National parochialism is ubiquitous across 42 nations around the world

Human cooperation is remarkable, enabling the creation of public goods among unrelated individuals. However, parochialism—favoring ingroup members—limits cooperation across groups. National parochialism, specifically, poses a significant challenge to global public goods provision, such as pandemic control, environmental conservation, and international collaborations. While its prevalence is hypothesized, the extent of national parochialism's variation across nations and its determinants remain unclear. Evolutionary theories suggest parochialism stems from expectations of ingroup cooperation, reputational concerns, and indirect benefits. Alternatively, cultural evolution posits that norms and institutions shape cooperation, potentially influencing national parochialism's variation through factors like rule of law, government effectiveness, and the influence of world religions. Prior research, often limited to self-report measures or WEIRD (Western, Educated, Industrialized, Rich, and Democratic) nations, provides an incomplete picture. This study aims to address these gaps by conducting a large-scale experiment across 42 nations, encompassing a significant portion of the global population. The study will examine the ubiquity and variability of national parochialism and test hypotheses about its cultural and institutional determinants.

Existing literature on parochialism offers competing explanations. Evolutionary perspectives highlight the role of reputation and indirect reciprocity within groups, predicting widespread ingroup favoritism. In contrast, cultural evolutionary theories emphasize the influence of norms and institutions, suggesting greater variation in parochialism across societies. Institutions such as strong rule of law or effective governance might promote cooperation beyond national borders, reducing national parochialism. Similarly, the spread of world religions, with their emphasis on broader moral frameworks, could mitigate ingroup biases. Empirical evidence has been limited, often restricted to specific regions or relying on self-reported measures. This study builds upon prior research by using a large-scale experimental design across a diverse range of nations, thereby offering a more comprehensive and robust test of these competing hypotheses.

This study involved an online experiment conducted in December 2018 with 18,411 participants representing 42 nations, stratified by age, gender, and income. Participants completed a prisoner's dilemma game, making 12 independent decisions with different partners (ingroup, outgroup, or stranger) without feedback. The value of monetary units (MUs) used in the game was standardized across nations based on the average hourly wage. The experiment included two blocks of six decisions each, varying whether choices were public or private. In the public block, participants used pseudonyms, and their choices were visible online; in the private block, choices remained anonymous. This manipulation tested whether the possibility of indirect reciprocity influences national parochialism. Three nations (Brazil, India, and Poland) had a subset of participants randomly assigned to either incentivized or hypothetical treatments, testing the robustness of findings to the nature of incentives. Participants also completed measures of relational mobility. National-level data on institutional quality, cultural values, ecological factors, and historical influences were collected from various publicly available sources. Mixed-effects models were employed to analyze the data, incorporating random effects for participants nested within nations. Meta-analysis was used to synthesize findings across nations.

The study revealed that national parochialism—greater cooperation with ingroup members—is a pervasive phenomenon, present in 39 out of 42 nations. The remaining three nations showed non-significant but still positive levels of national parochialism. The degree of national parochialism showed surprisingly little variation across nations, despite differences in wealth, political stability, and other factors. Cultural distance between nations did not predict differences in national parochialism. National parochialism occurred similarly in public and private conditions, contradicting the hypothesis that it serves primarily as a reputational strategy. While cooperation was generally higher in public situations, this effect was independent of partner nationality. In contrast to the relatively consistent levels of national parochialism, impersonal cooperation (cooperation with strangers) showed greater cross-national variation. This variation in impersonal cooperation was significantly associated with several ecological, social, and institutional factors, including historical exposure to Western churches, low pathogen stress, egalitarian values, and fluid social relations. Individual-level analyses showed that the relationship between national parochialism and cooperation was stronger for men than women, and weaker for individuals with higher education levels.

The findings challenge hypotheses predicting substantial cross-national variation in national parochialism. The ubiquity of national parochialism aligns with theories emphasizing the pervasive nature of ingroup favoritism. The lack of significant differences between public and private conditions suggests that reputational concerns alone do not fully explain this phenomenon. The greater variation observed in impersonal cooperation, compared to national parochialism, points to the distinct nature of these two types of cooperation. Ecological, social, and institutional factors seem to significantly influence impersonal cooperation, providing support for cultural evolutionary theories. The observed inconsistencies with some previous findings from less industrialized societies warrant further investigation into potential methodological differences and the role of specific experimental tasks.

This large-scale cross-national study demonstrates the widespread and relatively consistent presence of national parochialism across diverse societies. The findings highlight the importance of considering the distinct mechanisms underlying ingroup favoritism and impersonal cooperation. Future research should explore the interplay between these two forms of cooperation and the potential for interventions that promote cooperation beyond national boundaries. Further research is needed to reconcile the current findings with previous research in less industrialized societies, particularly by examining methodological differences in the design and tasks used to assess ingroup favoritism.

The study's use of a within-subjects design and online data collection might have introduced demand characteristics, although incentive manipulation did not alter the main findings. The prisoner's dilemma paradigm might not fully capture motivations to harm outgroups. The observed effect sizes, while statistically significant, were relatively small to medium; however, even small effects can have substantial cumulative societal impacts.