When and why will states adopt more (or less) cooperative bargaining strategies? Standard answers to this question focus on the role of state power. Other scholars highlight socialization effects. I argue that in most international negotiations, the institutional bargaining structure will mitigate the effects of power and socialization, and drive state bargaining behavior. Factors highlighted by formal models of international bargaining should therefore best explain the variation in the strategies states adopt. I introduce empirical measures of these abstract concepts, and test their effects against those of power and socialization using an original dataset of state bargaining strategies in the European Union (EU). The results show that structural factors best explain variation in the EU states’ bargaining strategies. I conclude by highlighting the conditions under which these effects should explain state bargaining behavior in other international negotiations, and discuss the implications of this argument for the study of international bargaining.