Groups and organizations face a fundamental problem: They need cooperation but their members have incentives to free ride. Empirical research on this problem has often been discouraging, and economic models suggest that solutions are unlikely or unstable. In contrast, the authors present a model and 4 studies that show that an unwaveringly consistent contributor can effectively catalyze cooperation in social dilemmas. The studies indicate that consistent contributors occur naturally, and their presence in a group causes others to contribute more and cooperate more often, with no apparent cost to the consistent contributor and often gain. These positive effects seem to result from a consistent contributor's impact on group members' cooperative inferences about group norms.