Group-level ethnic composition influences altruistic punishment: public goods experiments among Gitanos and non-Gitanos in southern Spain

Humans often punish non-cooperators in one-shot interactions among genetically-unrelated individuals. This so-called altruistic punishment poses an evolutionary puzzle because it enforces cooperation norms that benefit the whole group, but is costly for the punisher. Under the “big mistake” (or “mismatch”) hypothesis, social behaviors such as punishment evolved by individual selection at a time when repeated interactions with kin prevailed, and modern humans “mistakenly” apply it in one-shot interactions with non-kin. In contrast, cultural group selection (CGS) theories emphasize cultural differences in normative behavior and the role of intergroup competition and punishment for the emergence of large-scale cooperation in the absence of genetic relatedness. We conducted a series of multilateral-cooperation economic experiments with a sample of Spanish Romani people (Gitanos or Calé), who represent a unique cultural group to analyze the nature of punishment: Gitano communities rely heavily on close kin-based networks, maintain high consanguinity rates, and display a particularly strong sense of ethnic identity. We observe that Gitano non-cooperators were not punished by co-ethnics in only-Gitano (ethnically) homogeneous groups but were harshly punished by both Gitano and non-Gitano males in mixed groups. Although largely consistent with CGS-related theories, these results can help better qualify some of their interpretations and predictions.