Peace depends on the precarious balance between the shared benefits it brings to a whole community, and what an individual may gain from disrupting that peace. Glowacki provides an elegant explanation for the origin of peace between human groups, and why the conditions tipping this balance in favour of peace are rare. Peace, that is, long-lasting positive-sum intergroup relationships, is unknown among other mammal species, even those capable of short-term intergroup cooperation (Connor, Krützen, Allen, Sherwin, & King, Reference Connor, Krützen, Allen, Sherwin and King2022; Fruth & Hohmann, Reference Fruth and Hohmann2018), and yet, peace is not a uniquely human phenomenon. Certain ant species engage in long-term, nonaggressive, mutual resource exchange between nests (Burns et al., Reference Burns, Franks, Parr, Hawke, Ellis and Robinson2020; Robinson, Reference Robinson2014; Robinson & Barker, Reference Robinson and Barker2017). Indeed, both ants and humans display the full range of intergroup behaviours, from extreme hostility to remarkable harmony (Pisor & Surbeck, Reference Pisor and Surbeck2019; Rodrigues, Barker, & Robinson, Reference Rodrigues, Barker and Robinson2022). This raises intriguing questions about why peace arises in these two ecologically and taxonomically distinct groups.

Glowacki's model of differing payoffs for individuals and groups provides a compelling framework for considering the emergence and maintenance of peace in humans (target article, sect. 2). This framework is generalizable to ants: In ant colonies consisting of close kin, the payoffs are more homogenous, because workers gain from successful conflicts only through benefits to their queen. This interdependence relaxes the tension between the individual-level and group-level benefits (Rodrigues et al., Reference Rodrigues, Barker and Robinson2022). Where within-group relatedness is relatively low, “policing” is a potential mechanism, in both humans and ants, repressing the competitive tendencies of belligerent individuals. In social insects, policing is typically associated with multiple-mating and the concomitant decrease in within-group relatedness (Foster & Ratnieks, Reference Foster and Ratnieks2000; Liebig, Peeters, & Hölldobler, Reference Liebig, Peeters and Hölldobler1999). Among humans, there is suggestive evidence for a similar association between policing and lower within-group relatedness (Kümmerli, Reference Kümmerli2011). Moreover, kinship between groups means that asymmetric payoffs need not compromise intergroup cooperation, and may even favour between-group altruism (Pisor & Surbeck, Reference Pisor and Surbeck2019; Rodrigues et al., Reference Rodrigues, Barker and Robinson2022).

Spatial context shapes intergroup relationships: Within human societies, group isolation can lead to peace, while the presence of neighbours promotes conflict (target article, sect. 1.1). The relationship between geographical distance and the potential for peace becomes more complex when we make explicit the dynamics of groups: Both the value of resources and the risk of conflict with kin are higher when interacting with near neighbours (Taylor, Reference Taylor1992). In the simplest scenario, these effects cancel each other out, and thus geographical distance does not have a straightforward effect on intergroup relationships (Rodrigues et al., Reference Rodrigues, Barker and Robinson2022). Between-group movement changes the payoff distributions by inflating between-group relatedness. In some contexts, this raises the cost of intergroup conflict due to the increased risk of harm to kin, which promotes the maintenance of peaceful local intergroup interactions, especially during conflict with other unfamiliar groups (Rodrigues, Barker, & Robinson, Reference Rodrigues, Barker and Robinson2023).

Across societies, proclivity for warfare differs among group members (Glowacki & McDermott, Reference Glowacki and McDermott2022), often with older members attempting to curb younger members' higher inclination towards conflict (target article, sect. 3.3). Social insects also show age-related behaviour, but in contrast, riskier tasks, including fighting, are typically undertaken by older individuals (Cammaerts-Tricot, Reference Cammaerts-Tricot1975; Robinson, Reference Robinson1987; Uematsu, Kutsukake, Fukatsu, Shimada, & Shibao, Reference Uematsu, Kutsukake, Fukatsu, Shimada and Shibao2010). The key difference here lies in the individual-level age-dependent costs and benefits (Rodrigues, Reference Rodrigues2018). Among humans, a younger male's fairly low risk of injury during intergroup raids is outweighed by the benefits of acquiring additional resources and/or reputation, despite the community-level costs arising from the loss of peace and subsequent likelihood of retaliatory raids. Older individuals may face a greater risk of injury, and stand to gain less from accruing additional resources or reputation, particularly after they have already reproduced (target article, sect. 2). Among social insects, workers' reproductive potential is highest when young; even in societies with reproductive division of labour, young workers often have active ovaries (Page & Peng, Reference Page and Peng2001). Young workers thus incur individual costs if killed in intercolony conflicts, whereas older workers have no potential for direct fitness through reproduction, and therefore their behaviour is driven solely by the inclusive-fitness benefits of group defence. At the group level, the mode of group formation strongly affects group composition and cohesion over time. In ants that exhibit intergroup cooperation, groups founded by a few individuals split into networks of related interconnected nests, whereas in humans, both group formation and development are more fluid. Thus, age-specific individual-level payoffs differ in humans and ants, and a group's demographic composition may have a species-specific influence on the emergence of peace.

Peace is more than simply the absence of war (target article, sects. 1, 3 and 7). In human societies, we have cultural information that helps us distinguish peaceful coexistence from fearful avoidance, although both might result in superficially similar behaviours (Pisor & Surbeck, Reference Pisor and Surbeck2019). Among nonhuman animal groups, conflict avoidance is a major behavioural driver (Morris-Drake, Kennedy, Braga Goncalves, & Radford, Reference Morris-Drake, Kennedy, Braga Goncalves and Radford2022; Rodrigues et al., Reference Rodrigues, Barker and Robinson2022; Triki, Daughters, & De Dreu, Reference Triki, Daughters and De Dreu2022) and many apparent examples of intergroup tolerance may be the product of ongoing active conflict reduction. Although we are currently limited to identifying peaceful outcomes rather than peaceful intentions in nonhuman animals, evidence is accumulating for the cognitive complexity of social insects, including emotion-like states in bees that are consistent with those in vertebrates, measured behaviourally and chemically (Chittka & Rossi, Reference Chittka and Rossi2022). As methods of measurement continue to become more sophisticated, we may get closer to determining the mental states of nonhuman animals during peaceful interactions.

Parallels between human sociocultural evolution and comparable processes in social insects provide an opportunity to relinquish an anthropocentric perspective and identify the essence of a behavioural phenomenon (DeSilva, Traniello, Claxton, & Fannin, Reference DeSilva, Traniello, Claxton and Fannin2021; Gowdy & Krall, Reference Gowdy and Krall2013; Robinson & Barker, Reference Robinson and Barker2017). Some similarities between ants and human societies are likely due to the emergent properties of complex social systems, irrespective of the nature of their component parts, others are due to an intriguing convergence, where similar endpoints are reached through differing evolutionary mechanisms. Such convergent examples offer the opportunity to identify necessary and sufficient steps and alternative pathways to a given endpoint. The occurrence of long-lasting, positive-sum, interdependent intergroup relationships in both humans and ants has the potential to provide new insights into the evolution of peace.