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.

Fig. 1.

Distributions of PGG contributions across 16 villages. For each village on the y axis, the areas of the black bubbles represent the proportion of individuals from the village that made a contribution of the value on the x axis. To indicate scale, the numbers in some bubbles are the percentage proportions represented by those bubbles. Gray horizontal bars indicate the mean contributions for villages. Villages are ordered by their mean contributions; the bottom village (Semar Kona) has the lowest mean. Counts on the right (n) represent the number of players from each village (total n = 301). The overall mode across villages is 10 rupees (mean ± SD = 10.40 ± 5.48).

Fig. 2.

Distributions of salt deviations (amount available per person − amount taken) across 16 villages. For each village on the y axis, the areas of the black bubbles represent the proportion of individuals from the village with salt deviation of the value on the x axis. Note the break in the x axis. To indicate scale, the numbers in some bubbles are the percentage proportions represented by those bubbles. Gray horizontal bars indicate the mean salt deviations for villages. Villages are ordered by their mean salt deviations; the bottom village (Kharranagar) has the highest mean. The dashed line below the x axis marks whether a value of salt deviation indicates an “equal share taker” (salt taken = amount available per person), a “selfish” individual (salt taken > amount available per person), or a “generous” individual (salt taken < amount available per person). Counts on the left (n) represent the number of salt takers from each village (total n = 302). The overall mode across villages is 0 g (mean ± SD = −913.33 ± 2,619.02).