ABSTRACT: Social sanctioning is widely considered a successful strategy to promote cooperation among humans. In situations in which individual and collective interests are at odds, incentives to free-ride induce individuals to refrain from contributing to public goods provision. Experimental evidence from public goods games shows that when endowed with sanctioning powers, conditional cooperators can discipline defectors, thus leading to greater levels of cooperation. However, extant evidence is based on peer punishment institutions, whereas in complex societies, systems of control are often centralized: for instance, we do not sanction our neighbors for driving too fast, the police do. Here we show the effect of centralized sanctioning and legitimate authority on cooperation. We designed an adaptation of the public goods game in which sanctioning power is given to a single monitor, and we experimentally manipulated the process by which the monitor is chosen. To increase the external validity of the study, we conducted lab-in-the-field experiments involving 1,543 Ugandan farmers from 50 producer cooperatives. This research provides evidence of the effectiveness of centralized sanctioning and demonstrates the causal effect of legitimacy on cooperation: participants are more responsive to the authority of an elected monitor than a randomly chosen monitor. Our essay contributes to the literature on the evolution of cooperation by introducing the idea of role differentiation. In complex societies, cooperative behavior is not only sustained by mechanisms of selection and reciprocity among peers, but also by the legitimacy that certain actors derive from their position in the social hierarchy.