses for Human Prosociality
There has been much speculation to explain the “groupishness” of people, i.e., our
tendency to be spontaneously altruistic toward all in-group members and to readily
contribute to collective action. Traditional answers range from group selection and
cultural group selection to reputation-based individual benefits. Yet, their relative
merits remain subject of intense debate.
Group selection proposes that groups with altruists outcompete other groups
and replace them with colonists from their own group. The first problem facing
the group-selection explanation is that the frequent, lethal between-group competition it requires is probably far too recent to have overhauled our psychology, as
490
C.P. van Schaik and J.M. Burkart
argued above. Second, human groups are not usually as close to immigration as is
required. Even if other groups are being eliminated, many of their members are
absorbed into the dominant groups (Boyd and Richerson 2006), although perhaps
not always (Bowles 2006).
Cultural group selection argues that groups that adopt prosocial social norms
outcompete other groups, either in direct competition or simply by doing better in
the struggle for life in hostile habitats or by surviving lean periods (Richerson and
Boyd 2005). No genetic homogeneity is required and dispersal between groups is
allowed, but what is needed is that individuals abide by the social (moral) norms of
the group in which they find themselves (or select groups based on the reigning
norms there). Cultural group selection is a very plausible model, supported by much
evidence (Richerson and Boyd 2005), and it may explain much of the current
variation observed among human societies, as well as the maintenance of social
norms. However, Boyd and Richerson (2006) admit that it has difficulty explaining
why hominins were amenable to adopt prosocial norms rather than other ones, or
why they became so credulous or teachable. The cooperative breeding hypothesis
posits that this attitude was there to begin with, removing this weakness to the
cultural group selection hypothesis.
Much seemingly indiscriminate altruistic behavior in human groups may represent indirect reciprocity, in which actors of altruistic acts become recipients of
altruistic acts by third parties because of the reputation gained from these altruistic
acts (Milinski et al. 2002). However, while reputation may explain within-group
altruism, it is disputed whether it can also account for group-level cooperation, i.e.,
collective action (Panchanathan and Boyd 2004). Moreover, since reputation
effects can be far stronger in the presence of language (gossip), reputation may
not yet have been a major factor in early Homo.
We have argued that cooperative breeding can account for the origin and
maintenance of within-group prosociality and the tendency to engage in collective
action in the small groups containing mostly relatives and bonded non-kin pairs that
characterized humans for most of our evolutionary past. If that argument turns out
to be valid, then one might reasonably insist that the explanation for this development in humans must be the same as for the other cooperative breeders, given that
prosociality and generally also coordinated collective action (Burkart and van
Schaik in press) are observed in many cooperative breeders (reviewed in Burkart
and van Schaik in press). Cooperative breeders generally manage to prevent freeriding, and do so without obvious punishment (Snowdon and Cronin 2007),
even when non-kin are involved, and remain prosocial even when groups also
contain non-related helpers (Clutton-Brock 2002, 2006; Zahed et al. 2007;
McDonald et al. 2009).
It is unlikely that any of the three standard explanations for the evolution of
human prosociality discussed above can be applied directly to the cooperatively
breeding callitrichids, carnivores, elephants, or birds. Explaining the evolution of
cooperative breeding in general might, therefore, also solve the problem of the
origin of prosociality in the small forager groups of our ancestors, which consisted
of kin of variable relatedness and bonded partners. On the other hand, it is easy to
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491
see how cultural group selection could produce prosocial cultural norms in human
groups, once cooperative breeding had established prosociality, which facilitated
both cultural evolution and the establishment of social norms.
The maintenance of indiscriminate within-group prosociality in the larger
groups that ensued when foragers began to settle after the adoption of agriculture,
beginning at around 10,000 years ago, although perhaps earlier in some places
(Johnson and Earle 2000), is a thorny problem, but one that is distinct from that of
its origin and initial maintenance in mobile foragers. Accompanying these transitions, egalitarianism gave way to despotism, except within family units, and
uniform within-group sharing disappeared, again except within families (Johnson
and Earle 2000; Rowley-Conwy 2001), and specialized classes of norm enforcers
(punishers) arose. Here, almost certainly uniquely human processes are needed to
account for the maintenance of prosociality in these much larger units, which were
not only more likely to contain distant kin, and later non-kin (after states were
formed), but also routinely went to war with each other. This is probably when
prosocial preaching in the form of organized non-animistic religions arose, and
altruistic punishment by third parties became an important part of the package of
strong reciprocity (Marlowe et al. 2008).
22.4.3 Why Did Cooperative Breeding Arise and Why
was it Stable?
If cooperative breeding explains the origin of prosociality in the hominin lineage,
the key question is how it arose. The details will probably always remain unknown,
but it might be possible to develop a plausible scenario if we knew what conditions,
in general, favor the evolution and maintenance of cooperative breeding. Unfortunately, there is no consensus in the behavioral ecology literature, but from it one can
formulate a general condition. The origin is likely to lie in situations where helpers
gain better fitness return from helping than from dispersing and attempting to join
other units or found their own unit, or alternatively from trying to take over the natal
unit, provided there are non-related adults available as mates (Russell 2004). This
implies that cooperative breeding is likely where the following combination of
factors applies: (1) effective dispersal is very difficult, for a variety of reasons, but
at the same time (2) helping has a large impact on the fitness of the immatures
receiving the help (which may happen for a variety of reasons: Clutton-Brock
2006). Subsequent changes in female reproductive capacity will increase the
positive fitness impact of helping, making cooperative breeding increasingly
more specialized and less opportunistic.
Kin selection can easily account for the origin, but once the system is established, we regularly see some non-kin in the groups, operating as helpers (CluttonBrock 2002, 2006). The maintenance is, thus, difficult to explain against the risk
of free-riding, be it by non-kin group members or by helpers that compete for a
future breeding slot in the group. Theorists argue that free-riding is no threat in
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C.P. van Schaik and J.M. Burkart
cooperative breeders whenever fitness is positively linked to group size and all
group members share a stake in offspring survival, even the survival of offspring
they are not related to (Kokko et al. 2001). Harming a group member would, thus,
automatically diminish the group’s prospect of persisting, and thus make free-riding
not a viable option, even for non-kin. However, it is not clear how often these
stringent conditions are met.
Neither is it clear why potential rivals remain largely prosocial, given that
helping incurs some cost and by holding back in helping, pretenders might improve
their competitive ability, and thus their chances of achieving the breeding position.
The habitually tolerant and peaceful social relations among callitrichids are interrupted by aggression when disputes erupt over breeding status, which as in most
cooperative breeders can be quite serious (Digby et al. 2007). The same thing
seems to be true for other cooperative breeders, be they canids or small herpestids.
For instance, meerkat helpers clearly pay a short-term cost for helping, which
should affect their prospects of becoming a breeder, and thus creates some incentive
for free-riding that would seem to exist. Yet, all individuals seem to help according
to their ability (Clutton-Brock 2006). It is possible that there are constraints on the
extent to which individuals are capable of free-riding just enough to gather its
benefits without being attacked or evicted by others. Perhaps, the neuroendocrine
state that produces prosociality cannot be combined simultaneously with some level
of free-riding, unless the override is cognitively driven (cf. Wiltermuth and Heath
2009). Clearly, given its theoretical significance, it is an important issue for future
research to examine how other cooperative breeders deal with the problem of freeriding and can maintain their prosociality. The solution to this problem may go a
long way toward explaining the evolution of human prosociality.
22.5
Conclusion
In this chapter, we have examined the cooperative breeding hypothesis for the
evolution of human cooperation and cognition, in order to assess whether it is viable
enough to warrant more detailed evaluation. Having done this, we must now
proceed to systematically test its many assumptions and predictions, in a range of
cooperative breeders and in humans.
Acknowledgments We thank Charles Efferson, Kristen Hawkes, Sarah Hrdy, Karin Isler, and
Maria van Noordwijk for valuable discussion or comments on the manuscript, and Peter Kappeler
for the invitation to take part in the Freilandtage.
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Index
A
Access to resources, 173, 174, 183, 185, 189
Access to sexual partners, 201
Admiration, 267, 270
Advantageous inequity, 256, 257
Affiliative liking, 270
Aggrandizer, 147
Agriculturalist, 205
Agriculture, 491
Alliance, 110, 111, 116, 118 120, 123, 124,
126, 127, 129
Allocated power, 121, 123
Allomaternal care, 479, 480, 482, 486
Altruism, 223 240
Altruistic punishment, 230, 251
Amicability, 270
Anger, 266 268, 271 273
Anthropology, 20
Anti predator behavior, 410
Arms race, 209, 210, 216
Ateles, 187
Atom of kinship, 21 25, 39, 41, 42
Attachment, 268
Attitude, 261, 266 273
Autapomorphy, 396, 416
Authority, 140, 141, 146, 147, 149, 150
B
Baboon, 93, 284 291, 294
Battle, 172, 184, 188, 189, 191
Belief desire reasoning, 363, 364
Between group transfer, 22, 38
Bilocality, 30, 43
Biological determinism, 217
Biological market model, 227
Bipedalism, 4, 5
Bipedality, 487
Bonding, 319, 324
Bonobo, 431
Boundary patrol, 184
Brain evolution, 315, 316, 318 323
Brain growth, 317
Brain size, 397, 398, 401, 406, 410, 417,
479 482, 484 488
Brain size evolution, 319 321
Breast, 4
Breeding bond, 26 29, 33 40, 46
C
Callitrichinae, 226, 233 236
Call meaning relationship, 288
Call production, 283 287, 289, 292
Capuchin, 227, 230, 233, 234
Categorical perception, 305
Cebus olivaceus, 174, 183
Cercocebus, 172
Cercopithecine, 226
Cercopithecus, 172, 174
Ceremony, 441
Child, 284, 286, 293, 294
Chimpanzees, 20, 27 30, 32, 33, 35,
37 40, 42, 43, 431, 433 447
bonobo hypothesis, 29
genome, 4
Clan, 202, 203, 211, 213, 214
497
498
Coalition, 110, 111, 113, 114, 116, 117, 119,
120, 124, 125, 127 132, 406 408, 416
Coalitionary support, 226, 324
Coefficient of relatedness, 272
Cognition, 127 131, 397, 398, 406, 407,
413, 415, 416
Cognitive dissonance, 383
Cognitive empathy, 128, 355 358
Cognitive load, 374, 377 378, 383
Cognitive revolution, 299
Collective action, 140, 146, 148, 481, 483,
484, 489, 490
Collective action problem, 115, 184
Communal breeder, 235, 236
Companionate love, 269
Comparative method, 431, 433
Compassion, 223, 239
Competition, 110, 111, 114 118, 120, 122,
124, 128, 131, 133
Competition over resources, 198, 199, 205,
206
Concestor, 433, 434, 439, 440, 443, 446,
447
Conditioning, 292
Conformity, 444 446
Consensual power, 120, 121
Consolation, 232
Contempt, 271, 272
Contest competition, 114 116, 180
Contingent reciprocity, 224 228, 237
Continuity of mental functioning, 351
Convergence, 10
Convergent evolution, 431
Cooking, 488
Cooperation, 224, 226 228, 234, 236, 238,
239, 396, 406
Cooperative breeding, 234 236, 477 492
Cooperative breeding hypothesis,
479 482, 487 490, 492
Cooperative communication, 333, 338 342,
346
Correlated evolution, 319
Costs of aggression, 36
Cross cousin marriage, 24, 25, 38,
44 46
Cryptic female choice, 66
Cult, 211
Cultural adaptation, 453, 454
Index
Cultural evolution, 235, 237 240, 453, 456,
457, 470 472
Cultural evolutionary model, 456
Cultural group selection, 489 491
Cultural identity, 214
Cultural intelligence hypothesis, 332, 346,
446
Cultural norms, 333, 431
Cultural variation, 238, 239
Culture, 5 10, 12, 396, 397, 413, 429 447
Cumulation, 439, 447
Cumulative culture, 454, 460, 466 458, 472
D
Darwinian world, 306
Deception, 364, 365, 373 392, 406,
408 410, 416
Decision making, 245 247, 250, 252, 257
Delayed maturation, 235
Delayed reciprocity, 234
Delayed response paradigm, 403
Delegated power, 121, 123
Dependent power, 120, 121
Detection of deception, 375 377
Detour problem, 400
Development of social cognition, 353
Diana monkey, 288, 289
Dictator game, 251, 256
Diffusion experiments, 436, 437, 442, 444,
446
Discrimination learning, 403, 404, 413
Disgust, 264, 265, 271 274
Displacement activities, 378
Division of labor, 95, 98, 100, 481
Divorce, 89, 90, 93, 95, 97, 98
Dominance, 109 133, 139 141, 143, 149,
150, 267, 270, 275
hierarchy, 112, 113, 115, 128, 131, 140
relationship, 110 112, 118, 126 128
style, 113 119, 126, 127
Dual inheritance theory, 455
Dualism, 6
E
Economic rationality, 247
Ecstasy, 212 213
Index
Egalitarianism, 115, 116
Emotion, 205, 208, 212, 213, 215, 217,
261 276, 284, 286, 293, 397, 412, 416
Emotional empathy, 355
Emotional recognition, 355
Emotional state, 286
Empathy, 223, 231 232, 234, 235, 271,
353 358
Emulation, 442
Enculturation, 353
Endowment effect, 250, 255
Environment of evolutionary adaptedness, 8
Envy, 267
Equity, 245 258
Essentialist reasoning, 273
Ethnocentrism, 215
Eusociality, 154, 165
Evoked culture, 9
Evolutionary psychology (EP), 8 10
Exogamy, 21 27, 33, 34, 38, 39, 42,
44 47
Exogamy configuration, 27, 33, 34, 39, 46
Expected utility maximization, 247, 257
Expensive brain hypothesis, 284
Extractive foraging, 316
F
Facial expression, 412, 416
Fairness, 223, 231, 234, 237, 247, 250 252,
255, 257
False belief, 344, 345
False belief task, 352, 363, 364
Family, 213 215
Fear, 264
Female bonded group, 324
Female dominance, 113 119, 125, 128, 130
Fertility, 91 93, 100
Feuding, 188
Fission fusion sociality, 325, 326
Food calling, 230
Food finding, 316
Food sharing, 132, 226, 481, 485, 486
Forced copulation, 57 60, 64, 69
Formal dominance, 114, 125
FOXP2, 5
Free riding, 490 492
Frequency dependent selection, 376
499
Friendship, 71 73
Functional referentiality, 398, 411, 412
G
Gathering, 439
Gaze, 284, 290, 293, 294
Gaze following, 409, 410
Gender difference, 87
Gender role, 87
General intelligence, 332, 333
Genetic determinism, 7
Genetic similarity, 224
Genital coagulates, 66 67
Gestural communication, 332, 333
Gesture, 293, 336, 338 342, 412, 413, 416
Ghost experiment, 442
Goal emulation, 359 361
Gorilla hypothesis, 28, 29
Gossip, 490
Grandmother, 235, 236, 479, 480, 482, 488
Grandmothering, 482, 488
Granted power, 120, 121, 123
Gratitude, 269, 273
Grief, 271
Grooming, 223 240, 406, 408, 416
Grooming clique, 323, 324
Group coordination, 412
Group dominance, 174, 178, 182, 183
Group hunting, 334
Group level cooperation, 234, 236
Group selection, 215, 238, 239, 274,
489 491
Guided variation, 460
Guilt, 239, 269 271, 275
Guilty knowledge test, 376
H
Hamilton’s rule, 225, 226
Haplorrhine, 396 408, 411 413, 415, 416
Happiness, 380 382, 392
Helping, 164, 165
Homicide, 201, 203, 205
Hominid ancestors, 284
Hominin, 95
Homo economicus, 245 247, 249, 251
Homo erectus, 485, 486, 488
500
Homology, 10
Honest advertisement, 140
Honor, 206, 207, 211
Horticulturalist, 202, 204
Human foraging society, 235, 237
Human history, 162
Human intellectual evolution, 352
Human language, 283 295
Human revolution, 299 310
Human social group, 325
Human society, 19 47
Human state of nature, 197 217
Human universals, 54, 73, 85 100, 433
Human voice, 378
Hunter gatherers, 30, 35, 43, 326
Hunter gatherer society, 131, 200, 203, 211,
445
Hunting, 227, 236, 237, 435, 439, 478,
485 488
Hunting hypothesis, 487
I
Ideational resources, 122, 123
Imbalance of power hypothesis, 185 187,
189
Imitation, 359 362, 430, 432, 441 445, 447
Imitative learning, 332
Immune system, 380 383
Implicit association test, 387
Inbreeding avoidance, 265
Incest avoidance, 27, 41, 45
Incest taboo, 23, 39
Inclusive fitness, 41
Independent power, 120 122
Indirect reciprocity, 483, 484, 490
Individual learning, 430, 442
Individual recognition, 289
Inequity aversion, 256, 257
Infanticide, 68 73, 94, 95, 100
Inheritance system, 458, 459
Innovation, 316, 319, 406, 410 411, 416,
451 472
Instrumental helping, 337
Intelligence, 5, 386 387, 481, 489
Intention, 331 346
Intentional representation, 352
Intercommunity aggression, 185, 186
Index
Intergroup aggression, 171 191
Intergroup dominance, 173, 174, 181,
183 185, 187, 189, 190
Intergroup dominance hypothesis, 173, 174,
181, 183, 187, 189, 190
Intergroup encounter, 184, 185
J
Japanese macaque, 437
Jealousy, 267
Joint attentional frame, 341, 342, 345
K
Kin recognition, 27, 30 33, 37, 41, 225, 272
Kin selection, 224 226, 236, 491
Kinship, 214, 215
bond, 25, 42
network, 22
L
Language, 46, 299 310, 382, 387, 404, 405,
409, 411, 413
Leadership, 140, 145 150
Learning, 452 454, 456 472
Legitimacy, 120, 121
Lemur, 395 417
Lemur catta, 180 182
Levirate, 22, 23, 44 46
Life history, 478, 480, 482, 488
Life span, 478, 482, 485, 488
Limerance, 268, 269
Linguistic revolution, 294
Local adaptation, 454
Local enhancement, 358
Longevity, 482, 485
Loss aversion, 249, 250, 252, 254, 257
Lust, 265
Lying, 377, 382, 387, 390
M
Macaca
fuscata, 172, 178 180
sinica, 182
Machiavellian intelligence, 489
Male female association, 69 72
Index
Male philopatry, 29, 30, 43, 44, 226
Male provisioning, 236
Marmoset, 226, 233 236
Marriage, 87, 89 96, 98, 99, 201 203, 210,
214, 215, 441
Material culture, 440, 478
Mating competition, 87, 88, 97
Matriline, 30, 44
Matrimonial exchange, 21, 27, 38, 42,
44 46
Maze experiment, 400
Memes, 438
Memory, 378, 385, 399, 400, 403, 404
Menopause, 478, 480, 482
Menstruation, 308, 309
Mental image, 289
Meta cognition, 445
Military horizon, 188, 189, 191
Mimicry, 359, 360
Mirror neurons, 443
Monkey token economy, 253
Monogamy, 33 36
Moral altruism, 483
Moral approbation, 273, 274
Moral disgust, 273
Moralistic aggression, 132
Morality, 484
Moral norms, 441
Moral outrage, 273 275
Mortality rate, 174
Mother child interaction, 357
Multifamily community, 27 29
Multilevel selection, 318, 319
Multiple mating, 87, 88, 93, 100
Mutual support, 337
N
Natal attraction, 268
Natural selection, 6, 7, 224, 225, 452 454,
457, 463, 466, 467, 470
Neanderthals, 310, 485
Neocortex, 4
Nepotism, 30, 114 118, 226
Nervousness, 377, 378
Norm, 262, 272 275
Normative guilt, 275
Normative judgment, 343
501
Novel objects, 401
Numerical representation, 405
O
Object Choice task, 339 341
Object permanence, 399
Observational learning, 444, 446, 447
Oddity concept, 404
Offspring love, 268
Offspring survival rate, 174
Old age positivity, 383, 385 386
Orangutan, 57, 60, 69, 70
Origins of language, 300
Other regarding preferences, 232 236
P
Pair bond, 25, 27, 33 36, 38, 39, 41, 42,
44 46, 72, 87, 93 100, 478, 482
Pan homo split, 27 29, 34
Papio cynocephalus, 93
Parental collaboration hypothesis, 34, 35
Parental investment, 55, 73, 86 89, 94, 99
Parental love, 268
Paternal care, 35 37, 86, 87, 94, 96, 97, 99,
100
Paternal kin, 227
Paternity certainty, 86, 94
Patriline, 30
Patrilocality, 30, 39, 43
Pedagogy, 484
Penance, 270, 275
Penis morphology, 66, 67
Perception, 284, 285, 287, 289, 292
Personality, 111, 113, 117
Perspectival cognitive representation, 344
Perspective taking, 478
Phenotypic plasticity, 452, 460 462
Phonation, 284, 287, 292
Phylogenetic analysis, 26, 27, 35, 46
Physical cognition, 399, 416
Pity, 270, 271
Play, 302, 305 307
Playing, 209, 211, 212
Pleistocene hominin, 237
Plesiomorphy, 396, 397
Pointing, 304, 306, 338 341, 345
Policing behavior, 441
502
Political intelligence, 127, 130
Polygyny, 33 36, 44, 45, 89, 97, 201, 203,
204
Population density, 209
Power, 109 133
Predator alarm call, 285
Prestige, 139 150, 275
competition, 143 148
goods, 140, 143, 146 150
Price equation, 239
Pride, 267, 268, 274, 275
Primatology, 9 11
Priming effect, 388
Prisoner’s dilemma, 208 210, 216, 389, 390
Processualism, 120, 121, 123
Processualist paradigm, 121 122
Promiscuity, 61, 68, 69, 73
Prosocial behavior, 234, 236
Prosocial helping, 478, 479
Prosociality, 481, 483 485, 487 492
Proto pride, 267, 268, 274, 275
Proto shame, 267, 268, 274, 275
Punishment, 230, 238, 275, 483, 490, 491
R
Rape, 60
Ratchet effect, 226, 342, 438
Rational choice, 247, 248, 257
Rational imitation, 360
Rationality, 5, 6
Reciprocal altruism, 225
Reciprocal exogamy, 21 27, 42, 44, 46
Reconciliation, 408, 416
Reconciliatory behavior, 270
Red Queen effect, 210, 216
Reference dependence, 249, 250, 254, 255,
257
Referential communication, 353 355
Reflection effect, 249
Religion, 211, 435, 440, 484, 491
Reproductive division of labor, 154, 164
Reproductive potential, 484
Reproductive rate, 174, 179, 181 183
Reproductive skew, 154, 164 166, 204
Reproductive strategy, 60
Reproductive success, 87, 90 93, 164
Reputation, 231, 238, 239
Index
Revenge, 198, 207 210
Reversal learning paradigm, 403
Risk taking behavior, 205
Ritual, 210, 211
Role reversal, 335 337
Role taking, 336
Romantic love, 95
S
Scramble, 180
Scramble competition, 114
Security dilemma, 198, 209 211, 216
Self
deception, 373 392
esteem, 382
image, 382, 387, 388
inflation, 383, 384
Sensory integration, 291
Sex difference, 85 100
Sex role, 87, 88, 98, 99
Sexual access, 154, 155, 160, 164
Sexual conflict, 54 58, 60 62, 66 69,
72 74, 98, 99, 308
Sexual division of labor, 237
Sexual harassment, 57, 61 64, 70, 73
Sexual intimidation, 57 59, 61, 63, 64, 68
Sexually antagonistic coevolution, 56, 57, 72
Sexual maturity, 478
Sexual selection, 87, 88, 93
Sexual swelling, 70
Shame, 239, 267, 268, 274, 275
Shared ancestry, 432, 444
Shared attention, 293, 294, 353 355
Shared goal, 334 338
Shared identity, 272, 273
Shared intentionality, 331 346
Sharing, 478, 479, 481, 483, 485, 486,
488, 491
Signaling theory, 143, 149
Social brain hypothesis, 322
Social cognition, 406, 416
Social coherence, 319
Social complexity, 323
Social emotion, 239
Social identity, 273, 275
Social institution, 331
Social interaction, 110, 126
Index
Sociality, 319, 320, 324 326
Social learning, 142, 353, 358 362, 406,
410 411, 416, 430, 432, 433, 436,
441 447, 454, 457, 459 472, 481 483,
486, 488
Social mind, 303 305
Social mirroring, 359
Social monogamy, 70
Social network, 94, 100
Social norm, 230, 239, 240
Social power, 139 141, 148 150
Social referencing, 293, 354
Social science, 7, 8
Social structure, 20, 23, 26, 28, 29, 39, 46, 47
Social tolerance, 478, 481, 484, 488
Social tool, 339
Social transmission, 430, 432, 436
Sociobiology, 8, 217
Sororate, 22, 23, 44 46
Source sink dynamic, 190
Spatial mapping, 399
Spatial memory, 399, 400
Sperm competition, 56, 66, 67
Spider monkey, 187
Spiritual life, 210
Sport, 212
Status, 110 112, 114, 118, 120, 123 126,
128, 139, 141, 143 145, 147, 148, 150
Stimulus enhancement, 359
Strepsirrhine, 397, 398, 401 404, 406, 410,
412 416
Strong reciprocity, 483, 491
Stroop test, 383
Subsistence ecology, 478
Supernatural belief, 211
Support, 111 114, 116, 118 125, 128,
131, 132
Symbolic culture, 301, 305, 307, 310
Symplesiomorphy, 395 417
Syntax, 289 290, 293, 294
503
T
Tamarin, 226, 233, 234
Teaching, 342, 343, 353, 359, 362, 430, 441,
445, 446, 481, 483, 484
Technical cognition, 413
Territorial defense, 182
Territoriality, 199
Theory of mind, 235, 293 295, 352, 353,
356, 357, 362 365, 409
Theory of reality, 380
Third party punishing, 274
Tit for tat rule, 225, 389, 390
Tools, 36, 429, 431, 435, 438, 440, 447
Tool use, 316, 402, 411, 413, 415
Tradition, 430 439, 442, 446
Traditional society, 141, 144
Transitive inference, 406, 407
Tree thinking, 263
Triadic awareness, 111, 117, 128, 129
Tribe, 34, 39, 42 44, 202 205, 213, 214
Trust, 306, 307
U
Ultimatum game, 251, 390
Use of fire, 488
V
Vicarious emotion, 262, 272 273
Vocal communication, 284, 292,
293, 411
Vocal imitation, 287
W
War, 171 173, 188, 189, 191
Warfare, 171 173, 185, 188 191, 489
Weapon, 36, 130
Welfare valuation, 271