Substantial variation exists in the psychological beliefs and behaviors of populations across the globe. In particular, the proclivities of individuals in western, industrialized countries are unique. Previous research has shown that these societies, more recently characterized as Western, Educated, Industrialized, Rich and Democratic - or "WEIRD" - tend to be more individualistic, analytically oriented and trustful of others whilst demonstrating less conformity, obedience and solidarity.
Whether the driver of these traits is formal political institutions, for example, or something else, has been unclear. Jonathan Schultz and colleagues hypothesized that the Western Catholic Church's marriage and family program dissolved strong, cohesive kin networks, which then impacted psychology. To test this, they combined anthropological, historical and psychological data. For example, analyzing records kept by the Vatican, which informed the rate of cousin marriages, helped them to evaluate kinship intensity. To capture human psychology, they drew on a very broad set of data, including survey data, behavioral data, and ecologically relevant observational data such as voluntary blood donation.
Schulz et al.'s analysis points to the expanding Church's religious decrees on marriage systematically replacing extended kin-based family networks with smaller, more independent nuclear households with weak family ties. To rule out alternative hypotheses that could explain their results, they controlled for variables including geographic factors, income, wealth, and education.