Did sexual equality fuel the evolution of human cooperation?

When a massive earthquake struck Nepal 3 weeks ago, people around the world flooded the country with donations and other offers of support. Humans are among the most cooperative animals on the planet, yet scientists are unclear about how we got this way. A new study suggests the answer may be gender equality: When men and women have equal say in who they associate with, our social networks get larger.

Anthropologists used to think that we grew our social networks by associating with people who were genetically related to us. Families moved in with grandparents and cousins, who themselves lived close to other relatives. But a 2011 study of 32 hunter-gatherer societies found that most individuals living together in large groups were not genetically related.

Anthropologists Mark Dyble and Andrea Migliano of University College London wondered if human cooperation had less to do with genetics and more to do with sexual equality. If both men and women could decide who they lived with, they reasoned, husbands and wives wouldn’t always be living with their own relatives; they’d often hang out with folks they had no genetic or marital ties to.