Looking Under the Hood of Third-Party Punishment Reveals Design for Personal Benefit.

Third-party intervention, such as when a crowd stops a mugger, is common. Yet it seems irrational because it has real costs but may provide no personal benefits. In a laboratory analogue, the third-party-punishment game, third parties ("punishers") will often spend real money to anonymously punish bad behavior directed at other people. A common explanation is that third-party punishment exists to maintain a cooperative society. We tested a different explanation: Third-party punishment results from a deterrence psychology for defending personal interests. Because humans evolved in small-scale, face-to-face social worlds, the mind infers that mistreatment of a third party predicts later mistreatment of oneself. We showed that when punishers do not have information about how they personally will be treated, they infer that mistreatment of other people predicts mistreatment of themselves, and these inferences predict punishment. But when information about personal mistreatment is available, it drives punishment. This suggests that humans' punitive psychology evolved to defend personal interests.