Why do humans punish? According to standard evolutionary theories, punishment evolved to deter cheating and incentivize cooperation. In this talk, I argue that this view contradicts a central feature of our punitive psychology—that humans are intuitive retributivists. People punish to restore justice, by making transgressors suffer proportionately to the harm they have caused, a function that contradicts the requirements of optimal deterrence. I present data from several ongoing projects suggesting that retributivism is widespread across cultures; and propose to rethink the adaptive function of punishment to explain these patterns. I argue that retribution functions, not to deter cheating or enforce cooperation, but to restore mutual benefit between cooperative partners, by transferring proportionate benefits from the offender to the victim. I argue that this functional logic helps explain all the puzzling features of retributive justice, including its backward-looking character, proportionality to the tort inflicted, imperfect sensitivity to deterrence-relevant cues, the moral fungibility of punishment and material compensation, and the cognitive symmetry of retributive and distributive justice.