Richard McElreath

Costly punishment across human societies.

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.

Markets, religion, community size, and the evolution of fairness and punishment

A Fair Society Many of the social interactions of everyday life, especially those involving economic exchange, take place between individuals who are unrelated to each other and often do not know each other. Countless laboratory experiments have documented the propensity of subjects to behave fairly in these interactions and to punish those participants deemed to have behaved unfairly. Henrich et al. (p. 1480, see the Perspective by Hoff) measured fairness in thousands of individuals from 15 contemporary, small-scale societies to gain an understanding of the evolution of trustworthy exchange among human societies. Fairness was quantitated using three economic games. Various societal parameters, such as the extent to which food was purchased versus produced, were also collected. Institutions, as represented by markets, community size, and adherence to a world religion all predict a greater exercise of fairness in social exchange. The origins of modern social norms and behaviors may be found in the evolution of institutions. 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.

Shared norms and the evolution of ethnic markers

chone use of an “abundant” resource. Yukon Tourism Occasional Papers in Archaeology 3. o s w a l t , w e n d e l l h . 1963. Mission of change in Alaska. San Marino: Huntington Library. ———. 1990. Bashful no longer: An Alaskan Eskimo ethnohistory, 1778–1998. Norman: University of Oklahoma Press. o s w a l t , w e n d e l l h . , a n d j a m e s w. v a n s t o n e . 1967. The ethnoarchaeology of Crow Village, Alaska. Washington, D.C.: U.S. Government Printing Office. o w e n , l i n d a r . , a n d m a r t i n p o r r . Editors. 1999. Ethno-analogy and the reconstruction of prehistoric artefact use and production. Tübingen: Mo Vince Verlag. ro m a n o f f , s t e v e n . 1992. “Frasier River Lilloet salmon fishing,” in A complex culture of the British Columbia Plateau: Stl’atl’imx resource use. Edited by B. Hayden, pp. 222–65. Vancouver: University of British Columbia Press. s h a w, ro b e r t d . 1998. An archaeology of the central Yupik: A regional overview of the Yukon-Kuskokwim Delta, northern Bristol Bay, and Nunivak Island. Arctic Anthropology 35: 234–46. s t e n t o n , d o u g l a s r . 1991. The adaptive significance of caribou winter clothing for Arctic hunter-gatherers. Etudes/Inuit/Studies 15(1):3–28. t e s t a r t , a l a i n . 1982. The significance of food storage among hunter-gatherers: Residence patterns, population densities, and social inequalities. current anthropology 23: 532–37. t o r r e n c e , ro b i n . 1989. “Tools as optimal solutions,” in Time, energy, and stone tools. Edited by R. Torrence, pp. 1–6. Cambridge: Cambridge University Press. w h i t r i d g e , p e t e r . 2002. “Gender, households, and the material construction of social difference: Metal consumption at a Classic Thule whaling village,” in Many faces of gender: Roles and relationships through time in indigenous northern communities. Edited by L. Frink, R. S. Shepard, and G. A. Reinhardt, pp. 244–78. Boulder: University Press of Colorado. w o o d b u ry, a n t h o n y c . 1992. Cev’armiut qanemciit qulirait-llu: Eskimo narratives and tales from Chevak, Alaska. Anchorage: University of Alaska Press. w o l f e , ro b e r t j . 1984. Commercial fishing in the huntinggathering economy of a Yukon River Yup’ik society. Etudes/Inuit/Studies 8:159–84. ———. 1989a. Myths: What have you heard? Alaska Fish and Game 21(6):16–19. ———. 1989b. Tools: A crucial difference. Alaska Fish and Game 21(6):20–23. Shared Norms and the Evolution of Ethnic Markers

Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies

Origins of Egalitarianism Wealthy contemporary societies exhibit varying extents of economic inequality, with the Nordic countries being relatively egalitarian, whereas there is a much larger gap between top and bottom in the United States. Borgerhoff Mulder et al. (p. 682; see the Perspective by Acemoglu and Robinson) build a bare-bones model describing the intergenerational transmission of three different types of wealth—based on social networks, land and livestock, and physical and cognitive capacity—in four types of small-scale societies in which livelihoods depended primarily on hunting, herding, farming, or horticulture. Parameter estimates from a large-scale analysis of historical and ethnographic data were added to the model to reveal that the four types of societies display distinctive patterns of wealth transmission and that these patterns are associated with different extents of inequality. Some types of wealth are strongly inherited and, hence, contribute to long-term economic inequality. Small-scale human societies range from foraging bands with a strong egalitarian ethos to more economically stratified agrarian and pastoral societies. We explain this variation in inequality using a dynamic model in which a population’s long-run steady-state level of inequality depends on the extent to which its most important forms of wealth are transmitted within families across generations. We estimate the degree of intergenerational transmission of three different types of wealth (material, embodied, and relational), as well as the extent of wealth inequality in 21 historical and contemporary populations. We show that intergenerational transmission of wealth and wealth inequality are substantial among pastoral and small-scale agricultural societies (on a par with or even exceeding the most unequal modern industrial economies) but are limited among horticultural and foraging peoples (equivalent to the most egalitarian of modern industrial populations). Differences in the technology by which a people derive their livelihood and in the institutions and norms making up the economic system jointly contribute to this pattern.

Beyond existence and aiming outside the laboratory: estimating frequency-dependent and pay-off-biased social learning strategies

The existence of social learning has been confirmed in diverse taxa, from apes to guppies. In order to advance our understanding of the consequences of social transmission and evolution of behaviour, however, we require statistical tools that can distinguish among diverse social learning strategies. In this paper, we advance two main ideas. First, social learning is diverse, in the sense that individuals can take advantage of different kinds of information and combine them in different ways. Examining learning strategies for different information conditions illuminates the more detailed design of social learning. We construct and analyse an evolutionary model of diverse social learning heuristics, in order to generate predictions and illustrate the impact of design differences on an organisms fitness. Second, in order to eventually escape the laboratory and apply social learning models to natural behaviour, we require statistical methods that do not depend upon tight experimental control. Therefore, we examine strategic social learning in an experimental setting in which the social information itself is endogenous to the experimental group, as it is in natural settings. We develop statistical models for distinguishing among different strategic uses of social information. The experimental data strongly suggest that most participants employ a hierarchical strategy that uses both average observed pay-offs of options as well as frequency information, the same model predicted by our evolutionary analysis to dominate a wide range of conditions.

Ontogeny of prosocial behavior across diverse societies

Humans are an exceptionally cooperative species, but there is substantial variation in the extent of cooperation across societies. Understanding the sources of this variability may provide insights about the forces that sustain cooperation. We examined the ontogeny of prosocial behavior by studying 326 children 3–14 y of age and 120 adults from six societies (age distributions varied across societies). These six societies span a wide range of extant human variation in culture, geography, and subsistence strategies, including foragers, herders, horticulturalists, and urban dwellers across the Americas, Oceania, and Africa. When delivering benefits to others was personally costly, rates of prosocial behavior dropped across all six societies as children approached middle childhood and then rates of prosociality diverged as children tracked toward the behavior of adults in their own societies. When prosocial acts did not require personal sacrifice, prosocial responses increased steadily as children matured with little variation in behavior across societies. Our results are consistent with theories emphasizing the importance of acquired cultural norms in shaping costly forms of cooperation and creating cross-cultural diversity.

More ‘altruistic’ punishment in larger societies

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.

Culture rather than genes provides greater scope for the evolution of large-scale human prosociality

Whether competition among large groups played an important role in human social evolution is dependent on how variation, whether cultural or genetic, is maintained between groups. Comparisons between genetic and cultural differentiation between neighboring groups show how natural selection on large groups is more plausible on cultural rather than genetic variation.

The natural selection of bad science

Poor research design and data analysis encourage false-positive findings. Such poor methods persist despite perennial calls for improvement, suggesting that they result from something more than just misunderstanding. The persistence of poor methods results partly from incentives that favour them, leading to the natural selection of bad science. This dynamic requires no conscious strategizing—no deliberate cheating nor loafing—by scientists, only that publication is a principal factor for career advancement. Some normative methods of analysis have almost certainly been selected to further publication instead of discovery. In order to improve the culture of science, a shift must be made away from correcting misunderstandings and towards rewarding understanding. We support this argument with empirical evidence and computational modelling. We first present a 60-year meta-analysis of statistical power in the behavioural sciences and show that power has not improved despite repeated demonstrations of the necessity of increasing power. To demonstrate the logical consequences of structural incentives, we then present a dynamic model of scientific communities in which competing laboratories investigate novel or previously published hypotheses using culturally transmitted research methods. As in the real world, successful labs produce more ‘progeny,’ such that their methods are more often copied and their students are more likely to start labs of their own. Selection for high output leads to poorer methods and increasingly high false discovery rates. We additionally show that replication slows but does not stop the process of methodological deterioration. Improving the quality of research requires change at the institutional level.

When natural selection favors imitation of parents

It is commonly assumed that parents are important sources of socially learned behavior and beliefs. However, the empirical evidence that parents are cultural models is ambiguous, and debates continue over their importance. A formal theory that examines the evolution of psychological tendencies to imitate parents (vertical transmission) and to imitate nonparent adults (oblique transmission) in stochastic fluctuating environments points to forces that sometimes make vertical transmission adaptive, but oblique transmission recovers more quickly from rapid environmental change. These results suggest that neither mode of transmission should be expected to dominate the other across all domains. Vertical transmission may be preferred when (1) learned behavior affects fertility rather than survival to adulthood, (2) the relevant environment is stable, or (3) selection is strong. For those interested in the evolution of social learning in diverse taxa, these models provide predictions for use in comparative studies.