Abstract:Researchers from across the social sciences have found consistent deviations from the predictions of the canonical model of self-interest in hundreds of experiments from around the world. This research, however, cannot determine whether the uniformity results from universal patterns of human behavior or from the limited cultural variation available among the university students used in virtually all prior experimental work. To address this, we undertook a cross-cultural study of behavior in ultimatum, public goods, and dictator games in a range of small-scale societies exhibiting a wide variety of economic and cultural conditions. We found, first, that the canonical model -based on self-interest -fails in all of the societies studied. Second, our data reveal substantially more behavioral variability across social groups than has been found in previous research. Third, group-level differences in economic organization and the structure of social interactions explain a substantial portion of the behavioral variation across societies: the higher the degree of market integration and the higher the payoffs to cooperation in everyday life, the greater the level of prosociality expressed in experimental games. Fourth, the available individual-level economic and demographic variables do not consistently explain game behavior, either within or across groups. Fifth, in many cases experimental play appears to reflect the common interactional patterns of everyday life.

Recent behavioral experiments aimed at understanding the evolutionary foundations of human cooperation have suggested that a willingness to engage in costly punishment, even in one-shot situations, may be part of human psychology and a key element in understanding our sociality. However, because most experiments have been confined to students in industrialized societies, generalizations of these insights to the species have necessarily been tentative. Here, experimental results from 15 diverse populations show that (i) all populations demonstrate some willingness to administer costly punishment as unequal behavior increases, (ii) the magnitude of this punishment varies substantially across populations, and (iii) costly punishment positively covaries with altruistic behavior across populations. These findings are consistent with models of the gene-culture coevolution of human altruism and further sharpen what any theory of human cooperation needs to explain.

Large-scale societies in which strangers regularly engage in mutually beneficial transactions are puzzling. The evolutionary mechanisms associated with kinship and reciprocity, which underpin much of primate sociality, do not readily extend to large unrelated groups. Theory suggests that the evolution of such societies may have required norms and institutions that sustain fairness in ephemeral exchanges. If that is true, then engagement in larger-scale institutions, such as markets and world religions, should be associated with greater fairness, and larger communities should punish unfairness more. Using three behavioral experiments administered across 15 diverse populations, we show that market integration (measured as the percentage of purchased calories) positively covaries with fairness while community size positively covaries with punishment. Participation in a world religion is associated with fairness, although not across all measures. These results suggest that modern prosociality is not solely the product of an innate psychology, but also reflects norms and institutions that have emerged over the course of human history.

The application of evolutionary theory to understanding the origins of our species' capacities for social learning has generated key insights into cultural evolution. By focusing on how our psychology has evolved to adaptively extract beliefs and practices by observing others, theorists have hypothesized how social learning can, over generations, give rise to culturally evolved adaptations. While much field research documents the subtle ways in which culturally transmitted beliefs and practices adapt people to their local environments, and much experimental work reveals the predicted patterns of social learning, little research connects real-world adaptive cultural traits to the patterns of transmission predicted by these theories. Addressing this gap, we show how food taboos for pregnant and lactating women in Fiji selectively target the most toxic marine species, effectively reducing a woman's chances of fish poisoning by 30 per cent during pregnancy and 60 per cent during breastfeeding. We further analyse how these taboos are transmitted, showing support for cultural evolutionary models that combine familial transmission with selective learning from locally prestigious individuals. In addition, we explore how particular aspects of human cognitive processes increase the frequency of some non-adaptive taboos. This case demonstrates how evolutionary theory can be deployed to explain both adaptive and non-adaptive behavioural patterns.

If individuals will cooperate with cooperators, and punish non-cooperators even at a cost to themselves, then this strong reciprocity could minimize the cheating that undermines cooperation. Based upon numerous economic experiments, some have proposed that human cooperation is explained by strong reciprocity and norm enforcement. Second-party punishment is when you punish someone who defected on you; third-party punishment is when you punish someone who defected on someone else. Third-party punishment is an effective way to enforce the norms of strong reciprocity and promote cooperation. Here we present new results that expand on a previous report from a large cross-cultural project. This project has already shown that there is considerable cross-cultural variation in punishment and cooperation. Here we test the hypothesis that population size (and complexity) predicts the level of third-party punishment. Our results show that people in larger, more complex societies engage in significantly more third-party punishment than people in small-scale societies.

Synthesizing existing work from diverse disciplines, this paper introduces a culture-gene coevolutionary approach to human behavior and psychology, and applies it to the evolution of cooperation. After a general discussion of cooperation in humans, this paper summarizes Dual Inheritance Theory and shows how cultural transmission can be brought under the Darwinian umbrella in order to analyze how culture and genes coevolve and jointly influence human behavior and psychology. We then present a generally applicable mathematical characterization of the problem of cooperation. From a Dual Inheritance perspective, we review and discuss work on kinship, reciprocity, reputation, social norms, and ethnicity, and their application to solving the problem of cooperation.

No abstract

Lamba and Mace's critique (1) of our research (2-4) is based on incorrect claims about our experiments and several misunderstandings of the theory underpinning our efforts. Their findings are consistent with our previous work and lead to no unique conclusions.Lambda and Mace (1) incorrectly claimed that we "mostly" sampled from single communities within sites, and that we ignored "ecological" and "demographic" variables. In fact, much of our work focused on studying the variation among communities within sites. In phase I (2), 8 of 15 sites involved multiple communities, and in phase II (3, 4), 11 of our 16 sites sampled from multiple communities. Several sites included 5 communities*, and 1 site included 9 communities (5).During both phases, we conducted analyses like Lamba and Mace's (1) within each of our sites (2-4). Except for community size, which emerged as a focal predictor (4), we studied versions of all the key variables (age, network centrality, and siblings) of Lamba and Mace (1) in some populations. Occasionally, these were significant, but none had consistent effects across populations, games, or in retests years later. Lamba and Mace (1) also failed to find any predictors that were consistent across their experiments, despite trying dozens of variables. Moreover, their predictors are neither theoretically well-grounded nor particularly ecological. Nonetheless, Lamba and Mace (1) concluded that "ecology drives variation in cooperation."Our analyses show that the key claim of Lamba and Mace (1), that the fraction of variation they observed for monetary games (4%) among communities is comparable to the fraction we observe among our sites (12% for ultimatum game offers in phase I) is not correct, even allowing for their assumption that 12 ≈ 4. (1) is consistent with our data, but their data tell us nothing about the variation across sites (contrary to their claim).Our theoretical framework is also not made clear, because Lamba and Mace (1) claimed that our approach holds that only norms matter, and that norms cannot evolve in response to ecological variation. They further imply that if demographic or ecological variation is important, norms cannot be. Our research is based on the idea that people can acquire context-specific expectations (e.g., "one wife per man") and internalized motivations (e.g., "extramarital sex is wrong") as a consequence of cultural learning. These expectations and motivations then influence their decision making along with other factors, including evolved motivations linked to self-interest and genetic relatedness (2). Thus, as conditions vary, so too does behavior, even if people share such norms.*