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.

Glowacki argues that the human capacity for peace emerged 100,000 years ago, and that the logic of peace is such that the traits and technologies that enable peace are the same that are used to wage war. In my commentary I raise some concerns about these points as well as about Glowacki's understanding of peace.

Glowacki's work meshes well with our view of human nature as having evolved to use culture to improve survival and reproduction. Peace is a cultural achievement, requiring advances in social organization and control, including leaders who can implement policies to benefit the group, third-party mediation, and intergroup cooperation. Cultural advances shift intergroup interactions from negative-sum (war) to positive-sum (trade).

Economic games provide models of real-world contexts in which researchers can probe dispositional and structural determinants of intergroup relations. Most intergroup games focus on determinants of aggression between groups and constrain the possibilities for peace. However, paradigms such as the intergroup parochial and universal cooperation game allow for peaceful intergroup relations and can be adapted for the study of peace.

Glowacki offers many new directions for understanding and even eliminating the problem of war, especially creating positive interdependencies with out-group members. We develop Glowacki's intriguing proposition that in-group dynamics provide a route to peace by describing a prosocial motivational system, the caregiving system, that aligns individual interests and eliminates the need to use coercion to achieve mutually beneficial outcomes.

Glowacki explored the conditions required for peace and argued its preconditions arose only within the last 100,000 years. The present commentary addresses some major brain changes that occurred only in Homo sapiens within that period of time and the verbal and nonverbal cognitive sequelae of those neurological changes that may have aided the diplomatic negotiations required for peaceful solutions.

Here we revise Glowacki's model by proposing a simple and empirically tested mechanism that is applicable to a comprehensive set of social interactions. This parsimonious mechanism accounts for the choice of both cooperative and peaceful alternatives and explains when each choice benefits the interacting parties. It is proposed that this mechanism is key to the evolution of both peace and conflict.

The capacities required for both peace and war predate 100,000 years ago in the genus Homo are deeply entangled in the modes by which humans physically and perceptually construct their worlds and communities, and may not be sufficiently captured by economic models.

Glowacki asserts that interdependent relationships beyond group boundaries are exceptionally rare among nonhuman mammals. However, rudimentary forms of interdependence can be seen in primate species that form multilevel societies, that is, core social units embedded within higher-level grouping categories. Studies of primate multilevel societies can enrich discussions about the evolutionary origins of peaceful between-group interactions in humans.

We elaborate on Glowacki's claim that humans are more capable of establishing peace than other mammals. We present three aspects suggesting caution. First, the social capabilities of nonhuman primates should not be underestimated. Second, the effect of these capabilities on peace establishment is nonmonotonous. Third, defining peace by human-centered values introduces a fallacy.

While effective institutional practices are critical for the evolution of peace certain factors deter their effectiveness. In-group and out-group dynamics may make peace difficult between culturally distinct groups. Critical ecological conditions often lead to intractable conflict over resources. And within group conflicts of interest most prominently between generations may inhibit effective peace making.

I challenge the idea by Glowacki that “strong sanctions” such as fines, physical punishment, or execution are more effective in promoting peace than “weak punishments” like social rejection. Reviewing evidence that social rejection can have significant social and psychological costs for norm violators, I propose that social rejection can serve as a powerful reputational sanction in fostering peace in society.

The target article offers a game-theoretical analysis of primitive intergroup aggression (i.e., raiding) and discusses difficulties in achieving peace. We argue the analysis does not capture the actual strategy space, missing out “do-nothing.” Experimental evidence robustly shows people prefer doing nothing against out-group members over cooperating with/attacking them. Thus, the target article overestimates the likelihood of intergroup aggression.

In this commentary, we set out the specifics of how Glowacki's game theoretical framework for the evolution of peace could be incorporated within broader cultural evolutionary approaches. We outline a formal proposal for prisoner's dilemma games investigating raid-based conflict. We also centre an ethnographic lens to understand the norms surrounding war and peace in intergroup interactions in small-scale communities.

Glowacki's account overlooks the role of religion in the regulation of cooperation, tolerance, and peace values. We interrogate three premises of Glowacki's argument and suggest that approaching religion as an adaptive system reveals how religious commitments and practices likely had a more substantial impact on the evolution of peace and conflict than currently presumed.

While necessary parts of the puzzle, cultural technologies are insufficient to explain peace. They are a form of second-order cooperation – a cooperative interaction designed to incentivize first-order cooperation. We propose an explanation for peacemaking cultural technologies, and therefore peace, based on the reputational incentives for second-order cooperation.

Glowacki discusses how humans regularly face collective action problems that may result in either peaceful or aggressive between-group interactions. Peace and war probably coevolved in humans. Using a gene–culture evolutionary framework is a powerful way to analyse why, when, and how humans have the capacity to build and maintain long-term peaceful interactions between groups and also to wage deadly wars.

Glowacki recognizes the importance of norms in enabling war and peace, but does not focus on the cultural evolutionary mechanisms by which these norms are maintained. We highlight how group-structured cultural selection shapes the scale and nature of peaceful intergroup interactions. The mechanistic perspective reveals that there are many more cases of peaceful intergroup relations than the current account implies.

If group norms and decisions foster peace, then understanding how norms and decisions arise becomes important. Here, we suggest that neither norms nor other forms of group-based decision making (such as offering restitution) can be adequately understood without simultaneously considering (i) what individual psychologies are doing and (ii) the dynamics these psychologies produce when interacting with each other.

Enhanced cooperation increases the capacity for humans to engage in large-scale warfare. This ability provides the foundation for male coalitionary behavior, leaving open the question of whether cooperation evolved in the same way, or for the same purpose, in females. Such coalitionary behavior entrains hierarchical forms of leadership that remain inherently unstable, providing a spark for conflict to emerge.