Sarah Mathew

Human cooperation is motivated both by shared interests arising from kinship and friendship and by social norms, but whether these mechanisms substitute for or complement one another remains debated. Cooperative farming among Derung horticulturalists offers an ideal study context, because most cofarming occurs among kin or friends, and a norm prescribes equal harvest division by household regardless of contributions. We show that this norm solves two problems that shared interests cannot solve, or may even exacerbate. First, shared interests expand the range of acceptable divisions, making coordination harder. By specifying a salient division, the norm resolves this problem. Second, cofarmers often misperceive how much their partner benefits. By prescribing a division independent of cost-benefit assessments, the norm prevents these differences from unraveling cooperation. The complementary action of norms and shared interests may explain why humans cooperate with kin and friends more than other animals.

Reciprocity theory has a hallowed status in evolutionary social sciences because it provides a way to explain cooperation among un- related individuals. Theorists have created a large body of mathematical work based on the iterated prisoner’s dilemma game that explores which strategies are evolutionarily stable in a range of conditions including when there are behavioral errors. However, there is a mismatch between the vigorous pace of theoretical research and the trickle of empirical studies that have tested predictions generated by the theoretical work. Furthermore, findings from the empirical work on dyadic cooperation in humans deviate from theoretical predictions in important ways. We argue that this discrepancy suggests that the standard theoretical framework is poorly suited to account for key features of the real-world cooperation, and suggest possible avenues that the theoretical exploration of reciprocity needs to take to close the gap between theory and real-world cooperation.

Understanding how cultures evolve is critical in a world where anthropogenic cultural activities are driving rapid and profound social and environmental change, with far-reaching consequences for global futures. Yet, the study of cultural evolution is fragmented across disciplines and remains predominantly shaped by western perspectives. This Theme Issue stems from the Cultural Evolution Society Transformation Fund (CES-TF), a grant scheme designed to bridge the social and natural sciences, promote equity, diversity and inclusion (EDI) in research, and support work with policy relevance. In this introductory article, we highlight the grant scheme’s core commitments to equity, diversity and inclusion, and show how these gave rise to a collection of projects with emergent themes that converge on many of the UN’s Sustainable Development Goals. Collectively, the articles featured in this issue employ a variety of approaches and methods, draw on expertise from across the social sciences, natural sciences and humanities, include under-represented researchers and research settings beyond Northern America and Western Europe, and bring the field closer to policy. This diversification of the cultural evolution field is an ongoing process that ultimately strengthens its capacity to positively shape global futures. This article is part of the theme issue ‘Transforming cultural evolution research and its application to global futures’.

Norms are essential for social stability but can hinder adaptability in changing environments. Yet human societies have found ways to modify existing norms or create new ones in response to novel challenges. This paper proposes a framework for understanding adaptive norm evolution. First, drawing on a theory of legal order, we posit that societies balance normative stability and adaptability through metanorms—rules that govern the process by which norms are interpreted, changed and enforced. Second, we test this idea in the context of customary dispute resolution by elders among the Turkana, a pastoralist society in Kenya. Based on vignette experiments with 369 participants, we found that community members were significantly more willing to enforce decisions when elders aligned their conduct with metanorms. Elders are constrained in their ability to alter long-standing customs, but by following metanorms, they can create new rules for novel situations. These findings support our proposed mechanism: in the absence of centralized authority, metanorms governing normative institutions allow for adaptive norm change while preserving cultural continuity. We conclude by suggesting that group-level selection acts on cultural variation in metanorms, shaping the evolvability of normative systems and enabling societies to sustain adaptive legal order without coercive centralized power. This article is part of the theme issue ‘Transforming cultural evolution research and its application to global futures’.

Norms are essential for social stability but can hinder adaptability in changing environments. Yet human societies have found ways to modify existing norms or create new ones in response to novel challenges. This paper proposes a framework for understanding adaptive norm evolution. First, drawing on a theory of legal order, we posit that societies balance normative stability and adaptability through metanorms—rules that govern the process by which norms are interpreted, changed and enforced. Second, we test this idea in the context of customary dispute resolution by elders among the Turkana, a pastoralist society in Kenya. Based on vignette experiments with 369 participants, we found that community members were significantly more willing to enforce decisions when elders aligned their conduct with metanorms. Elders are constrained in their ability to alter long-standing customs, but by following metanorms can create new rules for novel situations. These findings support our proposed mechanism: in the absence of centralized authority, metanorms governing normative institutions allow for adaptive norm change while preserving cultural continuity. We conclude by suggesting that group-level selection acts on cultural variation in metanorms, shaping the evolvability of normative systems and enabling societies to sustain adaptive legal order without coercive centralized power

The Turkana pastoralists of Kenya inhabit arid, water-limited environments and rely largely on livestock for subsistence. Working with Turkana communities, we sequenced 367 whole genomes and identified eight regions with evidence for recent positive selection. One of these regions includes a putative regulatory element for STC1 —a kidney-expressed gene involved in metabolism and the response to dehydration. We show that STC1 is induced by antidiuretic hormone in human cells, is associated with urea levels in the Turkana themselves, and is under strong and recent selection in this population as well as a second East African population, the Daasanach. This work highlights how integrating anthropological and genomic approaches can lead to a new understanding of human physiology with biomedical relevance.

Essentialism has been hypothesized to be associated with intergroup hostility. However, it is unclear whether ethnic essentialism is a feature of human social cognition across a range of socio-ecological settings and what the causal direction of its association with intergroup hostility is. One possibility is that essentialist beliefs cause intergroup violence because they make violence on members of out-groups more palatable. An alternative hypothesis is that essentialist beliefs may develop as a response to intergroup violence. Using “switched at birth” vignettes to assess how intergenerationally inherited and fixed people’s identities are, we assessed the extent to which Turkana pastoralist warriors have essentialist beliefs about themselves and the neighboring Toposa ethnic group in South Sudan. The Turkana and Toposa engage in mutually lethal cattle raids, which account for about half of adult Turkana male mortality in our study area. We found that only four of 162 (2.5%) Turkana surveyed expressed essentialist beliefs about themselves and the Toposa, suggesting that ethnic essentialism may not be a cross-culturally widespread feature of social cognition nor a necessary ideology for engaging in intergroup hostilities. The four participants who expressed essentialist beliefs did not exhibit unusual combat-related behavior and attitudes towards the outgroup. Specifically, they were no different than non-essentialist survey respondents in their levels of combat exposure, killing of enemies in combat, witnessing morally injurious events, and moral judgments about killing Toposa. However, one of the four was an outlier in offensive raiding and killing, anecdotally suggesting that essentialism may contribute to, but be insufficient for, very prolific raiding in individual Turkana. The cultural similarity between the Toposa and Turkana, a common origin story, well-known examples of cross-group adoption and assimilation, and a tradition of cross-border friendships may explain the Turkana’s low frequency of essentialist beliefs despite widespread lethal violence.