About 10 percent of Asia can claim to be descended from Genghis Khan and they are absolutely correct, genetic studies show; the reason is that part of the benefit to rampaging across Asia, the mid-East and into Europe was a lot of sex.

But it isn't just conquering Mongols, even on a small scale violent conflict offers biological rewards to those those who take part in it, say anthropologists who correlated violent raids and combat to reproductive fitness. 

In their paper, the authors noted that members of an East African herding tribe who engaged in conflict, in the form of violent raids carried out on neighboring groups, had more wives, and thus more opportunities to increase their reproductive success through having more children. That benefit, the increased reproductive success enjoyed by raiders, is aided by powerful cultural forces, say Luke Glowacki, a Harvard doctoral student who worked with Richard Wrangham, an anthropology professor.

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"The currency of evolution is reproductive success," Glowacki said. "By having more wives you can have more children. What we found was that, over the course of their lives, those who took part in more raids had more children.

"What I really like about this paper is the finding isn't just 'biology made me do it. It's very clear what the pathway to greater reproductive success is - it's access to livestock, which are obtained through raiding and then used for marriage. But the cultural mechanism is mediated by the elders who control virtually all aspects of the society. After a raid young men give any livestock they capture to the elders and the raider cannot use them at that point even if he wants to get married. Later in life, as the raider gets older he can gain access to them, so there's a lag in receiving benefits from participating in a raid." 

Examining the connection between violence and reproductive capacity, however, is no easy feat.

For Glowacki, the project involved living with the Nyangatom, a group of nomadic herders living in a region of southwest Ethiopia and South Sudan, for more than a year, and observing virtually every part of day-to-day village life - from digging water holes to migrations.

Typically carried out by Nyangatom men between 20 and 40 years old armed with weapons like AK-47 rifles, the raids sometimes result in serious injuries and deaths. Those who take part in the raids, however, must turn over any livestock they obtain to village elders, who use them to obtain wives for themselves. It may not be until years later that elders agree to provide a raider with the cows necessary to obtain their first wife, or subsequent wives.

"In many cultures, particularly in east Africa, in order to get married you have to give livestock to the bride's family - we refer to it as bridewealth," Glowacki said. "If you don't have cows, you simply cannot get married - it doesn't matter how handsome you are or how much status you have, if you don't have cows to give the bride's family, you cannot get married."

Though he found clear evidence that violence offers a benefit to warriors, Glowacki's over-riding interest is in a much larger question.

"The overriding question I'm interested in is how humans cooperate, and one type of cooperation is participating in intergroup conflict," he explained. "Why do people do things that benefit their group if they have to pay a cost? For the Nyangatom there are no formal institutions governing society, and yet they manage to make a living from one of the toughest landscapes on Earth, and they do that through cooperation."

In fact, he said, cooperation plays a key role in virtually every aspect of Nyangatom life.

"I set out to study who herds together, who digs water holes together, who plants together, and also who participates in conflict events together," he added. "I conducted interviews about the raids, and collected reproductive histories by asking how many wives raiders have, how many children each has had, how many are alive, how many died and how they died."

Using that data, Glowacki was able to build conflict histories for the young men that took part in raids and village elders detailing who participated in raids, how often they participated, whether they were married and the number of wives and children they had.

In an analysis of 120 men, Glowacki said, the data was clear - those who participate in more raids had more wives and more children over the course of their lives.

But while raiders did benefit from taking part in conflict, the lack of an immediate payoff, Glowacki believes, helps to keep violence in check.

"We don't have quantitative data to that effect, but there are some groups in neighboring Kenya where raiders who capture cows in a raid don't have to give them to the elders or they can sell them at a market for money, and the violence is significantly greater" he said. "The Nyangatom have a mechanism that mediates the benefits the warriors receive," he added. "There is a lot of status and privilege that comes with participating in raids - when you come back to the village, the women are singing and people are parading. They're celebrating you, but you still go home alone."

Published in the Proceedings of the National Academy of Sciences.