Penalizing others for their selfishness might not be fun, but it wins out against the alternative, suggesting how social sanctions and the cooperation they entail might have emerged over time. German researchers instructed volunteers to play a game in which they could contribute to a pot of cash-redeemable tokens, which paid dividends to everyone regardless of who contributed. In one group, subjects could pay a token to punish free riders who did not ante up, making the freeloaders lose tokens; the other group forbade punishment. Before each round, everyone saw each group's dividends and chose one to participate in. Although most players started out in the nonpunishment crowd, almost all ended up with the punishers, who saw their dividends grow because free riding was less common. Check out the April 7 Science for more—or else.