Previous research has indicated that females are more likely than males to endorse collectivistic values and religious conservatism. The present research investigated an evolutionary explanation for these sex differences. More specifically, the sex differences in social conservatism may be due to variation in the behavioral immune system (BIS). The BIS is a set of psychological mechanisms that are proposed to be evolved solutions to disease threat. Four studies were conducted to examine this evolutionary explanation. In Study 1, BIS measures (e.g., disgust sensitivity) fully mediated sex differences in collectivism. This effect was specific to sexual disgust (Study 2). In Studies 3 and 4, the effect was extended to other forms of social conservatism (i.e., religious conservatism) and measures of the BIS. Together, these results suggest that sex differences in collectivism and religious conservatism may be explained in part by sex differences in the BIS.