From group-hunting by lions, wolves, or killer whales, to groups of chimpanzees raiding their neighbors, to hostile takeovers in the marketplace, and to territorial conflicts within and between nation states, intergroup conflict is often a clash between the antagonist’s out-group aggression and the opponent’s in-group defense.... In-group defense and out-group aggression appear to have distinct neurobiological origins, and may thus recruit different within-group dynamics. Whereas self-defense is impulsive and relies on brain structures involved in threat signaling and emotion regulation, offensive aggression is more instrumental and conditioned by executive control.... the motivation to avoid loss is stronger than the search for gain, suggesting that individuals more readily contribute to defensive, rather than offensive, aggression. Finally, self-sacrifice in combat is publicly rewarded more (e.g., with a Medal of Honor) when it served in-group defense rather than out-group aggression. Accordingly, in-group defense may emerge more spontaneously, and individuals may be more intrinsically motivated to contribute to in-group defense than to out-group aggression.Significance
Across a range of domains, from group-hunting predators to laboratory groups, companies, and nation states, we find that out-group aggression is less successful because it is more difficult to coordinate than in-group defense. This finding explains why appeals for defending the in-group may be more persuasive than appeals to aggress a rivaling out-group and suggests that (third) parties seeking to regulate intergroup conflict should, in addition to reducing willingness to contribute to one’s group’s fighting capacity, undermine arrangements for coordinating out-group aggression, such as leadership, communication, and infrastructure.Abstract
Intergroup conflict persists when and because individuals make costly contributions to their group’s fighting capacity, but how groups organize contributions into effective collective action remains poorly understood. Here we distinguish between contributions aimed at subordinating out-groups (out-group aggression) from those aimed at defending the in-group against possible out-group aggression (in-group defense). We conducted two experiments in which three-person aggressor groups confronted three-person defender groups in a multiround contest game (n = 276; 92 aggressor–defender contests). Individuals received an endowment from which they could contribute to their group’s fighting capacity. Contributions were always wasted, but when the aggressor group’s fighting capacity exceeded that of the defender group, the aggressor group acquired the defender group’s remaining resources (otherwise, individuals on both sides were left with the remainders of their endowment). In-group defense appeared stronger and better coordinated than out-group aggression, and defender groups survived roughly 70% of the attacks. This low success rate for aggressor groups mirrored that of group-hunting predators such as wolves and chimpanzees (n = 1,382 cases), hostile takeovers in industry (n = 1,637 cases), and interstate conflicts (n = 2,586). Furthermore, whereas peer punishment increased out-group aggression more than in-group defense without affecting success rates (Exp. 1), sequential (vs. simultaneous) decision-making increased coordination of collective action for out-group aggression, doubling the aggressor’s success rate (Exp. 2). The relatively high success rate of in-group defense suggests evolutionary and cultural pressures may have favored capacities for cooperation and coordination when the group goal is to defend, rather than to expand, dominate, and exploit.