Humans often cooperate, voluntarily paying an individual cost to supply a benefit to others. Public good experiments show that punishment induces a high level of cooperation, even when it is costly to the punisher.1 It is unclear, however, what motivates individuals to engage in costly punishment: a desire to retaliate against non-cooperators or a desire to reduce inequality among group members. Although both motives might have a positive effect on cooperation2-4, they cannot be separated in the conventional public good game.5 Here we conduct an experiment in which we add a randomly-generated fixed payoff to a public good game with punishment. This design allows us to determine whether punishment is aimed at low contributors or high earners...