Evolution and war: The "deep roots" theory of human violence

Other studies also point to the idea that inter-group warfare might not be beneficial, but intra-group violence is. Chimpanzee tribes, for example don’t often go to war with other tribes. Instead the most common types of violence involve a group of males ganging up on one individual male. This often happens when conditions are crowded or there were increased numbers of males in the tribe. And the researchers found that chimps’ participation in violence happened outside of the spheres of human influence, meaning violence was not a behavior the chimpanzees learned from us.

But other evidence suggests that humans likely didn’t participate in war as we know it until relatively recently. A 2013 survey of killings in 21 groups (foragers rather than shepherds) found that group warfare was rare compared to homicide.