In recent years, a person could be forgiven for feeling as if conflict is inescapable. Political polarization has increased. Levels of societal violence and terrorism are surging. Endless wars continue, with no conclusion in sight.

Why is there so much chaos? The history of violence offers one possible answer.

Scholars—from psychologists to political scientists specializing in conflict—are starting to understand that the desire to belong among humans plays an outsized role in generating group violence of all kinds. This evolutionary desire to belong does not mean belonging to just any group of humans, but to a cohesive social group that protects you from violence, and gives you access to resources and sexual partners. And for a social group to remain cohesive it needs to have norms and rules that solve five basic coordination problems inherent to groups. These five problems are: hierarchy (who makes the decisions), identity (who is in the group and who is out), trade (how do we trade or share resources), disease (how do we manage disease with so many individuals living in close proximity) and punishment (who are we allowed to punish as a group, and for what). If a group fails to solve these five problems, violence ensues and the group splinters and cleaves into smaller groups.

As average human group sizes have grown over macro history—from family, to tribe, to the mega societies in which we now live—we’ve solved these problems at progressively larger scales. And as groups are by definition mostly non-violent internally, it is easy to see why bigger groups lead to lower levels of violence for most people.