Reproductive inequality in humans and other mammals

Abstract

To address claims of human exceptionalism, we determine where humans fit within the greater mammalian distribution of reproductive inequality. We show that humans exhibit lower reproductive skew (i.e., inequality in the number of surviving offspring) among males and smaller sex differences in reproductive skew than most other mammals, while nevertheless falling within the mammalian range. Additionally, female reproductive skew is higher in polygynous human populations than in polygynous nonhumans mammals on average. This patterning of skew can be attributed in part to the prevalence of monogamy in humans compared to the predominance of polygyny in nonhuman mammals, to the limited degree of polygyny in the human societies that practice it, and to the importance of unequally held rival resources to women's fitness. The muted reproductive inequality observed in humans appears to be linked to several unusual characteristics of our species-including high levels of cooperation among males, high dependence on unequally held rival resources, complementarities between maternal and paternal investment, as well as social and legal institutions that enforce monogamous norms.

Keywords: egalitarian syndrome; inequality; mating systems; monogamy; reproductive skew.

Fig. 1.

Polygyny intensity and reproductive skew as a function of male rival resource inequality, rival resource importance, and mating system norms using the generalized polygyny threshold model introduced by Oh et al. (19). Male rival resource inequality, R, is measured using the Gini coefficient and ranges ∈(0.12, 0.64). Rival resource importance, μ, is measured using the fitness elasticity of rival resources and ranges ∈(0.15, 0.95). Nonrival resources, G, are held constant with a Gini coefficient of 0.12. Nonrival resource importance, γ, is measured using the fitness elasticity of nonrival resources and is given by the equation γ = 1 − μ to ensure constant returns to scale. For further methodological and mathematical details, SI Appendix, S3. (A) Male polygyny (e.g., percentage of married men with more than one wife) and female polygyny (e.g., percentage of women with cowives) as a function of male rival resource inequality, rival resource importance to fitness, and mating system norms. (B) Reproductive skew, M, as a function of male rival resource inequality, rival resource importance to fitness, and mating system norms.

Fig. 2.

Raw data on male and female reproductive skew in mammals (Left) and humans (Right). In panel (A), polygynous nonhuman mammal values are plotted with red circles, monogamous nonhuman mammal values are plotted with blue triangles, and human values are plotted with goldenrod squares. In panel (B), polygynous human values are plotted with red circles, and monogamous human values are plotted with blue triangles. In both panels, points on the dashed diagonal line represent groups with equal male and female skew values. Points below the line indicate groups where male skew exceeds female skew, and vice versa for points above the line. Because M values are very high for some species, we visualize the data using the signed square root transform: $M^{\ast }=\text{sign}(M)\sqrt{|M|}$. (A) Male and female reproductive skew across species. (B) Male and female reproductive skew across human populations.

Fig. 3.

Posterior distributions of the difference in reproductive skew between humans and nonhuman mammals/nonhuman primates. Points represent posterior mean differences, and horizontal bars represent 89% credible regions. The dashed vertical line at zero indicates no difference. Humans stand out from both nonhuman mammals, generally, and nonhuman primates, specifically, in terms of having lower values of average male reproductive skew and lower sex differences in skew. Female reproductive skew, however, appears similar in humans and both nonhuman mammals and nonhuman primates—on average. Sample sizes: N = 90 human populations, N = 49 nonhuman mammal species, N = 12 nonhuman primate species.

Fig. 4.

Posterior distributions of reproductive skew values (M^*) in nonhuman mammals as a function of mating 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 under a random model. Monogamous nonhuman mammals stand out from polygynous nonhuman mammals, in terms of having significantly higher absolute values of male and female reproductive skew, and significantly lower sex differences in skew. Female reproductive skew, in particular, is strongly damped in polygynous species and elevated in monogamous species. Sample sizes: N = 49 nonhuman mammal species (8 monogamous, 41 polygynous).

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).

Fig. 6.

A strong positive relationship between male skew (Top frame) and sex differences in skew (Bottom frame), as a function of percent age-adjusted female polygyny in humans. Percent age-adjusted female polygyny is the predicted fraction of women married to men with more than one total wife by age 60 (see ref. , for details). The solid line plots the posterior mean regression, while the shaded area plots the 95% posterior credibility region. The black points give the data. Sample size: N = 19 human populations.

Fig. 7.

A different patterning of skew in polygynous human populations and polygynous nonhuman mammals. 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. Male reproductive skew in polygynous humans is substantially lower than in polygynous nonhuman mammals: The contrast is −0.59 (89%CI: −0.87, −0.32). Female skew is also higher in polygynous human populations than in polygynous nonhuman mammals: The contrast is 0.2 (89%CI: −0.05, 0.43). Sex differences in skew are therefore much lower in polygynous human populations than in polygynous nonhuman mammals: The contrast is −0.80 (89%CI: −1.03, −0.53). Sample sizes: N = 14 polygynous human populations and N = 41 polygynous nonhuman mammal species.

Fig. 8.

The distribution of sex-specific reproductive skew values as a function of subsistence mode in N = 90 human populations. We find that reproductive skew does not vary strongly by subsistence mode.