Demography and ecology drive variation in cooperation across human populations

Abstract

Recent studies argue that cross-cultural variation in human cooperation supports cultural group selection models of the evolution of large-scale cooperation. However, these studies confound cultural and environmental differences between populations by predominantly sampling one population per society. Here, we test the hypothesis that behavioral variation between populations is driven by environmental differences in demography and ecology. We use a public goods game played with money and a naturalistic measure of behavior involving the distribution of salt, an essential and locally valued resource, to demonstrate significant variation in levels of cooperation across 16 discrete populations of the same small-scale society, the Pahari Korwa of central India. Variation between these populations of the same cultural group is comparable to that found between different cultural groups in previous studies. Demographic factors partly explain this variation; age and a measure of social network size are associated with contributions in the public goods game, while population size and the number of adult sisters residing in the population are associated with decisions regarding salt. That behavioral variation is at least partly contingent on environmental differences between populations questions the existence of stable norms of cooperation. Hence, our findings call for reinterpretation of cross-cultural data on cooperation. Although cultural group selection could theoretically explain the evolution of large-scale cooperation, our results make clear that existing cross-cultural data cannot be taken as empirical support for this hypothesis.evolution of cooperation | cultural norms | common-pool resource | real-world measure | economic game S everal recent cross-cultural studies in small-scale (1-3) and large-scale (4-6) societies demonstrate variation in patterns of cooperation across cultural groups. This behavioral variation is attributed to culturally inherited cooperative norms and taken as support for cultural group selection models of large-scale cooperation (1, 2, 7). However, these studies have mostly sampled from one population per culture. Thus, they confound cultural and environmental differences between populations and cannot determine whether the behavioral variation across populations is driven by conformism to cultural norms or by environmental (demographic and ecological) differences. Crucially, the evolution of large-scale cooperation via cultural group selection (7-13) depends on behavior being acquired via cultural transmission, such that behavioral variation between populations is maintained by conformism to group norms.We examine whether there are differences in levels of cooperation across discrete populations of the same endogamous cultural group, and we find that environmental drivers (local ecology and demography) are responsible for behavioral variation across our study populations. Moreover, variation between these populations of the same cultural group is comparable to that found between differen...

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