Humans: Unusually Murderous Mammals, Typically Murderous Primates

A new study looks at rates of lethal violence across a thousand species to better understand the evolutionary origins of humanity’s own inhumanity.

Two Barbary Macaque monkeys in Tiergarten Schoenbrunn zoo, Austria (Stefan Huwiler / Getty )

Which mammal is most likely to be murdered by its own kind? It’s certainly not humans—not even close. Nor is it a top predator like the grey wolf or lion, although those at least are #11 and #9 in the league table of murdery mammals. No, according to a study led by José María Gómez from the University of Granada, the top spot goes to… the meerkat. These endearing black-masked creatures might be famous for their cooperative ways, but they kill each other at a rate that makes man’s inhumanity to man look meek. Almost one in five meerkats, mostly youngsters, lose their lives at the paws and jaws of their peers.

Gómez’s study is the first thorough survey of violence in the mammal world, collating data on more than a thousand species. It clearly shows that we humans are not alone in our capacity to kill each other. Our closest relatives, the chimpanzees, have been known to wage brutal war, but even apparently peaceful creatures take each other’s lives. When ranked according to their rates of lethal violence, ground squirrels, wild horses, gazelle, and deer all feature in the top 50. So do long-tailed chinchillas, which kill each other more frequently than tigers and bears do.

The point of this macabre census was to understand the origins of our own behavior. Gómez typically studies plants and insects, but he realized that the techniques he uses to study their evolution can be used to study our own. In particular, he noted that closely related species tend to show similar levels of lethal interpersonal violence. He could use those similarities to predict how violent any given mammal should be, and whether it meets, exceeds, or defies those expectations.

Humans do all three. Gómez’s team calculated that at the origin of Homo sapiens, we were six times more lethally violent than the average mammal, but about as violent as expected for a primate. But time and social organizations have sated our ancestral bloodthirst, leaving us with modern rates of lethal violence that are well below the prehistoric baseline. We are an average member of an especially violent group of mammals, and we’ve managed to curb our ancestry.

Gómez’s team predicted that when our species arose, around 2 percent of us (1 in 50) would have been murdered by other people.

Thomas Hobbes would have approved. In the 17th century, he argued that modern society protects us from our brutish nature, lived in “continual fear, and danger of violent death.” Not so, said Jean-Jacques Rousseau, who felt that civilization corrupts our neutral nature. These opposing views on violence—the former emphasizing an innate proclivity, and the latter focusing on cultural influences—preceded Hobbes and Rousseau by many centuries, and outlived them by many more. “Consensus does not exist, and positions are polarized,” says Gómez. “We hope that our study will shed light to the role that both evolution and culture have played in human lethal violence.”

First, he and his team compiled everything they could find on causes of death for various mammals, accumulating some 3,000 studies over two years. Their work revealed that lethal violence aimed at others from the same species is rare but widespread. It exists in almost 40 percent of the 1,024 mammal species that the team surveyed, and varies from group to group. Contrary to Watership Down, rabbits rarely kill each either. Neither do bats or whales. As you might expect, carnivores like lions, tigers, and bears, do so more frequently. But “it was striking that lethal violence wasn’t concentrated in those groups,” says Gómez.

The primates—the order that includes us, apes, monkeys, and lemurs—seem to be especially violent. While just 0.3 percent of mammal deaths are caused by members of the same species, that rate rose to 2.3 percent in the common ancestor of primates, and dropped slightly to 1.8 percent in the ancestor of great apes. That’s the lethal legacy that humanity inherited.

That isn’t to imply determinism. Even within the apes, chimps are notably more aggressive than bonobos, which suggests that group-wide capacities for violence can be tempered by other factors. And history shows that humans have also varied greatly in our violent tendencies. We are influenced by our history, but not saddled to it.

Gómez’s team showed that by poring through statistical yearbooks, archaeological sites, and more, to work out causes of death in 600 human populations between 50,000 BC to the present day. They concluded that rates of lethal violence originally ranged from 3.4 to 3.9 percent during Paleolithic times, making us only slightly more violent than you’d expect for a primate of our evolutionary past. That rate rose to around 12 percent during the bloody Medieval period, before falling again over the last few centuries to levels even lower than our prehistoric past.

Why? Probably because, as Hobbes suggested, we became organized.

It’s likely that primates are especially violent because we are both territorial and social—two factors that respectively provide motive and opportunity for murder. So it goes for humans. As we moved from small bands to medium-sized tribes to large chiefdoms, our rates of lethal violence increased.

But once we formed large states, “institutions like the rule of law reduced rates of lethal violence below what one would expect for a mammal with our ancestry and ecology, and below what has been observed in human societies in earlier periods and with simpler forms of social organization,” says Steven Pinker from Harvard University. He argued as much in his book The Better Angels of Our Nature, but says that Gómez’s team have done so “with greater precision, rigor, and depth; I wish this study had been available when I wrote the book.”

“We’ve known that for a long time! Volumes have been written on this,” says Polly Wiessner, an anthropologist from the University of Utah who is unimpressed with the study’s human half. “They have created a real soup of figures, throwing in individual conflicts with socially organized aggression, ritualized cannibalism, and more. The sources of data used for prehistoric violence are highly variable in reliability. When taken out of context, they are even more so.”

Richard Wrangham from Harvard University has similar concerns about the mammalian data, noting that Gómez have folded a lot of different kinds of killing—infanticide, adult deaths, and more—into a single analysis. And from an evolutionary standpoint, it matters less whether two related species kill their own kind at a similar rate, but whether they do so in a similar way.

“In the primates, infanticide is undoubtedly the commonest type,” says Wrangham. However, we humans “belong to a club of species that kill adults at an exceptionally high rate—a small club that includes a few social and territorial carnivores such as wolves, lions and spotted hyenas. That’s worth stressing in order to avoid readers leaping to the conclusion that there is nothing surprising about human violence. Humans really are exceptional.”

“We tried to separate violence in different types, but we couldn’t find enough data,” Gómez admits. “But right now, we’re re-evaluating our database to explore whether different causes of mortality have different evolutionary patterns. I hope to tell you something more in the future.”

The study demonstrates the importance of recognizing humans as animals more generally, and primates more specifically,” says Patricia Lambert, an anthropologist at Utah State University. There is a tendency to see human behavior as distinct from that of all other members of the animal kingdom and I think this hinders our understanding of the human brain and behavior.”

That being said, she adds that rates of lethal violence vary considerably between different human populations, ranging from 0 to 65 percent. Average values “do not characterize the spectrum of human violence all that well, she says. “I wonder if this is also true for other mammals, or if this degree of variance is uniquely human.”