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Sexual Equality and Cooperation, Hunter-Gatherer Style

A two-year study of modern hunter-gatherer societies suggests that sexual equality led people to live with fewer kin and, in turn, the evolution of cooperation.
An Agta household in the Philippines. (Photo: Mark Dyble)

An Agta household in the Philippines. (Photo: Mark Dyble)

One of the great mysteries of the universe is how humans manage to cooperate with each other. There's no shortage of explanations, including the possibility that cooperation co-evolved with conflict and war, but a new study suggests an entirely different possibility: Perhaps cooperation is an indirect consequence of sexual equality in early humans.

That idea stems from a curious feature of modern hunter-gatherer societies. In most societies, members of the same extended family often live near each other, sometimes even in the same house. Biologically speaking, that makes sense. Families that live together can more easily help one another out (a.k.a. survival), which increases their chances of reproducing and passing the family genes on to another generation. Thus, natural selection often favors those organisms—not just people—predisposed to supporting their families. By extension, it ought to favor organisms prone to living with their families as well.

But hunter-gatherer societies don't fit that pattern. Modern hunter-gatherers tend to live alongside fewer of their own kin, compared with, say, agricultural societies. And no one's sure why.

Maybe hunter-gatherers live with fewer of their or their spouse's family members because they're balancing the wishes of both husband and wife.

In hopes of solving that puzzle, University College London anthropology student Mark Dyble and his colleagues first studied two groups of modern hunter-gatherers: the Palanan Agta in the Philippines and the Mbendjele BaYaka in Central Africa. On average, the team discovered, any one person in those communities was genetically related to about a quarter of the others in the same community, related by marriage to another quarter, and distantly related or completely unrelated to the remaining half.

Those results were consistent with an earlier study of two other hunter-gatherer societies, but stand in contrast with the Paranan, a farming community neighboring the Mbendjele. The Paranan people, the anthropologists found, live with more family members overall, and they live with about twice as many of the husband's family than they do the wife's.

That gave rise to a hypothesis: Maybe hunter-gatherers live with fewer of their or their spouse's family members because they're balancing the wishes of both husband and wife. To test the idea, the researchers built a computer simulation describing how people made decisions about where to live. In one version, the choice was egalitarian, and in another it was dominated by the husband. According to those simulations, when husband and wife decided on living arrangements together, they ended up living in communities with fewer relatives overall, but a more even split between the husband's family and the wife's.

So what's the connection to cooperation? Following on others' work, Dyble and his team argue that communities made up of people largely unrelated to each other have to learn to cooperate; if they can't, they might well die. If their theory is correct, and if our hunter-gatherer ancestors had reasonably progressive views on power sharing in marriages, sexual equality may have led indirectly to another of humanity's better instincts, cooperation.

Quick Studies is an award-winning series that sheds light on new research and discoveries that change the way we look at the world.