In the fascinating article, Glowacki proposes that peace can be understood as the solution to iterated prisoner's dilemma interactions between members of different groups. Following a game-theoretic approach and a systematic review of conflict in small-scale societies, Glowacki concludes that peace is a consequence of cultural complexity, and that in turn cultural complexity has been favored by the formation of social identities within groups. Here, we take issue with (i) the conceptualization of intergroup relations as solely characterized by positive-sum interactions, and (ii) the lack of attention to how bottom-up processes such as direct and indirect reciprocity can lead to the formation and dissolution of groups, group identities, and institutions.

Glowacki starts from the assumption that intergroup relations are characterized by positive-sum interactions. While it is true that humans possess remarkable abilities to cooperate within and between groups, interdependencies between groups can range from negative-sum to positive-sum interdependencies, or reflect independent coexistence (De Dreu & Gross, Reference De Dreu and Gross2019; De Dreu, Gross, Fariña, & Ma, Reference De Dreu, Gross, Fariña and Ma2020; Deutsch, Reference Deutsch1973). For example, how groups relate to each other depends on (changes in) socioecological systems, resource scarcities, and subsistence style (De Dreu, Gross, & Reddmann, Reference De Dreu, Gross and Reddmann2022). And different interdependencies dramatically shape and change intergroup interactions (De Dreu et al., Reference De Dreu, Gross, Fariña and Ma2020). While peaceful coexistence is by definition present between independent groups and possible in positive-sum interactions, it is less trivial to reach peace in negative or zero-sum environments (Romano et al., Reference Romano, Giardini, Columbus, De Kwaadsteniet, Kisfalusi, Triki and Hagel2021; Romano, Gross, & De Dreu, Reference Romano, Gross and De Dreu2022a). Rather than defining peace as the solution to problems of cooperation, peace can be more parsimoniously defined as lack of conflict – something possible in any type of interdependence among groups. Crucially, however, the presence of peace neither translates into the establishment of cooperation, nor does a lack of cooperation translates to the presence of conflict. Rather than a consequence, peace becomes a necessary but not sufficient prerequisite for intergroup cooperation.

When peace is defined as the absence of conflict, there is no need to hypothesize that the mechanisms evolved to establish peace between groups evolved after the mechanisms that sustain cooperation. In fact, humans evolved strategies to cope with different interdependencies, including strategies to avoid conflict in negative- and zero-sum interactions (Aktipis et al., Reference Aktipis, Cronk, Alcock, Ayers, Baciu, Balliet and Winfrey2018; Balliet, Tybur, & Van Lange, Reference Balliet, Tybur and Van Lange2016). This is not to say that cultural complexity does not contribute to the current features of peace that, for example, characterize international relations in the modern world. It is to say, however, that the psychological mechanisms that can bring groups of individuals to abstain from conflict can have coevolved in parallel to and independent of the psychological mechanisms that foster cooperation.

Acknowledging and integrating the interdependence structure of intergroup relations help to understand when and why cultural complexity may actually foster conflict rather than peace. Indeed, Glowacki makes the interesting observation that cultural complexity is often associated with an increased ability of groups to act unanimously, by developing, for example, hierarchies, role differentiation, and a dedicated military class. In environments in which groups are locked in negative-sum relationships, the ability of groups to coerce their members to act together may actually increase the likelihood of conflict and its associated waste (De Dreu et al., Reference De Dreu, Gross, Méder, Giffin, Prochazkova, Krikeb and Columbus2016).

In the final analysis, the question remains how groups move from peaceful coexistence to cooperation and, perhaps group fusion and fission. One possible answer is suggested in work on direct and indirect reciprocity (De Dreu, Fariña, Gross, & Romano, Reference De Dreu, Fariña, Gross and Romano2021; Lehmann, Powers, & Van Schaik, Reference Lehmann, Powers and Van Schaik2022; Romano, Balliet, & Wu, Reference Romano, Balliet and Wu2017; Romano, Saral, & Wu, Reference Romano, Saral and Wu2022b; Wu, Balliet, & Van Lange, Reference Wu, Balliet and Van Lange2016; Yamagishi & Kiyonari, Reference Yamagishi and Kiyonari2000). When initial acts of cooperation are reciprocated and rewarded rather than exploited, cooperation can become sustainable both within and between groups. Indeed, recent simulations and experimental studies in our laboratory revealed how more frequent interactions between members of different groups promote the emergence of cooperation across group boundaries and the creation of public goods that connect groups (Gross et al., Reference Gross, Méder, De Dreu, Romano, Molenmaker and Hoenig2023). Importantly, frequent intergroup interactions also led members to start identifying more with a larger collective rather than their own group – social identification is an emergent property rather than a causal mechanism for group formation and intergroup cooperation. With more (frequent) intergroup exchanges, reciprocity permeates group boundaries and fosters the fusion of groups, resulting in larger, multilayered groups with complex institutions.

Taken together, we suggest, in line with extant work on cooperation and conflict within and between groups, to define peace as the absence of conflict and (in)direct reciprocity as key to building cooperative relations between groups. For intergroup cooperation and fusion to happen and cultural complexity to emerge, we need peace and (in)direct reciprocity.