Peace may be an invention designed to optimize the benefits of long-term cooperation, as Glowacki argues. Yet cooperation first served a more influential purpose. The ability to cooperate solved the repeated evolutionary challenge of fighting off predators whether environmental, animal, or human. These forms of cooperation facilitated and encouraged greater capacity to wage large-scale warfare. In short, humans learned to cooperate in order to engage in combat more effectively.

This cooperation rests on a foundation of male coalitionary behavior (Wrangham, Reference Wrangham1999). Such coalitions entrain hierarchical forms of leadership that remain inherently unstable, as each member continually vies for higher status within the group, providing a spark for intragroup conflict to emerge continually. The role of leadership in suppressing intragroup conflict while simultaneously facilitating more effective intergroup conflict is crucial in scaling arguments derived from more egalitarian societies to larger scale modern nation states.

The interconnection between the capacity for cooperation and conflict offers insight into how the evolution of peace, based on small-scale societies, can be scaled to a more generalized argument applicable to large modern nation states. This argument faces at least three important challenges.

First, extrapolating from small-scale societies to large states requires consideration of the incentives and constraints inherent in large institutions and organizations. What applies to small-scale societies may not hold within the context of large bureaucracies that are driven/mediated by processes at odds with basic human psychological architecture. Specifically, once the militarized threshold is crossed in combat, and weapons help neutralize the importance of physical strength and formidability, the risk and benefits associated with combat are changed. Physically smaller men can kill large numbers of physically stronger people with precision weapons. Given the importance of the role of leadership in hierarchical societies, followers need to be convinced that leaders have their best interests, and those of community, in mind when committing to war. Followers need to believe the risks they undertake are worth the prize they expect. In small-scale societies, keeping track of how benefits are distributed is easier. But is not clear if the selection pressures that operate on small-scale societies function in the same way in the context of large-scale state-sponsored conflicts. This limits the generalizability of the argument regarding incentives and capacity for stable peace in the context of modern large-scale societies.

Second, the argument for peace laid out in the target article deals primarily with the capacity for intergroup cooperation with little discussion of the challenges of maintaining in-group harmony and cooperation. Leadership hierarchies are inherently unstable. But violence within large-scale societies is no longer rare, unexpected, or quickly resolved, as suggested in this article. Rather, it remains endemic. One need only look at the enormous number of mass casualties from gun violence in the United States or the repression of ethnic minorities like the Uyghurs in China or the Rohingya in Myanmar to question the limits of peace and cooperation within large-scale societies.

Third, kin-based clan governance styles have been shown to produce dysfunctional and unstable patterns of state behavior, including decreased security (Hudson, Bowen, & Nielsen, Reference Hudson, Bowen and Nielsen2015). These dynamics of state behavior rely on extreme subordination of women, often through discriminatory family law practices. The current policies of the Taliban in Afghanistan provide a vividly heartbreaking example of these strategies. The destructive influence of clan-based governance models demonstrates the significance and implications of sex differences in aggression for large state stability and security (Hudson, Caprioli, McDermott, & Bowen, Reference Hudson, Caprioli, McDermott and Bowen2023; McDermott, Reference McDermott2015). The inherent instability of the hierarchical governance models upon which male coalitions depend opens the question of whether cooperation, or conflict, evolved in the same way, or for the same purpose, in females. One need not argue that women are inherently peaceful to recognize that women may choose to fight under different circumstances, and for different reasons, than men. For example, women have been shown to be more likely to join defensive, as opposed to offensive campaigns (Lopez, Reference Lopez2017).

Humans are not inherently either peaceful or bellicose by default. Rather, we have the capacity for cooperation to engage in both. An integrated and productive approach seeks to examine the conditions and circumstances under which both aggression and cooperation might have emerged and proved beneficial and productive for men and women, in different ways and for varied reasons. This allows the discussion to move beyond an entrenched either-or approach and encourages for a more nuanced understanding of human evolution. Appreciating the inherent adaptability of individuals also generates novel hypotheses regarding the potential for modern nation states to structure their institutions and bureaucracies in ways that are in more natural alignment with human cognitive architecture. Biology does not act in isolation; rather, various programs can be activated by different environment and social circumstances. For example, hierarchical leadership structures create incentives for conflict, but also constraints on the ability to overthrow the leader. Recognizing how these various structures offer opportunities and impose limits on human affiliation and aggression can provide insight into how best to build future institutions to encourage cooperation and diminish hostility. Evolution may have exerted selection pressures that allowed for the development of cooperation and the socially integrated norms mentioned in the target article. But evolution also exerted pressures that generated incentives for violence against those who pose threats or risks.

If peace is an invention, it is one that needs more work. It remains a distant dream not only in history but also in modern international relations. The current war in Ukraine stands as testament to how one man aspiring to preeminence can wreak havoc on many great nations seeking to thwart his desire for supremacy. An evolutionary lens provides insight into how humans might do a better job at overcoming internecine conflict in order to achieve more cooperation with less hostility, but also cautions humility in our ability to overcome our inherent drives and desires for dominance.