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This is the only theory that I am aware of that attempts to explain broad patterns of human psychology on a global scale

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.