Dutton and colleagues tested for left-handedness among religious people and atheists. Left-handedness is a good marker of a high mutational load. They found higher levels of left-handedness in atheists than in followers of most major religions.

Explaining the behavior behind these results, Dutton says: “Religiousness makes you more pro-social, and you become more religious when you’re stressed. Religious people would have been sexually selected for because their pro-social, moral, unstressed nature would be attractive.”

The authors argue that certain kinds of religious belief involving the worship of moral gods can thrive as societies develop. “Once groups become relatively complex—developing farming and then cities—you find yourself dealing with strangers more and more,” he says.