Socialization theory is a neglected source of explanations for cooperation in international relations. Neorealism treats socialization (or selection, more properly) as a process by which autistic non-balancers are weeded out of the anarchical international system. Contractual institutionalists ignore or downplay the possibilities of socialization in international institutions in part because of the difficulties in observing changes in interests and preferences. For constructivists socialization is a central concept. But to date it has been undertheorized, or more precisely, the microprocesses of socialization have been generally left unexamined. This article focuses on two basic microprocesses in socialization theory—persuasion and social influence—and develops propositions about the social conditions under which one might expect to observe cooperation in institutions. Socialization theories pose questions for both the structural-functional foundations of contractual institutionalist hypotheses about institutional design and cooperation, and notions of optimal group size for collective action.