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.

Jeans and language: kin networks and reproductive success are associated with the adoption of outgroup norms

“…We computed the multinomial reproductive success skew index (M value, [41,42]) for all the elder Mosuo and Han men and women, who had finished their reproduction (or at least most of the reproduction) before the governments' child policy in the 1980s, that limited births in this region. Based on Peter Nanac's binomial index, B [43,44] and Bayesian estimation, the M index accounts for variance in age across individuals and is not influenced by differences in sample or group size [41].…”

Mechanisms of equality and inequality in mammalian societies

“…Social dominance can have important consequences for animals, contributing to unequal access to resources [35] and-in some cases-reproductive inequality (skew) [36][37][38], which varies within and among species (e.g. [39,40]). Because the diversity and evolution of reproductive inequality across humans and other mammals has recently been characterized elsewhere [40] and reproductive skew is discussed extensively in other contributions in this special issue [16,19,41], the first section of this paper focuses on social dominance as a measure of inequality.…”

“…[39,40]). Because the diversity and evolution of reproductive inequality across humans and other mammals has recently been characterized elsewhere [40] and reproductive skew is discussed extensively in other contributions in this special issue [16,19,41], the first section of this paper focuses on social dominance as a measure of inequality. Social dominance reflects how individuals are organized into a dominance hierarchy based on the outcomes of pairwise agonistic interactions (e.g.…”

Toward an evolutionary ecology of (in)equality

“…Although human populations do certainly exhibit reproductive skew [50], extreme forms of reproductive suppression and altruism such as in mole rats [20] and social insects [51] have little human counterpart. One key difference in the human case is attributed to enhanced paternity certainty and resultant paternal investment, resulting in a major expansion of kinship ties and the option of patrilineal as well as matrilineal networks and inheritance pathways [27,28,52,53].…”