Populations that are Western, Educated, Industrialized, Rich and Democratic (WEIRD) are peculiar due to the medieval church's set of rules governing descent, marriage, residence, etc., leading to the predominance of nuclear families and impersonal institutions

Schulz, Jonathan, Duman Barahmi-Rad, Jonathan Beauchamp, and Joseph Henrich. 2018. “The Origins of WEIRD Psychology.” PsyArXiv. June 22. doi:10.17605/OSF.IO/D6QHU. Final version: The Church, intensive kinship, and global psychological variation, Science, Nov 2019, https://science.sciencemag.org/content/366/6466/eaau5141

Abstract: 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.

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Structured Abstract

INTRODUCTION
A growing body of research suggests that populations around the globe vary substantially along several important psychological dimensions and that populations characterized as Western, Educated, Industrialized, Rich, and Democratic (WEIRD) are particularly unusual. People from these societies tend to be more individualistic, independent, and impersonally prosocial (e.g., trusting of strangers) while revealing less conformity and in-group loyalty. Although these patterns are now well documented, few efforts have sought to explain them. Here, we propose that the Western Church (i.e., the branch of Christianity that evolved into the Roman Catholic Church) transformed European kinship structures during the Middle Ages and that this transformation was a key factor behind a shift towards a WEIRDer psychology.

RATIONALE
Our approach integrates three insights. First, anthropological evidence suggests that diverse kin-based institutions—our species’s most fundamental institutions—have been the primary structure for organizing social life in most societies around the world and back into history. With the origins of agriculture, cultural evolution increasingly favored intensive kinship norms related to cousin marriage, clans, and co-residence that fostered social tightness, interdependence, and in-group cooperation. Second, psychological research reveals that people’s motivations, emotions, and perceptions are shaped by the social norms they encounter while growing up. Within intensive kin-based institutions, people’s psychological processes adapt to the collectivistic demands of their dense social networks. Intensive kinship norms reward greater conformity, obedience, and in-group loyalty while discouraging individualism, independence, and impersonal motivations for fairness and cooperation. Third, historical research suggests that the Western Church systematically undermined Europe’s intensive kin-based institutions during the Middle Ages (for example, by banning cousin marriage). The Church’s family policies meant that by 1500 CE, and likely centuries earlier in some regions, Europe lacked strong kin-based institutions and was instead dominated by relatively independent and isolated nuclear or stem families.

Our theory predicts that populations with (i) a longer historical exposure to the medieval Western Church or less intensive kin-based institutions will be more individualistic, less conforming, and more impersonally prosocial today; and (ii) longer historical exposure to the Western Church will be associated with less-intensive kin-based institutions.

RESULTS
We test these predictions at three levels. Globally, we show that countries with longer historical exposure to the medieval Western Church or less intensive kinship (e.g., lower rates of cousin marriage) are more individualistic and independent, less conforming and obedient, and more inclined toward trust and cooperation with strangers (see figure). Focusing on Europe, where we compare regions within countries, we show that longer exposure to the Western Church is associated with less intensive kinship, greater individualism, less conformity, and more fairness and trust toward strangers. Finally, comparing only the adult children of immigrants in European countries, we show that those whose parents come from countries or ethnic groups that historically experienced more centuries under the Western Church or had less intensive kinship tend to be more individualistic, less conforming, and more inclined toward fairness and trust with strangers.

CONCLUSION
This research suggests that contemporary psychological patterns, ranging from individualism and trust to conformity and analytical thinking, have been influenced by deep cultural evolutionary processes, including the Church’s peculiar incest taboos, family policies, and enduring kin-based institutions.

As predicted by our theory, countries with a longer exposure to the medieval Western Church have lower rates of cousin marriage (A); countries with lower rates of cousin marriage have a more individualistic and impersonally prosocial psychology (B); and countries with a longer exposure to the medieval Western Church have a more individualistic and impersonally prosocial psychology (C). Blue dots, green diamonds, and gray triangles denote countries primarily exposed to the Western Church, to the Eastern Church, and with no church exposure, respectively. ρˆ denotes Spearman correlation.


Abstract

Recent research not only confirms the existence of substantial psychological variation around the globe but also highlights the peculiarity of many Western populations. We propose that part of this variation can be traced back to the action and diffusion of the Western Church, the branch of Christianity that evolved into the Roman Catholic Church. Specifically, we propose that the Western Church’s transformation of European kinship, by promoting small, nuclear households, weak family ties, and residential mobility, fostered greater individualism, less conformity, and more impersonal prosociality. By combining data on 24 psychological outcomes with historical measures of both Church exposure and kinship, we find support for these ideas in a comprehensive array of analyses across countries, among European regions, and among individuals from 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 (1–3). Often occupying the extremes of global distributions, Western Europeans and their cultural descendants in North America and Australia tend to be more individualistic, independent, analytically minded, and impersonally prosocial (e.g., trusting of strangers) while revealing less conformity, obedience, in-group loyalty, and nepotism (3–12). Although these patterns are now well documented, efforts to explain this variation from a cultural-evolutionary and historical perspective have just begun (12–16). In this study, we develop and test a cultural evolutionary theory that aims to explain a substantial portion of this psychological variation, both within and across nations.

Our approach begins by considering how religions have evolved in ways that shape people’s institutions, social practices, economic outcomes, and psychology (17–22). Research in this area has, for example, documented the effect of Christian missions on both formal schooling and economic prosperity in places as diverse as Africa, China, and South America (23–26). Here, highlighting a less conspicuous channel, we go deeper into history and test the theory that the Western Catholic Church, primarily through its influence on marriage and family structures during the Middle Ages, had an important impact on psychological variation. Not only does our approach contribute to explaining why European and European-descent societies so often occupy the tail ends of global psychological distributions, it also helps explain variation within Europe—among countries, across regions within countries, and among individuals in the same country and region but with different cultural backgrounds.

To develop these ideas, our theory integrates three insights, drawing principally on anthropology, psychology, and history (27). First, anthropological research suggests that kin-based institutions represent the most fundamental of human institutions and have long been the primary framework for organizing social life in most societies (28–31). These institutions are composed of culturally transmitted norms that influence a broad range of social relationships by endowing individuals with sets of obligations and privileges with respect to their communities (supplementary text, section S1). Many kinship systems, for example, extend our species’s innate aversion to inbreeding (incest) to create taboos on marriage to more distant relatives, usually including particular types of cousins (32). By shaping patterns of marriage, residence, relatedness, and alliance formation, these norms organize interpersonal interactions and configure social networks in ways that profoundly influence social incentives and behavior (27, 33–35).

Although all premodern societies are organized primarily by kin-based institutions, evidence suggests that the character of these diverse institutions has been substantially influenced by ecological, climatic, and geographic factors (28, 30, 33, 34, 36–38). For instance, among mobile hunter-gatherers, cultural evolution has responded to ecological risk by favoring “extensive” kin ties, which create sprawling relational networks that can be tapped when local disasters strike (30, 37, 39). However, with the emergence of food production roughly 12,000 years ago, cultural evolution increasingly favored “intensive” kin-based institutions that permitted communities to unify larger groups to defend territories and organize production (30, 40–43). By constructing denser, tighter, and more interdependent social networks, these kin-based institutions intensified in-group loyalty, conformity, obedience to elders, and solidarity. For example, instead of favoring marriages to distant kin, cultural evolution often favored some form of cousin marriage, which tightened existing bonds among families (28). Cultural evolution thus led to a diversity of intensive kin-based institutions, including clans and kindreds (28, 32, 44), which dramatically restructured people’s social environments (27, 45, 46).

Our second insight, drawing on psychology and neuroscience, recognizes how aspects of our cognition, emotions, perceptions, thinking styles, and motivations adapt—often over ontogeny—to the normative demands, reputational incentives, and values of the interdependent social networks threaded together by kin-based institutions (3, 13, 27, 47–52). In particular, within intensive kin-based institutions, people’s psychological processes adapt to the collectivistic demands and the dense social networks in which they are enmeshed (53, 54). These institutions, thus, incentivize the cultivation of greater conformity, obedience, nepotism, deference to elders, holistic-relational awareness, and in-group loyalty but discourage individualism, independence, and analytical thinking (55). Because the sociality of intensive kinship is based on interpersonal embeddedness, adapting to these institutions reduces people’s inclinations toward impartiality, universal (nonrelational) moral principles, and impersonal trust, fairness, and cooperation; these institutions instead foster a contextually sensitive morality rooted in in-group loyalty.

Finally, drawing on historical research, our third insight incorporates the role of religion and its influence on kin-based institutions (27). By the start of the Common Era (CE), universalizing religions with powerful moralizing gods (or cosmic forces), universal ethical codes, and contingent afterlife beliefs had emerged across the Old World. However, these competing religions varied greatly in how their religious beliefs and practices shaped kin-based institutions (20, 56). In Persia, for example, Zoroastrians glorified the marriage of close relatives, including siblings, and encouraged widespread cousin marriage. Later, Islam curbed polygynous marriage (limiting a man to no more than four wives) but also adopted inheritance customs that promoted a nearly unique form of cousin marriage in which a daughter marries her father’s brother’s son—patrilineal clan endogamy (57–59). Beginning in Late Antiquity, the branch of Christianity that eventually evolved into the Roman Catholic Church—hereafter, the Western Church or simply the Church—systematically undermined Europe’s intensive kin-based institutions through a combination of religious prohibitions and prescriptions (46, 59–62). Prior to the Church’s efforts, the kin-based institutions of most European populations looked much like other agricultural societies and included patrilineal clans, kindreds, cousin marriage, polygyny, ancestor worship, and corporate ownership (27, 59, 60, 63–73). Meanwhile, although the branch of Christianity based in Constantinople that eventually evolved into the Orthodox Church—the Eastern Church—did adopt some of the same prohibitions as the Western Church, it never endorsed the Western Church’s broad taboos on cousin marriage, was slow to adopt many policies, and was unenthusiastic about enforcement.

The Western Church’s policies, which we call the Marriage and Family Program (MFP) (27), began with targeted bans on certain marriage practices used to sustain alliances between families (e.g., levirate marriage); however, by the Early Middle Ages, the Church had become obsessed with incest and began to expand the circle of forbidden relatives, eventually including not only distant cousins but also step-relatives, in-laws, and spiritual kin. Early in the second millennium, the ban was stretched to encompass sixth cousins, including all affines. At the same time, the Church promoted marriage “by choice” (no arranged marriages) and often required newly married couples to set up independent households (neolocal residence). The Church also forced an end to many lineages by eliminating legal adoption, remarriage, and all forms of polygamous marriage, as well as concubinage, which meant that many lineages began literally dying out due to a lack of legitimate heirs. As a result of the MFP, by 1500 CE (and centuries earlier in some regions), much of Europe was characterized by a virtually unique configuration of weak (nonintensive) kinship marked by monogamous nuclear households, bilateral descent, late marriage, and neolocal residence (59–62, 64, 74, 75).

Our theory, by synthesizing these insights, predicts that populations with a longer exposure to the medieval Western Church or less-intensive kin-based institutions will be less conforming but more individualistic and impersonally prosocial. At the same time, longer exposure to the Western Church should be associated with less intensive kin-based institutions. Of course, our theory does not preclude the existence of other important contributors to psychological variation, such as influences from the Church via channels other than kin-based institutions.

We emphasize that, in the absence of a decisive natural experiment in history, it is difficult to establish unassailable causal links between the Church’s MFP, kin-based institutions, and psychology (supplementary text S3). Our empirical approach has been to select both our psychological outcomes and explanatory variables ex ante on the basis of our theoretical predictions and then to repeatedly test for the expected relationships at different levels of analysis while controlling for an extensive battery of individual, regional, and historical covariates.