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Study suggests violence is an evolutionary adaptation

Chimp research rejects theory of human impact

Researchers found that lethal attacks among chimps were more common in crowded communities with lots of males.IAN GILBY

Is war a modern human invention, or does it have deep roots in biology? The age-old question has inspired theories by historians, anthropologists, and philosophers.

People have long known that chimpanzees occasionally attack and kill their own kind, and one theory holds that the violence is the result of humans’ impact on their environment.

But a new study on chimps, led by a prominent Harvard University evolutionary biologist, suggests that the origins of human violence may be found deep in evolution, and that chimps kill one another because such violent acts give them a reproductive and survival advantage, and a way of eliminating rivals.


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“The value of this report is it debunks once and for all the idea that this behavior is unnatural, an artifact of human presence,” Steven Pinker, a psychology professor at Harvard not involved in the research, wrote in an e-mail. “Romantics worry that if violence is a Darwinian adaptation, that must mean that it is good, or that it’s futile to work for peace, because humans have an innate thirst for blood that has to be periodically slaked. Needless to say, I think that all this is profoundly wrongheaded.”

The study, published in the journal Nature on Wednesday, was led by Richard Wrangham, a professor of human evolutionary biology at Harvard, who has been working at field sites in Africa since the 1970s. The researchers examined records from chimpanzee communities in Tanzania, Senegal, Guinea, Ivory Coast, the Republic of Congo, Democratic Republic of the Congo, and Uganda.

They found that human behaviors such as feeding animals or disturbing their habitat did not appear to cause more killings. Instead, lethal attacks seem to be more common in crowded communities with lots of males, suggesting that the violence may be an adaptation that helps in the competition for food and mates.


“The chimp model is a very helpful model for understanding how some of humans’ really challenging kinds of behavior have been favored by natural selection,” Wrangham said. “The aggressors can kill at incredibly low risk to themselves, and the reason is that they chose to attack only when they have an overwhelming imbalance of power.”

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Overall, the researchers chronicled 152 killings by chimpanzees that were directly observed, suspected, or inferred in 18 communities that had been followed for decades. The attackers were overwhelmingly male, and the majority were attacking other males from other groups. They attacked when they outnumbered their victims — by eight to one, on average.

The researchers used a variety of models and statistical analyses to gauge whether the killing patterns suggested a behavior that had evolved because it provided benefits or was instead a radical new behavior spurred by environmental changes triggered by people.

The rate of killing over time presumably should increase if it is a reaction to human influence. Primatologist Jane Goodall fed her chimps bananas, for example. In communities that receive food from humans delivered in a manner not found in nature, chimps might be expected to react by killing over the resource.

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If, on the other hand, killing is an adaptive strategy, lethal attacks should increase when the group is more crowded together and has more males. Killing could have benefits, such as eliminating competitors or providing access to females for mating.


Adult males from the Ngogo community in Uganda, studied since 1995, listening to vocalizations from distant members of a rival community.Courtesy of John Mitani/Credit: John Mitani

That is precisely what the data revealed. At a fairly undisturbed site called Ngogo, where researchers had never fed the animals, the rate of killing was the highest. When the number of males increased or the area was more crowded, the number of killings was higher.

Brian Ferguson, director of Peace and Conflict Studies at Rutgers University at Newark, said he disagrees with the new study’s interpretation of the data, as well as the methods used to rate the role of human influence on the chimp behavior. He suggests that each killing needs to be evaluated with context about how and why it occurred, and he believes that such a study would reveal that chimps begin killing as a result of human actions.

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“If people think that it is in our nature to go to war, that we’re somehow by evolution primed to go out and kill members of other groups, it leads to a kind of fatalism. You never can change that,” Ferguson said. Instead, he continues to believe that the evidence shows that war is a recent invention.

It can be tempting to take a dark view of the violent behavior of chimpanzees, but Joan Silk, a professor in the School of Human Evolution and Social Change at Arizona State University, said discovering the origins of human behaviors in other animals is not the same as learning our destiny.


“How do animals resolve conflict is interesting,” Silk said. “How do animals find out ways to cooperate? Those are general principles from which we can learn a lot, but it doesn’t mean we’re expecting them to be the same across species. I study baboons, and I love them dearly, but they do all kinds of things I think are sort of uncivilized. If they were my kids, I’d be very distressed.”

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Carolyn Y. Johnson can be reached at cjohnson@
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