Fig. 5.

Posterior distributions of reproductive skew values (M^*) in humans as a function of marriage system. Points represent posterior means, and lines represent 89% credible regions. The dashed vertical line at M^* = 0 indicates that reproduction is neither positively skewed, nor more equal than would be expected by a random model. In general, male reproductive skew appears fairly invariant to marriage system. Female skew appears slightly higher in human populations with socially imposed monogamy (normative monogamy) than populations in which polygyny is widely practiced (normative polygyny). Across all marriage system types, sex differences in skew are reliably different from zero—indicating that male reproduction is slightly more unequal than female reproduction, even where monogamy is imposed (normative monogamy) or frequent (polygyny rare, but tolerated). In contexts where polygyny is common, sex differences in skew are especially high. Sample sizes: N = 90 human populations (43 normative monogamy, 33 polygyny permitted, and 14 normative polygyny).