Costly third-party punishment in young children.

•By 6years of age, children begin to systematically punish unfair behavior even when they are unaffected third parties.•In our task, children sacrificed personal gain to prevent rewards from being distributed unfairly between other children.•Punishers showed sensitivity to the costs of punishment.•6-Year-olds were more likely to intervene against unfairness that resulted from selfishness than from generosity.•These findings show that costly third-party punishment of unfair behavior is present in young children.