A new study looks at the question of how much our branch of the evolutionary tree contributes to humans’ violent behavior.

By Kate Wheeling

Paul Cezanne’s The Murder, circa 1868. (Photo: Public Domain)

We may be living in one of the most peaceful periods in human history, but one need only glimpse the daily news to be confronted with humans’ propensity for violence.

Such reminders inevitably prompt questions about the nature of human violence, and researchers and philosophers have long hypothesized about the role genetics, culture, and evolutionary history play in our most virulent behaviors. Now, a new study seeks to quantify the degree to which our place on the evolutionary tree may contribute to humans’ violent behavior.

The study, published today in Nature, looked at the probability of dying at the hands of another person in more than 600 human populations dating back to the Paleolithic era. But since humans are not the only animals that kill members of the same species, the team collected information on violent behavior among related species as well.

“We started naively thinking that if man is a wolf for other men, maybe we have to think first to what extent wolves are in fact wolves to other wolves,” says José Maria Gómez Reyes, a researcher at Spain’s Estación Experimental de Zonas Aridas and lead author on the new study. “This eventually led to a comparative approach to this longstanding questions: Can we find out how violent humans are by looking at how violent other species are?”

Roughly 2 percent of human deaths should be caused by other humans.

Looking at lethal violence in over 1,000 species of mammals from 137 families, Gómez Reyes and his colleagues found evidence for intra-species killing in nearly 40 percent of them. “The biggest surprise was to realize how widespread lethal violence is among mammals,” Gómez Reyes says. “It was also striking that lethal violence was not concentrated in those groups that we would tend to consider as ‘violent’ a priori, such as carnivores. We found a high level of lethal violence in rhinos, ground squirrels, marmots, or horses, just to mention some examples.”

The authors found a significant phylogenetic signal for lethal violence, meaning that, for violent animals, more closely related species were more likely to share the trait. Lethal violence is common to both humans and our primate cousins, for example. but while chimpanzees often kill other chimps, the closely related Bonobos are a peaceful primate. This variation in violence among closely related species suggests evolutionary relationships are only part of the story: The team also found that other traits, such as sociality and territoriality, can influence violent tendencies by providing opportunities for conflict.

“Lethal violence does not come out from the blue, but has roots in conflicts among conspecifics,” he says, “and continuous interaction with conspecifics, such as in social species, or with neighbors, such as in territorial species, provide clear opportunities for conflict.” The study found that, based on our relationships to other violent species, and the fact that humans are a social and territorial bunch, roughly 2 percent of human deaths should be caused by other humans.

That number is in line with the predicted proportion of deaths from lethal violence for the shared ancestor of primates and apes, which suggests our violent traits have deep roots. But our evolutionary history is not necessarily indicative of the future. When the authors compared that prediction to the observed levels of lethal violence throughout human history, the level of violence varied over time and across sociopolitical systems. In other words, humans do exert some control over our behavior.

“Evolutionary history is not a total straitjacket on human conditions,” Gómez Reyes says. “The main message of our study is that no matter how violent or pacific we were in the origin, we can modulate the level of interpersonal violence by changing our social environment. We can build a more pacific society if we wish.”