The emergence of enforcement

How do mechanisms that enforce cooperation emerge in a society where none are available and agents are endowed with just raw power that allows a more powerful agent to expropriate a less powerful one? We study a model where expropriation is costly and agents can choose whether to engage in surplus-augmenting cooperation or engage in expropriation. While in bilateral relations, if cooperation is not overwhelmingly productive and expropriation is not too costly, the latter will prevent cooperation, when there are three or more agents, powerful ones can become enforcers of cooperation for agents ranked below them. In equilibrium they will expropriate smaller amounts from multiple weaker cooperating agents who in turn will not deviate for fear of being expropriated more heavily because of their larger expropriation proceeds. Surprisingly, the details of the power structure are irrelevant for the existence of equilibria with enforcement provided that enough agents are present and one is ranked above all others. These details are instead key to the existence of other highly noncooperative equilibria that are obtained in certain cases.