Many people gather on a beach in Pemba, Tanzania (image: Monique Borgerhoff Mulder)

Many evolutionary psychologists and popular science writers have argued that when it comes to mating, humans are rather like elephant seals and other polygynous mammals. Successful male elephant seals acquire large harems of females, while unsuccessful ones don’t manage to breed at all, a characteristic known as high reproductive skew. This is presumed to lead to competitive, aggressive males, and coy, choosy females.

Field anthropologists, in the meantime, have come to a starkly different perspective, proposing that human mating is unusually, even exceptionally egalitarian compared to other mammals, with most males having relatively equal opportunities to reproduce. In other words, they say, we have extraordinarily low reproductive skew. And this means that aggressive males aren’t more successful, leading to males and females that are more similar to one another.

So which is it? Are we an egalitarian species with limited differences between the sexes, or are we a winner-take-all species that rewards dominant males and reluctant females? Oddly enough, the question has not been directly tested — until now.

An SFI-affiliated team has finally settled it: Human reproductive skew is lower than most mammals, overall supporting the idea that human societies tend toward egalitarianism and fairly small differences between the sexes. But it’s also not exceptionally low, as some anthropologists have claimed. Quite a few mammals, such as the bottlenose dolphin, have lower skew. “Human exceptionalism has been greatly exaggerated,” SFI External Professor Monique Borgerhoff Mulder (UC Davis) says.

For the study, titled “Reproductive inequality in humans and other mammals,” former SFI Omidyar Fellow Paul Lovell Hooper (University of New Mexico) and former SFI Program Postdoctoral Fellow Cody Ross (Max Planck Institute for Evolutionary Anthropology), together with Borgerhoff Mulder, led a team of more than 100 scientists* to both empirically test the level of human reproductive skew, and to develop a working model to explain it.

The researchers gathered data from 90 societies from around the world, including such enormously varied cultures as royalty in Europe, Indonesian whale hunters, nomadic herders of northern Mongolia, and manioc-cultivating, fishing, and hunting communities of southern Guyana. They compared the data with those of 49 nonhuman mammals.

The model the team created suggests that reproductive skew across species is driven primarily by two factors: How much male investment in parenting helps offspring, and how unequally resources are divided. 

Remarkably, human reproductive skew remains quite similar across societies despite extraordinarily different cultural and reproductive practices. The key to this consistency appears to be that human children need enormous investments from both parents. A man might be able to produce fifty children, but he won’t be able to provide for or nurture them all. 

“This uncovers a new human universal,” Hooper says, “that reproductive inequality in our species is confined to a very narrow range, from the most equal to the most unequal conditions of material wealth. This is surprising, and points to something important about the nature of social relationships and reproduction in our societies.”

* The team also includes SFI researchers Stefani Crabtree (Utah State University), Samuel Bowles, and Eleanor Power (London School of Economics & Political Science).

Read the paper, “Reproductive inequality in humans and other mammals,” in PNAS (May 22, 2023). doi:10.1073/pnas.2220124120