Deities enforcing moral codes tended to appear in ancient societies after those societies had already developed into significant and complex political bodies.

That’s according to new research from Keio University in Kanagawa, Japan, published this week in Nature magazine.

A team of scientists analyzed the relationship between social complexity and watchful gods in 414 societies spanning the past 10,000 years from 30 regions across the globe. They found that belief in just gods punishing the guilty and rewarding the righteous usually followed increases in social complexity, generally appearing after the emergence of civilizations with populations of more than about 1 million people.

According to anthropologist Patrick Savage the research suggests that religion is playing a functional role throughout world history, helping stabilize societies and enabling strangers to cooperate with each other. “In really small societies, like very small groups of hunter-gatherers, everyone knows everyone else, and everyone’s keeping an eye on everyone else to make sure they’re behaving well. Bigger societies are more anonymous, so you might not know who to trust.”

At those sizes, you see the rise of beliefs in an all-powerful, supernatural deity watching over and keeping things under control, Savage added.