Multiferroics present a dynamic crossover between magnetic and electronic order parameters. Using both the conventional pyroelectric current (PC) and the bias electric field (BE) methods, the authors investigate the electric polarization of multiferroic MnO (= Ca, Sr, Cd, Pb). While the pyrocurrent measurements show intense peaks below the magnetic ordering temperature, the peaks are not observed in the biased electric field method, which indicates the peaks are produced by thermal stimulation, not ferroelectricity. In CaMnO, the electric polarizations polarization produced is quite small, which suggests that the large polarizations reported in previously are not associated with intrinsic ferroelectricity.