The world learned the details of the Islamic States systemic rape and slavery of women through shocking stories told to the New York Times in 2015.Our collective outrage also showed how war has changed. Rape, torture and slavery are considered beyond taboo; they are criminalized even in war. This archaic behavior is not supposed to happen in our modern world.

But thats a pretty recent development. Systemic rape used to go hand in hand with war as women, resources and landswere assimilated into the victors communities. The victorious menhad more children, more land and more power. Some researchers have argued that this is proof of the deep roots theory of war: Human males fight each other for reproductive advantage, proving that war is an evolutionary advantageous behavior.

But this theory has been hard to prove. In fact, studies of human groups and other primates have added to the evidence both for and against the controversial idea that humans were made for war, evolutionarily speaking. A January 2015study indicates that societies dont actually benefit from head-to-head action, though other forms of violence do pay off.

Harvard evolutionary biologists Luke Glowaki and Richard Wrangham studied the Nyangatom people of East Africa. The group are polygamous shepherds who raise small livestock and can have multiple wives. At times, the Nyangatom go to war with other groups. But there is a another pervasive and nearly constant form of violence in the group. Young riders make raids on nearby camps with the goal of stealing cattle. Glowaki and Wrangham asked if either or both of these types of violence was beneficial to the men who engaged in them. They measured by counting the the number of wives and kids they had.

This study is one of many that has heightened thedebate over how muchwar has had an impact on a warriors evolutionary success. At least in this society,sneaking around after dark and stealing cows may have beenmore consequential. Robert Sapolosky at the Wall Street Journal explained:

By contrast, lots of battle raidingopen-field, daytime combat with hundreds of participantsdid not serve as a predictor of elevated reproductive success, probably because such fighting carried a nontrivial chance of winding up dead. In other words, in this society, being a warrior on steroids did not predict reproductive success; being a low-down sneaky varmint of a cattle rustler did.

But researchers only discovered this by looking at the elders in the community. Stealthy animal raiding did lead to better outcomes but decades later. In Nyangatom culture, most of the stolen livestock goes to fathers and other paternal relatives rather than being kept by the young men who stole them. The male heads of families made marriage decisions for their younger relatives. So, while it this kind of violence makes a difference, the payoff is quite delayed. The researchers speculated the cattle-rustling effect would be stronger in a group where the raiders got to keep the livestock they stole and incentives were strengthened.

Other studies also point to the idea that inter-group warfare might not be beneficial, but intra-group violence is. Chimpanzee tribes, for example dont often go to war with other tribes. Instead the most common types of violence involve a group of males ganging up on one individual male. This often happens when conditions are crowded or there were increased numbers of males in the tribe. And the researchers found that chimps participation in violence happened outside of the spheres of human influence, meaning violence was not a behavior the chimpanzees learned from us.

But other evidence suggests that humans likely didnt participate in war as we know it until relatively recently. A 2013 survey of killings in 21 groups (foragers rather than shepherds) found that group warfare was rare compared to homicide. John Horgan categorized the evidence at Scientific American:

Some other points of interest: 96 percent of the killers were male. No surprise there. But some readers may be surprised that only two out of 148 killings stemmed from a fight over resources, such as a hunting ground, water hole or fruit tree. Nine episodes of lethal aggression involved husbands killing wives; three involved execution of an individual in a group by other members of the group; seven involved execution of outsiders, such as colonizers or missionaries. Most of the killings stemmed from what Fry and Soderberg categorize as miscellaneous personal disputes, involving jealousy, theft, insults and so on. The most common specific cause of deadly violenceinvolving either single or multiple perpetratorswas revenge for a previous attack.So it maybe that a proclivity for violence and an innate sense of revenge that perpetuates war, rather than war itself.

Another factor to consider is that while our common ancestors lived in groups like these thousands of years ago, almost no one does anymore. In fact, finding these undisturbed cultures is hard to do. Having more cows doesnt carry the same appeal it once did. Its unlikely stealing your neighbors TV for your uncle will fetch you a better bride. Some scientists worry that if we accept the idea that violence was a beneficial tool for our ancestors, it somehow overturns the societal progress that has moved us beyond the rape and pillage culture to something still imperfect, but largely more peaceful.

This is the biggest struggle with the deep roots theory of human violence. Just because something garnered an advantage thousands of years ago doesnt make it okay today. Harvard psychologist Steven Pinker, who has written a book on human violence, said in the Boston Globe:

romantics worry that if violence is a Darwinian adaptation, that must mean that it is good, or that its futile to work for peace, because humans have an innate thirst for blood that has to be periodically slaked. Needless to say, I think all this is profoundly wrongheaded.

Meredith Knight is a contributor to the human genetics section for Genetic Literacy Project and a freelance science and health writer in Austin, Texas. Follow her @meremereknight.

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Evolution and war: The 'deep roots' theory of human violence - Genetic Literacy Project

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