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Nov. 29, 2012 Volume 34, No. 14

MU study suggests evolutionary roots of violence among Amazon tribes

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URGE TO FIGHT Robert Walker, assistant professor of anthropology, holds two arrow points made by the Aches, an Amazon tribe. Walker says that the social structure of modern societies is based on evolutionary drives. Photo by Rob Hill


Professor examines anthropological reports of warfare in 44 tribes

Twenty-first century societies in developed countries seem pretty rational and sophisticated. But the cog deep in the societal machine appears to have primordial roots. 

Robert S. Walker, an anthropology assistant professor, has studied Amazonian tribal societies for 13 years. Among his discoveries is an in-group/out-group mentality that seems to harken back to social structures among higher primates.

But modern societies are equally tethered to their evolutionary past. “It pops up in all facets of life,” Walker said, offering examples of cliques, sports, politics, gangs and racism.

The Other

In August, Walker co-published a paper in the journal Evolution and Human Behavior on causes of death among people in South America’s Amazon tribes, such as the Yanomamo, Ayoreo, Ache, Hiwi and Kayapo. He examined anthropological studies undertaken between 1900 and 1970 on causes of death within tribes before contact with modern civilizations. The sample involved 44 lowland South American societies. 

He discovered that on average 30 percent of deaths were due not to health issues and accidents but violence. Seventy percent of violent deaths were male. Overall, 238 violent events (duels, homicides, raids, battles, revenge killings) accounted for 1,145 deaths. 

In-group warfare is between two villages with a shared culture, including language. Out-group warfare is between different cultures, which can mean different language, religion and dress.

Walker found that in-group raids were more frequent but had fewer casualties than out-group raids. Out-group conflict appeared to ratchet up the violence, he said, in that revenge raids involved higher body counts than the original attack. This implies that societies with different cultural signposts are more likely to engage in high-stakes warfare. 

“It’s really going to war against ‘The Other,’ ” Walker said.

But The Other can also be within the tribe, such as between two Yanomamo villages. The distance between villages is enough to break down cooperation.

Scientists have found in-group/out-group behavior among chimpanzees, who share a recent common ancestor with Homo sapiens. Neighboring chimpanzee groups often compete with each other for territory, female access and food. 

In the 1960s, Jane Goodall observed in Tanzania’s Gombe Stream National Park a chimpanzee group that split for seemingly arbitrary reasons, followed by violence between them. It appeared that the split was enough to unleash bloody conflict.

“The applicability of the chimpanzee model appears to fit Amazonian warfare up to a point in that violence is often ongoing between groups, mostly low cost for attackers during any particular event, and includes some benefits in terms of access to captured females [for reproductive purposes] and goods and potentially to more territory,” wrote Walker and Drew Bailey, a psychology postdoc at Carnegie Mellon University in Pittsburgh, Pa.

Motivation for tribal conflict, however, can differ from the motivation for chimp conflict in that the former can also involve revenge, Walker said. What’s more, tribes engage in guile to slay enemies by befriending another tribe, then, with guard lowered, striking violently, he said.

Why were Amazonian tribes so warlike prior to outside contact? Walker said that the tribes generally didn’t trade with one another because most of their goods were perishables, so cooperation was limited. Another factor is lack of overarching government. 

When the Portuguese first made contact with Amazonian tribes in the 1500s, they pillaged and spread European diseases. But they also introduced a rudimentary government and the Catholic faith that offered alternatives to violence, Walker said. 

“After European contact, the dynamics of Amazonian tribal life changed dramatically,” he said. “Although the spread of Christianity and imposition of national legal structures resulted in great loss of cultural identity, they also reduced deadly raids.” In recently contacted tribes, a tribal government ideally resolves differences without violence.

Evolutionary drives toward conflict are not limited to hunter-gatherer tribes such as those in the Amazon, Walker said. Primal drives are the platform on which modern, sophisticated societies are built.

In American politics, and most recently in the general election, Democrats and Republicans seem reflexively to take polar views — perhaps a highbrow channeling of evolutionary aggression toward the out-group. “It’s almost comical if you take a third-person perspective,” Walker said of political jousting. “You can see the inconsistencies.”

Racism is probably the most obvious example of the predilection for division due to geographical distance, appearance or culture, Walker said.

Modern-day sports appear to be a positive way to vent the primal urge to battle and take sides, he said.

David Livingstone Smith, keynote speaker at the Mizzou Diversity Summit Oct. 29–30, wrote in his book The Most Dangerous Animal (St. Martin’s Press, 2007) that human societies have such a propensity for war that they will create reasons to engage in it. 

Beyond Conflict

Government rule, trade and culture can decrease violence and lay the groundwork for tolerance. 

The first step is to admit that the in-group/out-group mentality is an evolutionary dead-end, Walker said. The Civil Rights movement of the early 1960s is an example of overcoming the tendency to demonize those who look different from the majority ethnicity.

“We are all one country now, deal with it,” Walker said. “Move past the long evolutionary history of conflict.”