A sweeping theory published Thursday in the journal Science posits a new explanation for the divergent course of Western civilization from the rest of the world: The early Catholic Church reshaped family structures and, by doing so, changed human psychology forever after.

The researchers assert that they can trace all sorts of modern-day differences among cultures — from donating blood to strangers to paying parking tickets — to the influence of medieval Catholicism.

“The longer the duration under the church will predict greater individualism, less conformity and obedience, and more cooperation and trust with strangers. Our findings have big implications,” said Joseph Henrich, one of the researchers.

The research, conducted by George Mason University economists Jonathan Schulz and Jonathan Beauchamp and Harvard University evolutionary biologists Henrich and Duman Bahrami-Rad, tells a new story about how human cultures turned out so different from one another.

That 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 prohibiting marriage between relatives (which had been de rigueur), and eventually provoked a wholesale transformation of communities, changing the norm from large clans into small, monogamous nuclear families.

That cultural overhaul, the researchers argue, prompted tremendous changes to human psychology.

The team analyzed Vatican records to document the extent of a country’s 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.

“This is the only theory that I am aware of that attempts to explain broad patterns of human psychology on a global scale,” University of Pennsylvania associate professor of psychology Coren Apicella wrote in an email. Apicella, who was not associated with this research but has studied the evolution of religion, called the new paper “phenomenal.”

Marrying a cousin was common practice in the large, close-knit networks of kin that dominated societies before Catholicism, the researchers said, and remains normal in many parts of the world today. Bahmari-Rad, one of the researchers, said that he was raised in Iran, where 30 percent of marriages are to first or second cousins, and that he was surprised when he moved to the United States: “I thought it’s weird that Westerners don’t fall in love with their cousins.”

By contrast, the early Catholic Church was obsessed with preventing incest, even between distant relatives, Schulz said: “Thirteen out of 17 church councils in the 6th century were talking about incest and incest regulation.”

He argued that while some people, including white nationalists, tend to interpret any scientific study about concrete differences between cultures as evidence of Western superiority, this study should instead point to the randomness of differences between peoples. “There’s really nothing special, to start with, about Europe, except that the church creates this obsession,” Schulz said. “This could have happened with other places around the world. It’s just more or less coincidence this happened in Europe.”

Because their analysis examined a country’s exposure to the church before 1500, the researchers noted, they did not study the effects of the Protestant Reformation, which began in 1517.

As they studied places that were colonized by Christian nations in the years after 1500, they came up with a Catholicism-exposure score for them, based on the proportion of migrants to native population, and the length of exposure of the colonizing country before colonization. That led to some striking differences.

Much of Spain was under Muslim rule, not Christian, before 1492, so Spain’s Catholicism-exposure score is much lower than England’s — meaning Mexico, colonized by Spain, is much less influenced eventually than New England, colonized by Britain.

Scientists describe a select set of cultures as WEIRD: Western, educated, industrialized, rich and democratic. Researchers have known that these WEIRD countries — a concept introduced by Henrich and other researchers in 2010 — share a slate of odd psychological traits, including high levels of individualism and trust in strangers.

This paper claims to explain why.

The scientists say that the findings explain cultural differences on a strikingly precise level: “Just to give an example,” Schulz said, “we find that in the south of Italy where medieval church exposure was shorter, the rate of cousin marriage is also higher [in modern times] and voluntary blood donations are lower compared to the north of Italy.”

University of Chicago associate professor of behavioral science Thomas Talhelm, who was not involved in this work and who has previously tried to explain cultural differences in his own work by looking at which regions grew wheat and which regions grew rice, called the paper “interesting and important.”

“I think this is really trying to get at where human culture comes from,” Talhelm said. But he wondered whether preexisting differences between Europeans and others laid the groundwork for the transformative church to take root: “Where did the church come from then? What caused the church?”