Chimpanzees

Chimpanzees (Pan troglodytes) showing aggressive behavior in a tropical forest in western Uganda.

Suzi Eszterhas/Minden Pictures

Why do we kill? Controversial study blames our distant ancestors

Human bloodlust—from war to murder—traces back millions of years to our primate ancestors. That’s the conclusion of a controversial new study, which reaches far back into our family tree to uncover the evolutionary roots of lethal violence among more than 1000 mammalian species.

“It’s nice to see where humans fall in relation to other species,” says Polly Wiessner, an anthropologist at the University of Utah in Salt Lake City who studies violence and peacemaking in Papua New Guinea.

But she says the sweeping analysis relies on imprecise data and doesn’t provide new information about the cultural subtleties that affect when and how humans deploy violence.

Humans are far from the only species that kills its own. Murder has been observed in animals ranging from chimpanzees to wolves to marmots, a type of oversized squirrel. José Maria Gómez, an evolutionary biologist at the University of Granada and the Spanish National Research Council’s campus in Almería, wondered whether each species had developed its capacity for lethal violence on its own, or whether the tendency had been passed down from their evolutionary ancestors.

So for 2 years, he and his team scoured decades of scientific research to create a database of how 1024 species of mammals die, including what proportion of each one was killed by other members of its own species. Forty percent of the species in the study had been observed engaging in lethal violence, but rates varied widely. The researchers found that some species, like bats and whales, hardly ever kill each other. Others, like ground squirrels and tree shrews, do so relatively often. Animals that live in groups and defend territories, such as wolves and chimps, tend to be more violent. Both violence and nonviolence tended to clump along certain branches of the mammalian family tree. Statistically, the more violent your close relatives are, the more violent your species is likely to be.

That association meant that Gómez and his colleagues could use their extensive database to predict a given species’s rate of lethal violence—and that’s what they did for humans. Though group-living primates are relatively violent, the rates vary. Nearly 4.5% of chimpanzee deaths are caused by another chimp, for example, whereas bonobos are responsible for only 0.68% of their compatriots’ deaths. Based on the rates of lethal violence seen in our close relatives, Gómez and his team predicted that 2% of human deaths would be caused by another human.

To see whether that was true, the researchers dove into the scientific literature documenting lethal violence among humans, from prehistory to today. They combined data from archaeological excavations, historical records, modern national statistics, and ethnographies to tally up the number of humans killed by other humans in different time periods and societies. From 50,000 years ago to 10,000 years ago, when humans lived in small groups of hunter-gatherers, the rate of killing was “statistically indistinguishable” from the predicted rate of 2%, based on archaeological evidence, Gómez and his colleagues report today in Nature.

Later, as human groups consolidated into chiefdoms and states, rates of lethal violence shot up—as high as 12% in medieval Eurasia, for example. But in the contemporary era, when industrialized states exert the rule of law, violence is lower than our evolutionary heritage would predict, hovering around 1.3% when combining statistics from across the world. That means evolution “is not a straitjacket,” Gómez says. Culture modulates our bloodthirsty tendencies.

The study is “innovative and meticulously conducted,” says Douglas Fry, an anthropologist at the University of Alabama, Birmingham. The 2% figure is significantly lower than Harvard University psychologist Steven Pinker’s much publicized estimate that 15% of deaths are due to lethal violence among hunter-gatherers. The lower figure resonates with Fry’s extensive studies of nomadic hunter-gatherers, whom he has observed to be less violent than Pinker’s work suggests. “Along with archaeology and nomadic forager research, this [study] shoots holes in the view that the human past and human nature are shockingly violent,” Fry says.

Wiessner says that the database of lethal violence among mammals is an impressive contribution. But the data on humans is “very weak,” she says. “The stuff we have on prehistory is very thin.” She notes that the study gives descriptions of isolated cultures by missionaries, excavations of ancient battlefields, and modern homicide rates equal weight, when they likely have different biases and margins of error. “I don’t think these averages are really telling us anything.”

Richard Wrangham, a biological anthropologist at Harvard who studies the evolutionary relationship between chimpanzees and humans, agrees the data set, although “new and amazingly large,” lacks crucial context, because all lethal violence is not the same. Infanticide, for example, is extremely common among chimpanzees, but humans tend to kill other adults. In order to draw conclusions about the evolutionary history of violence, it’s not just numbers that matter, Wrangham says. 

Gómez agrees that the inability to distinguish between types of lethal violence “is the biggest problem with our work.” He says there simply weren't enough data to calculate the rates of different types of lethal violence for humans, but he and his team are working on adding that information for the other mammals. He is also planning to make the full data set public and hopes that anthropologists will contribute. “The global result is robust,” Gómez says. But when it comes to studying the specifics of human violence, the devil is always in the details.

For more coverage of the history of human conflict, check out our Special Issue.

*Correction, 29 September 2016, 1:44 p.m.: This story has been updated with a new photograph to reflect the fact that lethal violence is much more common in chimpanzees than bonobos.