The story begins with kinship networks — the tribes and clans of densely connected, insular groups of relatives who formed most human societies before medieval times. Catholic Church teachings disrupted those networks, in large part by vehemently prohibiting marriage between relatives (which had been de rigeur), and eventually provoked a wholesale transformation of communities, changing the norm from large clans into small, monogamous nuclear families.
The team analyzed Vatican records to document the extent of a country or region’s exposure to Catholicism before the year 1500, and found that longer exposure to Catholicism correlated with low measures of kinship intensity in the modern era, including low rates of cousins marrying each other. Both measures correlated with psychology, the researchers found by looking at 24 different psychological traits of people in different cultures: Countries exposed to Catholicism early have citizens today who exhibit qualities such as being more individualistic and independent, and being more trusting of strangers.From Gelfand:
...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 (Western, Educated, Industrialized, Rich and Democratic) psychological shifts, even when controlling for alternate explanations (e.g., the influence of Roman political institutions, schooling, migration).