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.

Together with several colleagues, the duo created two versions of a mathematical model: one in which both men and women had an equal say about who their families lived with, and another in which only one or the other sex made that decision

As the researchers report online today in Science, gender equality led to more diverse living arrangements. Even in groups as small as 20, people under the egalitarian model had a 12% chance of being unrelated to another individual, whereas those living in a nonegalitarian model had a less than 1% chance of being unrelated to any other individual.

Armed with this model, the anthropologists went into the field during a period of 2 years, collecting data from two contemporary hunter-gatherer groups. They gathered information from 191 adults spread across 11 camps of the Palanan Agta people of the Philippines, and from 103 adults in nine camps of the Mbendjele pygmies of Central Africa. In both groups, households move often between bands that include the families of either the husband or the wife. For comparison, the team also took data from 49 adults from the Paranan, a farming group that lives near the Agta. Among the Paranan, males are dominant and families usually live with the father’s kin.

The overall results matched the model’s predictions: An average of 16.7% of the egalitarian hunter-gatherers were unrelated either genetically or by marriage, versus only 4.2% of the nonegalitarian farmers.

“If all individuals seek to live with as many kin as possible,” and all individuals have equal say, “no one ends up living with many kin at all,” Dyble explains.

But it’s a win-win situation for both sexes, Dyble adds, because both husbands and wives get the contact they need with their families when it is most important. For example, he says, “the Agta normally move close to the wife’s family when she is going to give birth, but tend to move close to the husband’s kin” after several children are born and males need to cooperate in hunting for food.

Having unrelated people in a band was important in our evolution: The team argues that if early human ancestors had a similar social structure to today’s hunter-gatherers, then the increase in nonrelated group members would have set the stage for widespread cooperation and social networks that extended far beyond kin. “In forming mainly unrelated camps, hunter-gatherers evolved the capacity to cooperate with unrelated individuals,” Migliano says.

Only later, with the rise of agriculture and its systems of property and inherited wealth, did sexual inequality reappear, the researchers say.

But some experts caution that sexual equality might be only one of a number of factors that fueled human hypercooperation. “The paper is a very interesting and useful exercise,” says Kim Hill, an anthropologist at Arizona State University, Tempe. “But I would be reluctant to accept their model as the [only] answer.” He is also wary of assuming that today’s hunter-gatherers behave like ancient humans, in part because many have disturbed social systems as a result of contact with modern civilization. Polly Wiessner, an anthropologist at the University of Utah in Salt Lake City, who has studied the !Kung hunter-gatherers and related groups in Africa, agrees. “The extreme egalitarianism we see today” in many hunter-gatherer groups “is due to a breakdown in more formal structures and cultural rules,” she says.