Imagine you’re a Yanomamö man, growing crops and hunting in the Amazon rainforest of southern Venezuela and northern Brazil. Someone from your village has been murdered, and you’re organizing a raiding party to do a revenge killing. Whom do you choose to fight alongside you?

Your brothers, guessed cultural anthropologist Shane Macfarlan of the University of Utah in Salt Lake City. Based on evolutionary theory and the importance of patrilineages in the Yanomamö’s social structure, Macfarlan assumed that a group of Yanomamö warriors, or unokai, was a literal band of brothers—a group of close male relatives who had lived in the same community their whole lives.

Macfarlan checked that assumption with data gathered back in the 1960s and 1970s by Napoleon Chagnon, an extremely divisive figure in anthropology who nevertheless amassed a valuable database on Yanomamö genealogy and warfare. Macfarlan discovered to his surprise that most co-unokais, or men who participated in the same killing, weren’t brothers. At best, they were cousins. But most were related by marriage: Seventy percent of married unokais had a wife directly related to one of their co-unokais.

The paper, published online today in the Proceedings of the National Academy of Sciences, offers a twist on Chagnon’s original ideas about how being a warrior boosts men’s fitness, and fits current notions that war is a mode of cooperation. “Humans are able to cooperate even in costly settings like warfare beyond the residential community and beyond kin,” says Sarah Mathew, an anthropologist at Arizona State University (ASU), Tempe, who studies the evolution of human cooperation. That’s “a real exceptional feature of Homo sapiens,” notes Kim Hill, a human behavioral ecologist also at ASU who has studied small-scale tribal societies in South America.

During 3 decades of living with and studying the Yanomamö, Chagnon, now at the University of Missouri, Columbia, gathered reams of information about their marriages, wars, alliances, and other relationships. But some anthropologists said his 1968 ethnography, Yanomamö: The Fierce People, exploited the tribe by breaking their taboo on using the names of the dead; others argued that its emphasis on violent encounters helped governments justify invasions of Yanomamö territory. Chagnon also faced criticism for infractions such as giving gifts, including machetes, to his closest Yanomamö associates, and thus distorting the social structure he aimed to analyze. In 2000, journalist Patrick Tierney even accused him of worsening a measles epidemic among the Yanomamö as part of a biomedical experiment. The American Anthropological Association, however, cleared Chagnon of that and most other charges in 2001 and rescinded their remaining criticisms in 2005.

For his part, Macfarlan calls Chagnon’s data “one of the most unique data sets in the world,” capturing the Yanomamö at a time before outside influences had altered their way of life. Listening to Chagnon’s extensive recorded interviews with his Yanomamö informants assuaged any doubts he had about data collection, Macfarlan says.

Macfarlan believes that his new view of how warriors benefited from becoming unokai could explain one of Chagnon’s most controversial findings. In 1988, Chagnon reported in Science that unokais of every age had more wives and children than their nonunokai counterparts. He hypothesized that being a warrior gave men high social standing and so made them more attractive as marriage partners and fathers. His suggestion that warfare and violence are “adaptive” behaviors that help drive human evolution quickly attracted fiery criticism.

Some of those ideas are no longer so startling. But in contrast to Chagnon’s focus on warriors’ reputations, Macfarlan found that the key advantage held by unokais was that they “are plugged into this social scene” that allows them to make strong alliances outside of their immediate paternal family, often in different villages. That gives them access to their friends’ female relatives as potential wives. They are “getting the resources and marriage opportunities of [their] allies,” not their defeated enemies, Macfarlan says.

The study shows that for the Yanomamö and possibly other small-scale societies, “warfare is about cooperation,” agrees Stephen Beckerman, an anthropologist and professor emeritus at Pennsylvania State University, University Park. He plans a similar analysis of alliances among the Waorani, another small-scale society in the Amazon with a tradition of waging war.

Macfarlan acknowledges that Chagnon’s data don’t rule out the possibility that Yanomamö warriors formed raiding parties with men who were already their in-laws, confirming existing bonds rather than forging new ones. In fact, Hill notes, men in many small-scale societies often prefer to ally with their brothers-in-law because by definition these men aren’t competing for access to the same women.

Although Hill doesn’t expect the new paper to sway any of Chagnon’s most committed critics, he says it represents “a more subtle and nuanced view of human society” than the anthropologist has articulated in the past. “It’s nice to see Chagnon reanalyzing some of his data 30 or 40 years later and coming up with a slightly different view of things.”