For Members Only: Ingroup Punishment of Fairness Norm Violations in the Ultimatum Game

Although group membership has many privileges, members are expected to reciprocate those privileges. We tested whether in-group members would be punished more harshly than out-group members for marginal fairness norm violations within ultimatum game bargaining interactions. Participants considered monetary splits (of US$20) from in-group and out-group proposers, which ranged in proportion. Accepting an offer yielded the proposed payout; rejecting it caused each player to earn nothing-a punishment of the proposer at a personal cost. Participants exacted stricter costly punishment on racial in-group than out-group members for marginally unfair offers (Study 1), an effect that was replicated with college group membership and magnified among strong in-group identifiers (Study 2). Importantly, ultimatum game decisions were driven by fairness perceptions rather than proposer evaluations (Study 3), suggesting our effects reflected norm enforcement and not esteem preservation. These findings illuminate a previously unexplored process for maintaining group-based norms that may promote in-group favoritism.