A fundamental problem in groups and organizations is the difficulty of encouraging cooperation when group 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, we present a model and three studies which show that an unwaveringly consistent contributor can effectively solve such social dilemmas. The data indicate that consistent contributors emerge naturally, and their presence in a group leads others to cooperate more and more often, with little or no apparent cost to the consistent contributor, and sometimes gain.