What norms trigger punishment?

Many experiments have demonstrated the power of norm enforcement-peer monitoring and punishment-to maintain, or even increase, contributions in social dilemma settings, but little is known about the underlying norms that monitors use to make punishment decisions, either within or across groups. Using a large sample of experimental data, we empirically recover the set of norms used most often by monitors and show first that the decision to punish should be modeled separately from the decision of how much to punish. Second, we show that absolute norms often fit the data better than the group average norm often assumed in related work. Third, we find that different norms seem to influence the decisions about punishing violators inside and outside one's own group.