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

Why some coalitions benefit from historical myths more than others

Behavioural ecologies in themselves can create variation in fitness interdependencies among individuals, and hence modulate the functionality of invoking historical myths. We develop this possibility for the case where coalitions form to attack and exploit enemies, or to defend and protect against hostile out-groups. We propose that invoking historical myths are functional and observed especially when groups aggressively expand.

Prosocial preferences can escalate intergroup conflicts by countering selfish motivations to leave

When defending against hostile enemies, individual group members can benefit from others staying in the group and fighting. However, individuals themselves may be better off by leaving the group and avoiding the personal risks associated with fighting. While fleeing is indeed commonly observed, when and why defenders fight or flee remains poorly understood and is addressed here with three incentivized and preregistered experiments (total n = 602). In stylized attacker-defender contest games in which defenders could stay and fight or leave, we show that the less costly leaving is, the more likely individuals are to abandon their group. In addition, more risk-averse individuals are more likely to leave. Conversely, individuals more likely stay and fight when they have pro-social preferences and when fellow group members cannot leave. However, those who stay not always contribute fully to group defense, to some degree free-riding on the efforts of other group members. Nonetheless, staying increased intergroup conflict and its associated costs.

Group Formation and the Evolution of Human Social Organization

Humans operate in groups that are oftentimes nested in multilayered collectives such as work units within departments and companies, neighborhoods within cities, and regions within nation states. With psychological science mostly focusing on proximate reasons for individuals to join existing groups and how existing groups function, we still poorly understand why groups form ex nihilo, how groups evolve into complex multilayered social structures, and what explains fission-fusion dynamics. Here we address group formation and the evolution of social organization at both the proximate and ultimate level of analysis. Building on models of fitness interdependence and cooperation, we propose that socioecologies can create positive interdependencies among strangers and pave the way for the formation of stable coalitions and groups through reciprocity and reputation-based partner selection. Such groups are marked by in-group bounded, parochial cooperation together with an array of social institutions for managing the commons, allowing groups to scale in size and complexity while avoiding the breakdown of cooperation. Our analysis reveals how distinct group cultures can endogenously emerge from reciprocal cooperation, shows that social identification and group commitment are likely consequences rather than causes of group cooperation, and explains when intergroup relations gravitate toward peaceful coexistence, integration, or conflict.

The Influence of Group Favoritism on Moral Judgment -- Evidence From Event-Related Potential

When judging the morality of an action, individuals may exhibit a bias stemming from group favoritism. It leads to the expression of different moral evaluations of the same behaviors performed by different social groups. The current study used event-related potentials (ERPs) to investigate the influence of in-group favoritism (in-group vs. homogeneous out-group) and out-group favoritism (in-group vs. high-quality out-group) on moral judgments. Two experiments were conducted, showing a higher moral score of the in-group compared to the homogeneous out-group by participants. ERP data indicated that in-group stimulation triggered larger P2 amplitudes than homogeneous out-group. Additionally, a larger N400-like amplitude, localized to the right hemisphere, was observed during the evaluation of the homogeneous out-group. Significantly, when comparing the in-group to the high-quality out-group, there was no discernible difference in moral scores or ERP amplitudes. Overall, the results indicate that in-group favoritism can affect moral judgments, especially when assessing homogeneous out-groups, but it can be reduced by the characteristics of groups.

Leader-follower behavioural coordination and neural synchronization during intergroup conflict

Leaders can launch hostile attacks on out-groups and organize in-group defence. Whether groups settle the conflict in their favour depends, however, on whether followers align with leader's initiatives. Yet how leader and followers coordinate during intergroup conflict remains unknown. Participants in small groups elected a leader and made costly contributions to intergroup conflict while dorsolateral prefrontal cortex (DLPFC) activity was simultaneously measured. Leaders were more sacrificial and their contribution influenced group survival to a greater extent during in-group defence than during out-group attacks. Leaders also had increased DLPFC activity when defending in-group, which predicted their comparatively strong contribution to conflict; followers reciprocated their leader's initiatives the more their DLPFC activity synchronized with that of their leader. When launching attacks, however, leaders and followers aligned poorly at behavioural and neural levels, which explained why out-group attacks often failed. Our results provide a neurobehavioural account of leader-follower coordination during intergroup conflict and reveal leader-follower behavioural/neural alignment as pivotal for groups settling conflicts in their favour.

Cultural threat perceptions predict violent extremism via need for cognitive closure

Understanding the psychological processes that drive violent extremism is a pressing global issue. Across six studies, we demonstrate that perceived cultural threats lead to violent extremism because they increase people's need for cognitive closure (NFC). In general population samples (from Denmark, Afghanistan, Pakistan, France, and an international sample) and a sample of former Mujahideen in Afghanistan, single-level and multilevel mediation analyses revealed that NFC mediated the association between perceived cultural threats and violent extremist outcomes. Further, in comparisons between the sample of former Afghan Mujahideen and the general population sample from Afghanistan following the known-group paradigm, the former Mujahideen scored significantly higher on cultural threat, NFC, and violent extremist outcomes. Moreover, the proposed model successfully differentiated former Afghan Mujahideen participants from the general Afghan participants. Next, two preregistered experiments provided causal support for the model. Experimentally manipulating the predictor (cultural threat) in Pakistan led to higher scores on the mediator (NFC) and dependent variables (violent extremist outcomes). Finally, an experiment conducted in France demonstrated the causal effect of the mediator (NFC) on violent extremist outcomes. Two internal meta-analyses using state-of-the-art methods (i.e., meta-analytic structural equation modeling and pooled indirect effects analyses) further demonstrated the robustness of our results across the different extremist outcomes, designs, populations, and settings. Cultural threat perceptions seem to drive violent extremism by eliciting a need for cognitive closure.

Anticipatory Threat Mitigates the Breakdown of Group Cooperation

Humans are exposed to environmental and economic threats that can profoundly affect individual survival and group functioning. Although anecdotal evidence suggests that threat exposure can increase collective action, the effects of threat on decision-making have been mainly investigated at the individual level. Here we examine how threat exposure and concomitant physiological responses modulate cooperation in small groups. Individuals (N = 105, ages 18-34 years) in groups of three were exposed to threat of electric shocks while deciding how much to contribute to a public good. Threat of shock induced a state of physiological freezing and, compared with no-threat conditions, reduced free riding and enabled groups to maintain higher cooperation over time. Exploratory analyses revealed that more cooperative responses under threat were driven by stronger baseline prosociality, suggesting that habitual prosociality is reinforced under threat. The current results support the view that human groups respond to outside threat with increased cooperation.

Conflict misperceptions between citizens and foreigners across the globe

In a globalizing world, conflict between citizens and foreigners hinders cooperation and hampers how well the global community can tackle shared problems. Here, we study conflict between citizens and foreigners and find that people substantially misperceive how competitive foreigners are. Citizens (from 51 countries; N = 12,863; 656,274 decisions) interacted with foreigners in incentivized contest experiments. People across the globe systematically failed to anticipate the competitiveness of foreigners and either competed too much or too little. Competition was poorly explained by differences in cultural values or environmental stress. By contrast, competition and concomitant conflict misperceptions were robustly accounted for by differences in the wealth of nations, institutions, and histories of engaging in international conflict. Our results reveal how macro-level socio-economic differences between countries create false stereotypes and might breed conflict.

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’.

Intergroup conflict: origins, dynamics and consequences across taxa

Although uniquely destructive and wasteful, intergroup conflict and warfare are not confined to humans. They are seen across a range of group-living species, from social insects, fishes and birds to mammals, including nonhuman primates. With its unique collection of theory, research and review contributions from biology, anthropology and economics, this theme issue provides novel insights into intergroup conflict across taxa. Here, we introduce and organize this theme issue on the origins and consequences of intergroup conflict. We provide a coherent framework by modelling intergroup conflicts as multi-level games of strategy in which individuals within groups cooperate to compete with (individuals in) other groups for scarce resources, such as territory, food, mating opportunities, power and influence. Within this framework, we identify cross-species mechanisms and consequences of (participating in) intergroup conflict. We conclude by highlighting crosscutting innovations in the study of intergroup conflict set forth by individual contributions. These include, among others, insights on how within-group heterogeneities and leadership relate to group conflict, how intergroup conflict shapes social organization and how climate change and environmental degradation transition intergroup relations from peaceful coexistence to violent conflict.

This article is part of the theme issue ‘Intergroup conflict across taxa’.