Human large-scale cooperation as a product of competition between cultural groups

Abstract

A fundamental puzzle of human evolution is how we evolved to cooperate with genetically unrelated strangers in transient interactions. Group-level selection on culturally differentiated populations is one proposed explanation. We evaluate a central untested prediction of Cultural Group Selection theory, by assessing whether readiness to cooperate between individuals from different groups corresponds to the degree of cultural similarity between those groups. We documented the normative beliefs and cooperative dispositions of 759 individuals spanning nine clans nested within four pastoral ethnic groups of Kenya-the Turkana, Samburu, Rendille and Borana. We find that cooperation between groups is predicted by how culturally similar they are, suggesting that norms of cooperation in these societies have evolved under the influence of group-level selection on cultural variation. Such selection acting over human evolutionary history may explain why we cooperate readily with unrelated and unfamiliar individuals, and why humans' unprecedented cooperative flexibility is nevertheless culturally parochial.

Fig. 1. Data collection sites.

GPS locations of the clans and territorial sections in the sample superimposed on Google Earth map of northern Kenya. TS, territorial section.

Fig. 2. Cultural differentiation between groups.

Top portion shows cultural F_ST values averaged across 49 traits between pairs of clans within ethnic groups (blue), pairs of territorial sections within Turkana (green) and pairs of ethnic groups (brown) in our study. Estimates are based on a sample of 759 individuals as shown in Table 1. Bottom portion shows cultural F_ST values (gray) and genetic F_ST values (yellow) from the literature. Source data are provided as a Source Data file.

Fig. 3. Predictors of cooperation rates between groups.

Logistic regression model of the effect of cultural F_ST and geographic distance between actor and target’s group, on whether subject endorses the cooperative act. a Log-odds estimates with 95% CI (***p < .001). b Predicted probabilities of subjects endorsing cooperative act conditioned on cultural F_ST value and geographic distance between actor and target’s group. c Average rate of endorsement of cooperative act by vignette condition for each subpopulation. For the Turkana (Tur) there are a minimum of 12 subjects in each vignette condition per clan per territorial section. For the Rendille, Samburu and Borana there are a minimum of 17 subjects per clan in each vignette condition. Source data are provided as a Source Data file.