Common-enemy effects: multidisciplinary antecedents and economic perspectives

A disparate literature hypothesizes what can broadly be described as the common-enemy effect: the fact that the interaction with a common enemy (formed by Nature, an individual, or a group) increases cooperation. This review identifies the multidisciplinary antecedents of this effect, and then distinguishes between several strands of literature applying noncooperative game theory to account for it. A first strand argues that the threat posed by a common enemy makes each player's cooperative effort more critical. In a second strand a behavioral common-enemy effect caused by group interaction is studied experimentally. A third strand models the common-enemy effect as the formation of a coalition of players against another player in a contest. A fourth strand formalizes the principle that the ‘enemy of my enemy is my friend’, either in a model of social relations, interdependent altruistic preferences, or indirect reciprocity in repeated games. The connections between these strands of literature are investigated, and questions for future research are proposed.