Our Roots Run Deep: The Psychological Significance of Perceived Shared Ancestry in Social Behavior – Main Study n°1

Humans are a uniquely cooperative species (Henrich, 2017; Henrich & Muthukrishna, 2021). Yet, their generosity is not distributed equally towards all their conspecifics. Rather, human cooperation is typically parochial: people tend to favor socially close individuals—family, friends, and community members—over strangers and outsiders (Cappelen et al., 2022; Enke, 2019; Hruschka et al., 2014; Hruschka & Henrich, 2013; Kirkland et al., 2023; Schulz et al., 2019; Tomasello, 2020; Waytz et al., 2019). Of course, parochialism is highly variable across individuals and can manifest in many different forms: humans have been found to discriminate on the basis of religion, nationality, language, and many other social categories (Cappelen et al., 2022; DeJesus et al., 2017; D. J. Hruschka & Henrich, 2013; Romano et al., 2017; Romano et al., 2021; Shaver et al., 2018). Despite this diversity, one sort of parochialism is especially widespread: favoritism on the basis of shared ancestry. Across many cultures, large human groups are defined and organized on the basis of their shared biological descent from the same ancestors. Many of the typical social units that structure the social organization of many populations are descent groups: lineages, clans, tribes or phratries (Gil-White, 2005). Likewise, the belief in shared ancestry is also one of the defining characteristics of ethnicity and nationality—whenever nationalism is expressed in ethnic terms (Gil-White, 2005; Smith, 2009). In most of these social units, people use their shared ancestry to justify the boundaries of their group and, most importantly, the high degree of cooperation that is expected within these boundaries (Wimmer, 2019; Horowitz, 1985; Smith, 2009; Van den Berghe, 1987). Nation-states, for instance, require high levels of commitment on the part of citizens—they must contribute taxes, serve in the military and commit to respect the political rights of other citizens—despite strong incentives to shirk these obligations (Hechter, 2000). As such, nationalist propaganda typically relies heavily on myths of shared ancestry to increase the consent of the population to these forms of large-scale cooperation (Bouchard, 2013). More dramatically, social divides based on shared ancestry can motivate ethnic clientelism, exclusionary racism, warfare, and even genocide (Coakley, 2012; Habyarimana et al., 2007; Hall, 1997; Zefferman & Mathew, 2015). The prevalence of shared ancestry as a criterion for parochial cooperation is often considered natural by virtue of its similarity with kinship—as manifested by the frequent use of kin terms like “brotherhood” or “motherland”—to refer to group members (Cronk et al., 2019). Yet, descent groups routinely number in the thousands or even in the millions, in the case of ethnic groups and nation-states. At such scales, the genetic relatedness between group members is so low that it should not significantly affect the distribution of prosocial behavior (Jones, 2018). Consequently, the cross-cultural significance of shared ancestry should puzzle social scientists. Why do humans so readily build communities on the basis of fictive kinship ties with unrelated strangers? Why would such a remote sense of biological descent unite entire ethnic groups and nation-states into a special moral contract? This research project seeks to understand why shared ancestry is such a powerful organizing principle in many human societies, both in traditional and modern industrial contexts. Our main hypothesis is that the psychological significance of shared ancestry for human social behavior is not a natural extension of kin altruism. Shared ancestry, we argue, is not perceived as a cue for genetic relatedness, but as a proxy indicator for information that is relevant for organizing collective action—like cultural homogeneity, efficient coordination norms, or the presence of efficient sanctioning institutions . Importantly, our framework can simultaneously explain why perceived shared ancestry delimits cooperative boundaries in many social contexts, and why it can be so readily dismissed in others. To test this hypothesis, we designed a vignette experiment to examine the interaction effects of shared ancestry with other information of high importance for cooperation on perceived group cohesion. The objective of our experimental design is to show that the effect of shared ancestry on perceived group cohesion can be readily dismissed when combined with information that is actually relevant for cooperation—focusing on cultural homogeneity in the present study. If shared ancestry is indeed nothing but an indirect cue for inferring group cohesion, then information about cultural homogeneity should make it significantly less relevant. The logic of our experiment follows that of Kurzban et al., 2001, who showed that skin color ceased to be encoded by the social cognition of participants when this information was combined with information that was more relevant for predicting coalitional affiliations (Kurzban et al., 2001). In brief, we predict that information about shared ancestry should have a significantly reduced effect on perceived group cohesion when it is incongruent with more relevant information, such as cultural homogeneity and cooperation efficiency.