Group leaders establish cooperative norms that persist in subsequent interactions.

The temptation to free-ride on others' contributions to public goods makes enhancing cooperation a critical challenge. Solutions to the cooperation problem have centered on installing a sanctioning institution where all can punish all, i.e., peer punishment. But a new, growing literature considers whether and when the designation of a group leader-one group member, given the sole ability to administer punishment-is an effective and efficient alternative. What remains unknown is whether and to what extent these group leaders establish cooperative norms in their groups via their own contributions to the public good, their use of sanctions, or both. Nor has past work examined whether leaders' behaviors have lasting effects on non-leaders' cooperation in subsequent interactions, outside of the leader's purview. Here I show that leaders' contributions to the public good predict non-leaders' subsequent cooperation. Importantly, the effect is not limited to cooperation within the institution: the effect of leaders' contributions continue to predict non-leaders' contributions in a later interaction, where sanctions are removed. This process is mediated by non-leaders' increased contributions in the institution, suggesting that leaders have effects on followers that shape followers' subsequent behaviors. These effects occur above and beyond a baseline tendency to be influenced by non-leader group members; they also occur above and beyond the influence of peers in groups under a peer punishment institution. Results underscore how critical it is that groups install cooperative leaders: followers model their leaders' cooperation choices, even in decisions external to the original institution and outside of the leader's watch