Although Glowacki proposed that peace developed from the relatively recent advent of intergroup norms and tolerance for out-group members, we submit that (a) positive intergroup relations developed from a psychology grounded in the regulation of intragroup relations, (b) the “default” intergroup orientation is uncertainty, and (c) positive intergroup relations likely existed early in our evolutionary history.

Glowacki defines peace as harmonious relationships between groups maintained without the threat of violence, where groups can be anything from families to nation states. However, defining such contentious concepts like “peace” and “groups” is a difficult task, and we discuss the implications of Glowacki's definitions for understanding intergroup relationships and their evolutionary history.

The emergence of social norms would have been dependent on the evolution of the cognitive capacity for mentalising to multiple orders of intentionality. Common knowledge is a related phenomenon that can solve coordination problems. That the same cognitive and social mechanisms should facilitate both peace and war is resonant with Girard's scapegoat hypothesis on the relationship between violence and religion.

Peace is a hallmark of human societies. However, certain ant species engage in long-term intergroup resource sharing, which is remarkably similar to peace among human groups. We discuss how individual and group payoff distributions are affected by kinship, dispersal, and age structure; the challenges of diagnosing peace; and the benefits of comparing convergent complex behaviours in disparate taxa.

We take issue with Glowacki's assumption that intergroup relations are characterized by positive-sum interactions and suggest to include negative-sum interactions, and between-group independence. As such, peace may be better defined as the absence of negative-sum interactions. Rather than being a consequence of cooperation, peace emerges as a necessary but not sufficient prerequisite for positive (in)direct reciprocity between groups that, in turn, is key to social identities and cultural complexity.

Glowacki's detailed account of small-scale societies’ endogenously emerging tendencies to oscillate between phases of peace and war highlights a need for understanding better the incentives governing “internal” policing for “external” peacekeeping. Here, I sketch some of these incentives and point out a resulting dilemma which Glowacki's account leaves unresolved for the time being.

Reconstructing pathways to human peace can be hampered by superficial evaluations of similar processes in nonhuman species. A deeper understanding of bonobo social systems allows us to reevaluate the preconditions for peace to gain a greater insight on the evolutionary timescale of peace emergence.

Peace, the article shows, is achieved by culturally evolved institutions that incentivize positive-sum relationships. We propose that this insight has important consequences for the design of human social cognition. Cues that signal the existence of such institutions should play a prominent role in detecting group membership. We show how this accounts for previous findings and suggest avenues for future research.

By focusing on peace, Glowacki provides a fresh perspective on warfare. Why did humans evolve peace? Other animals aggregate peacefully when resources are not economically defendable. The human capacity for peace may arise from two key factors: Multilevel societies and psychology shaped by within-group exchanges, which may have begun when tools enabled hominins to extract foods, including tubers and roots.

Based on evidence of selection against alpha-male behavior in the earliest Homo sapiens, I suggest that by 300,000 ya (years ago) language would have been sufficiently sophisticated to contribute to peacemaking between groups. Language also influenced the social landscape of peace and war, and groups' ability to form coalitions.

Humans are predisposed to form in-groups and out-groups that are remarkably flexible in their definition due largely to the complex language that has evolved in them. Language has allowed for the creation of shared “background stories” that can unite people who do not know each other. Second, the discovery of agriculture has resulted in the critical need to negotiate boundaries, a process that can lead to peace (but also war).

The 30 commentators are largely sympathetic to the account I develop for the origins of peace in humans, though many suggest that peace has deeper roots and that humans share characteristics of peace with other species. Multiple commentators propose how to extend my framework or focus on the cognitive and psychological prerequisites for peace. In my reply, I discuss these considerations and further my account of why I think peace as defined here was unlikely prior to behavioral modernity which emerged approximately 100,000 years ago. In general, there seems to be a consensus that moving the debate beyond “war versus peace” in human evolution and instead focusing on the conditions that enable war or peace is a fruitful direction for the field to take.

The dominant paradigm of experiments in the social and behavioral sciences views an experiment as a test of a theory, where the theory is assumed to generalize beyond the experiment's specific conditions. According to this view, which Alan Newell once characterized as “playing twenty questions with nature,” theory is advanced one experiment at a time, and the integration of disparate findings is assumed to happen via the scientific publishing process. In this article, we argue that the process of integration is at best inefficient, and at worst it does not, in fact, occur. We further show that the challenge of integration cannot be adequately addressed by recently proposed reforms that focus on the reliability and replicability of individual findings, nor simply by conducting more or larger experiments. Rather, the problem arises from the imprecise nature of social and behavioral theories and, consequently, a lack of commensurability across experiments conducted under different conditions. Therefore, researchers must fundamentally rethink how they design experiments and how the experiments relate to theory. We specifically describe an alternative framework, integrative experiment design, which intrinsically promotes commensurability and continuous integration of knowledge. In this paradigm, researchers explicitly map the design space of possible experiments associated with a given research question, embracing many potentially relevant theories rather than focusing on just one. Researchers then iteratively generate theories and test them with experiments explicitly sampled from the design space, allowing results to be integrated across experiments. Given recent methodological and technological developments, we conclude that this approach is feasible and would generate more-reliable, more-cumulative empirical and theoretical knowledge than the current paradigm – and with far greater efficiency.

Creating an integrated design space can be successful only if researchers agree on how to define and measure a certain phenomenon of interest. Adversarial collaborations and mathematical modeling can aid in reaching the necessary level of agreement when researchers depart from different theoretical perspectives.

Integrative experiments, as described, seem blindly empirical, as if the question of generality of effects could not be understood through controlled one-at-a-time experiments. But current research using such experiments, especially applied research, can resolve issues and make progress through understanding of cause–effect pathways, leaving to engineers the task of translating this understanding into practice.

We propose a friendly amendment to integrative experiment design (IED), adversarial-collaboration IED, that incentivizes research teams from competing theoretical perspectives to identify zones of the design space where they possess an explanatory edge. This amendment is especially critical in debates that have high policy stakes and carry a strong normative-political charge that might otherwise prevent free exchange of ideas.

Demonstrating the limitations of the one-at-a-time approach, crowd initiatives reveal the surprisingly powerful role of analytic and design choices in shaping scientific results. At the same time, cross-cultural variability in effects is far below the levels initially expected. This highlights the value of “medium” science, leveraging diverse stimulus sets and extensive robustness checks to achieve integrative tests of competing theories.

Almaatouq et al. claim that the integrative experiment design can help “develop a reliable, cohesive, and cumulative theoretical understanding.” I will contest this claim by challenging three underlying assumptions about the nature of scientific theories. I propose that the integrative experiment design should be viewed as an exploratory framework rather than a means to build or evaluate theories.

Integrative experiment design should be extended to thought-experiments. Thought-experiments are closely connected to “real” experiments. They are involved in devising the design space of theories and possible experiments. The latter may be partitioned into experiments to be really performed and mere thought-experiments. The proposed extension of integrative experiment design lends guidance to a more methodical performance of thought-experiments.

In many areas of the social and behavioral sciences, the nature of the experiments and theories that best capture the underlying constructs are themselves areas of active inquiry. Integrative experiment design risks being prematurely exploitative, hindering exploration of experimental paradigms and of diverse theoretical accounts for target phenomena.