Empirical evidence for the spread of Cooperation through copying successful groups.

Large-scale cooperation among unrelated humans remains an evolutionary puzzle. Cultural group selection theory suggests that strong intergroup selection drives the evolution of cooperation, given substantial behavioral variation between groups. This study investigated payoff-biased imitation of successful out-groups as an intergroup selection process, specifically as an alternative to warfare-based mechanisms. We thus conducted two experiments manipulating exposure to cooperative out-group information in public goods games, demonstrating that cooperative behavior can indeed transmit across group boundaries, thereby influencing decision-making processes. However, this effect gradually diminished within groups, and transmitted cooperation was not likely to spread. Additionally, we performed a cross-cultural survey examining relationships between press freedom, which is a proxy for accessibility to out-group information, and different types of prosociality. The results revealed contrasting relationships, particularly in democratic countries: higher press freedom positively predicted impersonal prosociality (e.g., generalized trust) while negatively predicting personal prosociality (e.g., trust in family). These findings suggest that cultural processes such as payoff-biased imitation can facilitate the spread of cooperation beyond group boundaries, serving as a potential channel for intergroup selection. Finally, we discuss the implications for cultural group selection theory and additional mechanisms' potential role in sustaining within-group cooperation.