Recent research not only confirms the existence of substantial psychological variation around the globe but also highlights the peculiarity of populations that are Western, Educated, Industrialized, Rich and Democratic (WEIRD). We propose that much of this variation arose as people psychologically adapted to differing kin-based institutions-the set of social norms governing descent, marriage, residence and related domains. We further propose that part of the variation in these institutions arose historically from the Catholic Church's marriage and family policies, which contributed to the dissolution of Europe's traditional kin-based institutions, leading eventually to the predominance of nuclear families and impersonal institutions. By combining data on 20 psychological outcomes with historical measures of both kinship and Church exposure, we find support for these ideas in a comprehensive array of analyses across countries, among European regions and between individuals with different cultural backgrounds. A growing body of research suggests that populations around the globe vary substantially along several important psychological dimensions, and that people from societies characterized as Western, Educated, Industrialized, Rich and Democratic (WEIRD) are particularly unusual (1-6). Often at the extremes of global distributions, people from WEIRD populations tend to be more individualistic, independent, analytically-minded and impersonally prosocial (e.g., trusting strangers) while revealing less conformity, obedience, in-group loyalty and nepotism (3, 5-13). While these patterns are now well documented, efforts to explain this variation from a cultural evolutionary and historical perspective have just begun (13-20). Here, we develop and test a cultural evolutionary theory that aims to explain a substantial portion of this psychological variation, both within and across nations. Not only does our approach contribute to explaining global variation and address why WEIRD societies so often occupy the tail ends of global distributions, but it also helps explain the psychological variation within Europe-among countries, across regions within countries and between individuals with different cultural backgrounds within the same country and region. Our approach integrates three insights. The first, drawing on anthropology, reveals that the institutions built around kinship and marriage vary greatly across societies (21-23) and that much of this variation developed as societies scaled up in size and complexity, especially after the origins of food production 12,000 years ago (22, 24-29). In forging the tightly-knit communities needed to defend agricultural fields and pastures, cultural evolution gradually wove together social norms governing marriage, post-marital residence and ingroup identity (descent), leading to a diversity of kin-based institutions, including the organizational forms known as clans, lineages and kindreds (21, 27, 30). The second insight, based on work in psychology, is that people's motivations, em...

Recent research not only confirms the existence of substantial psychological variation around the globe but also highlights the peculiarity of populations that are Western, Educated, Industrialized, Rich and Democratic (WEIRD). We propose that much of this variation arose as people psychologically adapted to differing kin-based institutions—the set of social norms governing descent, marriage, residence and related domains. We further propose that part of the variation in these institutions arose historically from the Catholic Church’s marriage and family policies, which contributed to the dissolution of Europe’s traditional kin-based institutions, leading eventually to the predominance of nuclear families and impersonal institutions. By combining data on 20 psychological outcomes with historical measures of both kinship and Church exposure, we find support for these ideas in a comprehensive array of analyses across countries, among European regions and between individuals with different cultural backgrounds.