Combat stress in a small-scale society suggests divergent evolutionary roots for posttraumatic stress disorder symptoms.

Military personnel in industrialized societies often develop posttraumatic stress disorder (PTSD) during combat. It is unclear whether combat-related PTSD is a universal evolutionary response to danger or a culture-specific syndrome of industrialized societies. We interviewed 218 Turkana pastoralist warriors in Kenya, who engage in lethal cattle raids, about their combat experiences and PTSD symptoms. Turkana in our sample had a high prevalence of PTSD symptoms, but Turkana with high symptom severity had lower prevalence of depression-like symptoms than American service members with high symptom severity. Symptoms that facilitate responding to danger were better predicted by combat exposure, whereas depressive symptoms were better predicted by exposure to combat-related moral violations. The findings suggest that some PTSD symptoms stem from an evolved response to danger, while depressive PTSD symptoms may be caused by culturally specific moral norm violations.

Group-structured cultural selection can explain both war and peace.

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.

Integrating models of cognition and culture will require a bit more math.

We support the goal to integrate models of culture and cognition. However, we are not convinced that the free energy principle and Thinking Through Other Minds will be useful in achieving it. There are long traditions of modeling both cultural evolution and cognition. Demonstrating that FEP or TTOM can integrate these models will require a bit more math.

Cultural group selection follows Darwin's classic syllogism for the operation of selection.

The main objective of our target article was to sketch the empirical case for the importance of selection at the level of groups on cultural variation. Such variation is massive in humans, but modest or absent in other species. Group selection processes acting on this variation is a framework for developing explanations of the unusual level of cooperation between non-relatives found in our species. Our case for cultural group selection (CGS) followed Darwin's classic syllogism regarding natural selection: If variation exists at the level of groups, if this variation is heritable, and if it plays a role in the success or failure of competing groups, then selection will operate at the level of groups. We outlined the relevant domains where such evidence can be sought and characterized the main conclusions of work in those domains. Most commentators agree that CGS plays some role in human evolution, although some were considerably more skeptical. Some contributed additional empirical cases. Some raised issues of the scope of CGS explanations versus competing ones.

Cultural group selection plays an essential role in explaining human cooperation: A sketch of the evidence.

Human cooperation is highly unusual. We live in large groups composed mostly of non-relatives. Evolutionists have proposed a number of explanations for this pattern, including cultural group selection and extensions of more general processes such as reciprocity, kin selection, and multi-level selection acting on genes. Evolutionary processes are consilient; they affect several different empirical domains, such as patterns of behavior and the proximal drivers of that behavior. In this target article, we sketch the evidence from five domains that bear on the explanatory adequacy of cultural group selection and competing hypotheses to explain human cooperation. Does cultural transmission constitute an inheritance system that can evolve in a Darwinian fashion? Are the norms that underpin institutions among the cultural traits so transmitted? Do we observe sufficient variation at the level of groups of considerable size for group selection to be a plausible process? Do human groups compete, and do success and failure in competition depend upon cultural variation? Do we observe adaptations for cooperation in humans that most plausibly arose by cultural group selection? If the answer to one of these questions is "no," then we must look to other hypotheses. We present evidence, including quantitative evidence, that the answer to all of the questions is "yes" and argue that we must take the cultural group selection hypothesis seriously. If culturally transmitted systems of rules (institutions) that limit individual deviance organize cooperation in human societies, then it is not clear that any extant alternative to cultural group selection can be a complete explanation.

An evolutionary theory of large-scale human warfare: Group-structured cultural selection.

When humans wage war, it is not unusual for battlefields to be strewn with dead warriors. These warriors typically were men in their reproductive prime who, had they not died in battle, might have gone on to father more children. Typically, they are also genetically unrelated to one another. We know of no other animal species in which reproductively capable, genetically unrelated individuals risk their lives in this manner. Because the immense private costs borne by individual warriors create benefits that are shared widely by others in their group, warfare is a stark evolutionary puzzle that is difficult to explain. Although several scholars have posited models of the evolution of human warfare, these models do not adequately explain how humans solve the problem of collective action in warfare at the evolutionarily novel scale of hundreds of genetically unrelated individuals. We propose that group-structured cultural selection explains this phenomenon.

Many important group-level traits are institutions.

Smaldino makes a solid contribution to the literature on the evolution of human social organization by pointing out that group-level-traits (GLTs) often emerge from the interactions of group members in such a way that their effects are not easily partitioned into individual selection. However, we argue that he too readily dismisses institutional analysis as a tool for understanding these traits.

Constraints on cooperation shape hierarchical versus distributed structure in human groups.

Some human groups are organized hierarchically and some are distributed. Both types of groups occur in economic, political, and military domains, but it is unclear why hierarchical organizations are favored in certain contexts and distributed organizations are favored in others. I propose that these different organizational structures can be explained by human groups having different constraints on their ability to foster cooperation within the group. Human within-group cooperation is often maintained by monitoring and punishment. In hierarchical groups, monitoring and punishment are organized into tree-like command-and-control structures with supervisors responsible for monitoring the cooperation of their subordinates and punishing non-cooperators. By contrast, in distributed groups, monitoring is diffuse and punishment is collective. I propose that the organization of cooperative human groups is constrained by the costs of monitoring and punishment. I formalize this hypothesis with a model where individuals in a group cooperate to produce public goods while embedded in a network of monitoring and punishment responsibilities. I show that, when punishment costs are high and monitoring costs are low, socially-optimal monitoring and punishment networks are distributed. The size of these distributed networks is constrained by monitoring costs. However, when punishment costs are low, socially-optimal networks are hierarchical. Monitoring costs do not constrain the size of hierarchical networks but determine how many levels of supervision are required to foster cooperation in the hierarchical group. These results may explain the increasingly large and hierarchical groups throughout much of human history. They also suggest that the recent emergence of large-scale distributed organizations has been possible because new technologies, like the internet, have made monitoring costs extremely low.