: Researchers argue that third party involvement is critical for sustaining humans’ unique levels of cooperation, yet it remains unclear precisely how third parties contribute, especially in small-scale, politically decentralized societies. In a detailed study of crime and punishment among the Mentawai rainforest-horticulturalists of Siberut Island, we test two leading hypotheses of third-party involvement: punishment and arbitration. Across a sample of ~400 transgressions, most of which were followed by the payment of a fine (usually paid in cooking pots, pigs, chickens, etc.), we found that third parties never directly punished. Fines were demanded by victims or their clanmates, and if an aggressor failed to pay, second-order punishment was always taken on by the victim or an aggrieved party and never by third parties. The magnitude of punishment reflected dyadic, rather than communal, concerns: A considerable number of transgressions were followed by no punishment, and, as predicted by dyadic accounts, punishment was less likely when transgressions were less severe and among related individuals. At the same time, third parties were often called as arbitrators and, as expected, third-party arbitrators were called more as cooperation was increasingly threatened. Moreover, government officials appear to fill similar roles to community arbitrators, demonstrating how governmental intervention might contribute to the decline of indigenous leadership institutions. These findings suggest that justice and institutionalized punishment, at least among the Mentawai, serve more to restore cooperation than to enforce norms.;