One of the biggest puzzles facing the social sciences is understanding our immense cultural variation. Over the past several thousand years, humanity has evolved to the point where there now exist 195 countries, more than 7000 languages, and thousands of religions. Research has begun to describe psychological variation across the globe (1–4), yet only recently have we begun to understand ecological, historical, and sociopolitical factors that produce such differences. Often absent from this mix is how religion and psychological variation are interrelated (5, 6). On page 707 of this issue, Schulz et al. (7) break new ground in showing how the specific practices of a branch of one of the world’s largest religions—Christianity—can in part explain widespread variation in human psychology around the world.
Schulz et al. present an intriguing thesis: The Western Catholic Church’s Marriage and Family Program (MFP), launched during the Middle Ages (in 506 CE), can partially explain the distinctively individualistic and nonconformist psychology of Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies in modern times. The MFP radically altered the institution of marriage by prohibiting nuptials within extended families and often requiring newly married couples to set up independent households. Schulz et al. predict that longer exposure to the MFP, along with weaker kinship ties that presumably arose from such practices, would drastically alter human psychology, from one that emphasized in-group loyalty, obedience, and conformity, to one that was more individualistic, prosocial toward strangers, and less conforming.
The authors compiled an enormous amount of data on psychological variation in the modern era, along with historical data on the MFP and kinship intensity, to examine their hypothesis across nations around the world, within regions in Europe, and among second-generation immigrants of various backgrounds living in Europe. At the national level, exposure was defined as the number of centuries the population was under the control of the Western Church prior to 1500 CE (when the MFP was at its most intense). Exposure at the regional level was quantified as the number of bishoprics—administrative centers in the Church’s hierarchy—across 440 regions of Europe.
The intensity of kin-based institutions was defined using two measures: a Kinship Intensity Index, which averaged five key variables from the Ethnographic Atlas (e.g., cousin marriage preference, polygynous marriage, co-residence of extended families), and a measure of the prevalence of cousin marriage in the 20th century at the country and regional levels. Psychological variation—the main outcome of interest—was captured by previously collected data on 24 variables that the authors grouped into three categories: individualism and independence, conformity and obedience, and impersonal prosociality. These captured not only self-reported behaviors and preferences, but also behavioral data such as that from measures of creativity, public goods games, and even unpaid New York City parking tickets issued to diplomats to the United Nations.
The results are striking. Cross-nationally, the authors found that both longer exposure to the Western Church and weaker kinship intensity (which were negatively related, as expected) were associated with greater individualism and independence, less conformity and obedience, and greater prosociality toward strangers—relationships that mostly held when controlling for a range of geographic variables. The results were replicated across 440 regions in 36 European countries: Longer exposure to the Western Church was generally associated with the same WEIRD psychological shifts, even when controlling for alternate explanations (e.g., the influence of Roman political institutions, schooling, migration).
To further test their hypothesis, Schulz et al. compared second-generation immigrants who grew up in the same European country but whose parents originated from different countries, effectively allowing the authors to isolate the effect of intergenerational transmission. Second-generation immigrants whose parents had more Western Church exposure and weaker kinship intensity evidenced more WEIRD psychology.
One major contribution of this research is that it expands the way we think about religion in a cultural evolutionary framework. Other research has shown that religion can affect many aspects of behavior, from prosociality (8, 9), to inequality (10), to intergroup conflict (11). For example, belief in a moralizing supernatural agent has been linked to cooperation (6) and cultural complexity (8, 12). Schulz et al. build on this psychological account by examining structural aspects of religion that go beyond belief to capture the top-down influence of Church practices on human psychology. Further, their paper focuses on characteristics that are specific to one of the largest religions in the world: Western Christianity. Although multiple religions have moralizing supernatural agents (6), the authors argue that Western Christianity is unique in its emphasis on specific marriage practices and kinship structures that have shaped psychological profiles lasting for generations. Schulz et al.‘s massive historical and geographic effort simultaneously expands upon the culture and religion literature and the cultural psychology literature on the origin of WEIRD psychology.
A couple being married by a clergyman, by Henricus von Assia (13th century). Marriage practices of the Church appear to influence modern psychology.
IAMGE: PHAS/UNIVERSAL IMAGES GROUP VIA GETTY IMAGES
To be sure, numerous critical questions await future investigation. The central question of interest—the impact of the Church on later psychological processes—requires longitudinal designs in order to provide a truly convincing causal account. Moreover, although exploratory mediational analyses tested the extent to which the Western Church influenced psychology via its impact on kinship intensity, many tests were subject to low power and do not provide precise estimates. The mechanisms through which Church exposure influences psychological variation remain an important open question. Empirical analyses are also needed to document the forces that led the Church to adapt such practices in the first place. It is possible, for example, that MFP practices were embraced in communities that were already evolving WEIRD social and psychological characteristics for other reasons. And although the Church’s practices were clearly related to variance in WEIRD psychological variables, to provide a full cultural evolution account of global psychological variation, it will be necessary to understand through causal modeling the impact of the Church along with other ecological and historical factors shown to be related to WEIRD psychological variation (4, 13).
Understanding human diversity is not only critical for the advancement of science, but also for bridging cultural divides. Illuminating the ways in which cultures vary—and why they have evolved in different ways given certain socioenvironmental forces—can help us to empathize with those who are different. By documenting how a very specific religious agenda in late antiquity may have had far-reaching effects on the development of cultural differences between the West and the rest of the world, Schulz et al. help us to decipher part of the puzzle of human diversity.
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