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.Keywords: competition; culture; evolution; group selection; heritable variation; institutions; norms BEHAVIORAL AND BRAIN SCIENCES (2016) KARL FROST is a Ph.D. candidate in Ecology at the University of California, Davis. He researches the cultural evolution of prosociality via religion and ritual practices, using behavioral experiments, gene-culture coevolution models, and field research in Canada looking at environmental activism in the face of the tar sands oil industry and an antagonistic government. He also directs the Body Research Physical Theater and is interested in cross-cultural exchange of theater practice as theater anthropology and arts-science fusion.
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.
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.
Clinicians in Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies are rethinking whether Post-traumatic Stress Disorder (PTSD) is caused solely by exposure to life-threatening experiences, or also by moral injury—witnessing or participating in acts that violate moral beliefs. However, while there are evolutionary hypotheses explaining PTSD as a response to physical danger, the evolutionary roots of moral injury lack an explanation. We posit that a subset of symptoms of combat-related PTSD is associated with moral injury and that these symptoms evolved in tandem with human’s norm-psychology. We can examine this hypothesis by comparing societies with different moral beliefs about warfare, norm enforcement mechanisms, and spheres of moral concern. To illustrate the utility of this framework, we describe combat trauma, war norms, and norm enforcement among Turkana pastoralist warriors in Kenya who participate in highly lethal raids of neighboring ethnic groups. We previously showed that depressive PTSD symptoms in Turkana warriors are more strongly associated with experiencing moral violations in combat, and that Turkana warriors with comparably high overall PTSD symptom-severity experience lower rates of depressive symptoms than US combat veterans. Here we detail aspects of Turkana warfare, moral beliefs, and post-battle rituals that differ from WEIRD societies, and that may ameliorate the symptoms of moral injury in Turkana warriors. Our findings highlight how further studies of combat trauma outside of WEIRD militaries can help evaluate this theory and illustrate the importance of cross-cultural research for identifying the evolutionary roots of combat stress and best practices for prevention and recovery.
Humans in many societies cooperate in economic experiments at much higher levels than would be expected if their goal was maximizing economic returns, even when their interactions are anonymous and one-shot. This is a puzzle because paying a cost to benefit another in one-shot interactions gives no direct or indirect benefits to the cooperator. This paper explores the logic of two competing evolutionary hypotheses to explain this behavior. The "norm psychology" hypothesis holds that a player's choice of strategy reflects socially-learned cultural norms. Its premise is that over the course of human evolutionary history, cultural norms varied considerably across human societies and through a process of gene-culture co-evolution, humans evolved mechanisms to learn and adopt the norms that are successful in their particular society. The "mismatch" hypothesis holds that pro-social preferences evolved genetically in our hunter-gatherer past where one-shot anonymous interactions were rare and these preferences are misapplied in modern laboratory settings. I compare these hypotheses by adopting a wellknown model of the mismatch hypothesis and show that selection for oneshot cooperation in the model is an artifact of agents being confined to only two strategies: Tit-for-Tat and Always Defect. Allowing for repentant and forgiving strategies reverses selection away from one-shot cooperation under all environmental parameters. Direct reciprocity does not necessarily lead to cooperation, but instead generates many different normative equilibria depending on a group's idiosyncratic evolutionary history. Therefore, an agent whose behavior is evoked solely from non-cultural environmental cues will be disadvantaged relative to an agent who learns the locally successful norms. Cooperation in one-shot laboratory experiments is thus more easily explained as the result of a psychology evolved for learning social norms than as a genetic mismatch. 2.
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