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

Abstract

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’.

Intergroup conflict: origins, dynamics and consequences across taxa

“…First, carrying-capacity stress can bind individuals within groups and promote parochial other-concern (α I in equation (3.1); see [153]). De Dreu et al [45] provide experimental evidence that environmental unpredictability and ensuing carryingcapacity stress lead to parochial in-group concerns and more out-group aggression. Their findings resonate with findings on clan formation in spotted hyaenas [154] and can explain how climate change and economic shocks link to political revolts and civil conflicts in humans [155][156][157].…”

“…Second, carrying-capacity stress increases the expected benefits from invading adjacent territories. The increased risk of starvation makes the risk of lethal injury from fighting comparatively low and aggressing out-groups more 'attractive' (element v in equation (3.1)) [45,158]. Gareta García et al [41] provide data conducive to this possibility where groups of vervet monkeys are more likely to meet and compete with others in areas with higher ecological value.…”

“…Likewise, Lemoine et al [18] review evidence that collective grooming and food sharing in chimpanzees prior to intergroup encounters can increase social ties among group members, and stronger social ties enable collective action during violent intergroup encounters. Finally, De Dreu et al [45] show how solidarity among group members and increased parochialism enable costly investment in out-group aggression, forcing out-groups to invest resources in group defence.…”

“…Second, carrying-capacity stress increases the expected benefits from invading adjacent territories. The increased risk of starvation makes the risk of lethal injury from fighting comparatively low and aggressing out-groups more ‘attractive’ (element v in equation (3.1)) [ 45 , 158 ]. Gareta García et al .…”

“…It provides, for the first time, to our knowledge, a side-by-side treatment of intergroup conflict across taxa, detailing the origins and consequences of intergroup conflict. Each in their own way, the contributions provide insight on intergroup conflict for social insects [32,33], social fishes [40], group-living birds [26,27], non-primate mammals such as banded mongoose [24], lions and wolves [17], several monkey species [41][42][43], chimpanzees [18,44] and humans [45][46][47].…”