Environmental stress increases out-group aggression and intergroup conflict in humans

Peaceful coexistence and trade among human groups can be fragile and intergroup relations frequently transition to violent exchange and conflict. Here we specify how exogenous changes in groups' environment and ensuing carrying-capacity stress can increase individual participation in intergroup conflict, and out-group aggression in particular. In two intergroup contest experiments, individuals could contribute private resources to out-group aggression (versus in-group defense). Environmental unpredictability, induced by making non-invested resources subject to risk of destruction (versus not), created psychological stress and increased participation in and coordination of out-group attacks. Archival analyses of interstate conflicts showed, likewise, that sovereign states engage in revisionist warfare more when their pre-conflict economic and climatic environment were more volatile and unpredictable. Given that participation in conflict is wasteful, environmental unpredictability not only made groups more often victorious but also less wealthy. Macro-level changes in the natural and economic environment can be a root cause of out-group aggression and turn benign intergroup relations violent. This article is part of the theme issue 'Intergroup conflict across taxa'

Shadow of conflict: How past conflict influences group cooperation and the use of punishment

Intergroup conflict profoundly affects the welfare of groups and can deteriorate intergroup relations long after the conflict is over. Here, we experimentally investigate how the experience of an intergroup conflict influences the ability of groups to establish cooperation after conflict. We induced conflict by using a repeated attacker-defender game in which groups of four are divided into two ‘attackers’ that can invest resources to take away resources from the other two participants in the role of ‘defenders.’ After the conflict, groups engaged in a repeated public goods game with peer-punishment, in which group members could invest resources to benefit the group and punish other group members for their decisions. Previous conflict did not significantly reduce group cooperation compared to a control treatment in which groups did not experience the intergroup conflict. However, when having experienced an intergroup conflict, individuals punished free-riding during the repeated public goods game less harshly and did not react to punishment by previous attackers, ultimately reducing group welfare. This result reveals an important boundary condition for peer punishment institutions. Peer punishment is less able to efficiently promote cooperation amid a ‘shadow of conflict.’ In a third treatment, we tested whether such ‘maladaptive’ punishment patterns induced by previous conflict can be mitigated by hiding the group members’ conflict roles during the subsequent public goods provision game. We find more cooperation when individuals could not identify each other as (previous) attackers and defenders and maladaptive punishment patterns disappeared. Results suggest that intergroup conflict undermines past perpetrators’ legitimacy to enforce cooperation norms. More generally, results reveal that past conflict can reduce the effectiveness of institutions for managing the commons

Shadow of conflict: How past conflict influences group cooperation and the use of punishment

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