Ensuring high-quality, civil social interactions remains a vexing challenge in many online spaces. In the present work, we introduce a novel approach to address this problem: using psychologically "embedded'' CAPTCHAs containing stimuli intended to prime positive emotions and mindsets. An exploratory randomized experiment (N = 454 Mechanical Turk workers) tested the impact of eight new CAPTCHA designs implemented on a simulated, politically charged comment thread. Results revealed that the two interventions that were the most successful at activating positive affect also significantly increased the positivity of tone and analytical complexity of argumentation in participants' responses. A focused follow-up experiment (N = 120 Mechanical Turk workers) revealed that exposure to CAPTCHAs featuring image sets previously validated to evoke low-arousal positive emotions significantly increased the positivity of sentiment and the levels of complexity and social connectedness in participants' posts. We offer several explanations for these results and discuss the practical and ethical implications of designing interfaces to influence discourse in online forums.