TULE RAFTS ON SAN FRANCISCO BAY. Credit: California Missions Resource Center.

I was always bugged by the fact that history in school seemed like a huge lesson about human violence. Compared to wars, the topics covering humanity’s landmark achievements like the invention of concrete or the microscope were a disheartening minority. Biases aside, we seem a species that seems not only immensely fascinated by war but heavily involved in it — up until the last century at least.

With this in mind, why do people kill each other? That may seem like an odd question but despite the numerous hypotheses, from those that posit natural selection rewards the aggressive to ethnic feud,  few are verifiable. Mark Allen, professor of anthropology at Cal Poly Pomona, published a recent study that hints a clear explanation for why people kill each other is resource scarcity. He and colleagues analyzed lethal trauma among hunter-gatherer populations in prehistoric central California to reach this conclusion.

This was a very challenging research because North American tribes didn’t keep written records, so the researchers looked for empirical evidence of trauma and the density of occurrence.

“You have to have something significant,” Allen said. “You have to have good evidence. As archeologists, you don’t get the data you want most of the time. We are typically dealing with fragmented evidence.”

The team used an archaeological database of human remains from Central California which covered thousands of burials, some going back more than 1,500 years. Allen and colleagues systematically looked at every burial for signs of physical trauma, whether it was bashed skull, broken or cut bones. They then made a map which also looked at the way communities were socially organized.

California used to be a very cosmopolitan region being home to a staggering 100 different languages. Interestingly, the data shows a correlation between resource scarcity and violence. That might seem obvious but what’s to be noted is that it was ultimately resource scarcity that could predict high homicide rates, not some political organization.

“When people are stressed out and worried about protecting the group, they are willing to be aggressive,” he says. “Violence is about resources for the group.”

Understanding what make people kill each is the first but also the most important step to making sure it doesn’t happen.  “If we want to reduce conflict, we need to figure out what to do about resource stress,” Allen says.

“Blunt force cranial trauma shows no correlation with NPP or political complexity and may reflect a different form of close contact violence. This study provides no support for the position that violence originated with the development of more complex hunter-gatherer adaptations in the fairly recent past. Instead, findings show that individuals are prone to violence in times and places of resource scarcity,” the researchers conclude in the journal PNAS.