Modelling behaviour in intergroup conflicts: a review of microeconomic approaches

The evolution of peace

While some species have affiliative and even cooperative interactions between individuals of different social groups, humans are alone in having durable, positive-sum, interdependent relationships across unrelated social groups. Our capacity to have harmonious relationships that cross group boundaries is an important aspect of our species' success, allowing for the exchange of ideas, materials, and ultimately enabling cumulative cultural evolution. Knowledge about the conditions required for peaceful intergroup relationships is critical for understanding the success of our species and building a more peaceful world. How do humans create harmonious relationships across group boundaries and when did this capacity emerge in the human lineage? Answering these questions involves considering the costs and benefits of intergroup cooperation and aggression, for oneself, one's group, and one's neighbor. Taking a game theoretical perspective provides new insights into the difficulties of removing the threat of war and reveals an ironic logic to peace - the factors that enable peace also facilitate the increased scale and destructiveness of conflict. In what follows, I explore the conditions required for peace, why they are so difficult to achieve, and when we expect peace to have emerged in the human lineage. I argue that intergroup cooperation was an important component of human relationships and a selective force in our species history beginning at least 300 thousand years. But the preconditions for peace only emerged in the past 100 thousand years and likely coexisted with intermittent intergroup violence which would have also been an important and selective force in our species' history.

Key individuals catalyse intergroup violence

Intergroup violence is challenging to understand: why do individuals cooperate to harm members of other groups when they themselves may be killed or injured? Despite progress in understanding the evolutionary and proximate mechanisms that underlie violence, we still have little insight into the processes that lead to the emergence of coalitionary aggression. We argue that an overlooked component is the presence of individuals who have a crucial role in initiating violence. In instigating intergroup violence, these key individuals may expect to face lower costs, receive greater benefits, or garner benefits that have a greater value to them than others. Alternatively, key individuals may be motivated by individual traits such as increased boldness, propensity for aggression or exploratory behaviour. Key individuals catalyse the emergence of coalitionary violence through one of several processes including altering the costs and benefits that accrue to others, paying a greater share of the startup costs, signalling privileged knowledge, or providing coordination, among other factors. Here we integrate diverse lines of empirical research from humans and non-human animals demonstrating that inter-individual variation is an important factor in the emergence of intergroup violence. Focusing on the role of key individuals provides new insights into how and why violence emerges. This article is part of the theme issue 'Intergroup conflict across taxa'.

Evolution of warfare by resource raiding favours polymorphism in belligerence and bravery

From protists to primates, intergroup aggression and warfare over resources have been observed in several taxa whose populations typically consist of groups connected by limited genetic mixing. Here, we model the coevolution between four traits relevant to this setting: (i) investment into common-pool resource production within groups (helping); (ii) proclivity to raid other groups to appropriate their resources (belligerence); and investments into (iii) defense and (iv) offense of group contests (defensive and offensive bravery). We show that when traits coevolve, the population often experiences disruptive selection favouring two morphs: 'Hawks', who express high levels of both belligerence and offensive bravery; and 'Doves', who express neither. This social polymorphism involves further among-traits associations when the fitness costs of helping and bravery interact. In particular, if helping is antagonistic with both forms of bravery, coevolution leads to the coexistence of individuals that either: (i) do not participate into common-pool resource production but only in its defense and appropriation (Scrounger Hawks) or (ii) only invest into common pool resource production (Producer Doves). Provided groups are not randomly mixed, these findings are robust to several modelling assumptions. This suggests that inter-group aggression is a potent mechanism in favouring within-group social diversity and behavioural syndromes. 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’.

Leaders of war: modelling the evolution of conflict among heterogeneous groups

War, in human and animal societies, can be extremely costly but can also offer significant benefits to the victorious group. We might expect groups to go into battle when the potential benefits of victory (V) outweigh the costs of escalated conflict (C); however, V and C are unlikely to be distributed evenly in heterogeneous groups. For example, some leaders who make the decision to go to war may monopolize the benefits at little cost to themselves ('exploitative' leaders). By contrast, other leaders may willingly pay increased costs, above and beyond their share of V ('heroic' leaders). We investigated conflict initiation and conflict participation in an ecological model where single-leader-multiple-follower groups came into conflict over natural resources. We found that small group size, low migration rate and frequent interaction between groups increased intergroup competition and the evolution of 'exploitative' leadership, while converse patterns favoured increased intragroup competition and the emergence of 'heroic' leaders. We also found evidence of an alternative leader/follower 'shared effort' outcome. Parameters that favoured high contributing 'heroic' leaders, and low contributing followers, facilitated transitions to more peaceful outcomes. We outline and discuss the key testable predictions of our model for empiricists studying intergroup conflict in humans and animals. This article is part of the theme issue 'Intergroup conflict across taxa'.