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Medieval church rules against marrying close relatives broke down strict, long-standing kinship structures, paving the way for a more individualistic modern Western society.

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How the early Christian church gave birth to today’s WEIRD Europeans

In September 506 C.E., the fathers of what would later become the Roman Catholic Church gathered in southern France to draw up dozens of new laws. Some forbade clergy from visiting unrelated women. Others forbade Christians from marrying anyone more closely related than their third cousin. The authors of a sweeping new study say that last, seemingly trivial prohibition may have given birth to Western civilization as we know it.

“If the authors are right, or even in the vicinity of being right, it couldn’t be bigger,” says Stephen Stich, a philosopher and cognitive scientist at Rutgers University in New Brunswick, New Jersey, who wasn’t involved in the work. “What they are offering to explain is the emergence of democratic institutions, of individualism in the West.”

The church’s early ban on incest and cousin marriage, the researchers say, weakened the tight kinship structures that had previously defined European populations, fostering new streaks of independence, nonconformity, and a willingness to work with strangers. And as the church’s influence spread, those qualities blossomed into a suite of psychological traits common today across Western industrialized nations, they argue.

“They’re looking at what created the modern Western world,” Stich says.

Before the Middle Ages, Europe was similar to other agrarian societies around the world: Extended kin networks were the glue that held everything together. Growing crops and protecting land required cooperation, and marrying cousins was an easy way to get it, explains Harvard University anthropologist and study co-author Joseph Henrich. Cousin marriages were even actively promoted in some societies because they kept wealth concentrated in powerful families.

But sometime around the sixth century C.E., the early church started to formulate strict marriage rules and become “obsessed” with incest, Henrich says. Historians aren’t sure why, although some religious thinkers of the time connected incest with the spread of the plague.

Lesser prohibitions against incest were already swirling around Europe when the church fathers formalized their marriage and family program. The new regulations prohibited people from marrying their first and second cousins and banned the practice of levirate marriage, in which a widow must marry her dead husband’s brother. “That part of a wedding where the officiant asks, ‘Does anybody here have any objections?’ goes back to the church asking, ‘Does anybody here know if these people are cousins?’” Henrich says.

Centuries living under these restrictions fundamentally reshaped European societies’ kinship structure—and their psychology, the authors say. Traditional kin networks stressed the moral value of obeying one’s elders, for example. But when the church forced people to marry outside this network, traditional values broke down, allowing new ones to pop up: individualism, nonconformity, and less bias toward one’s in-group.

“These are things that don’t come to mind when you think about the influence of the Catholic Church,” Stich says, which may explain why nobody had previously connected the church’s influence with the emergence Western psychology.

Working under the assumption that more time spent under church rule would ingrain those values more deeply, the researchers compared psychological and kinship traits of modern populations with the time their ancestors spent under Roman Catholic rule. The researchers built a vast database from historical records of church exposure in every nation on Earth, beginning in the first century and ending in 1500 C.E., when European society had become nearly fully Christianized.

Next, they consulted anthropological data to assign a kinship intensity score to each of the world’s major ethnolinguistic groups. This score was based on historical rates of cousin marriage, polygamy, and other factors. Finally, they drew on dozens of studies that used established psychological measures such as the World Values Survey to determine modern population-level scores for traits such as individualism, creativity, nonconformity, obedience, and ingroup/outgroup trust. (Two of the more unorthodox measures of obedience and outgroup trust, for example, were unpaid parking tickets issued to United Nations diplomats and participation in blood donation drives.)

Plotting these points together, they found that the longer a population spent under the rule of the Roman Catholic Church, the lower its kinship intensity score, meaning lower rates of cousin marriage and polygamy and looser familial and clan structures. And as kinship intensity drops off in their data, a certain suite of traits grows stronger, including individualism, nonconformity, and willingness to trust and help strangers, the researchers report today in Science.

This constellation of traits lines up with the dominant psychological profile of people living in Western, educated, industrialized, rich, and democratic countries—what Henrich and others have labeled WEIRD societies. The upshot, Henrich says, is that the church’s prohibitions against marrying close relatives weakened Europe’s traditional kinship networks and inadvertently replaced them with something very close to modern Western civilization’s cultural customs and norms. It also reinforces the notion that studying the behavior of people in WEIRD countries is not the same thing as studying human behavior more broadly, Henrich adds.

The traits identified in the study may also have paved the way for democratic governance. “You need a civic society to sustain democracy, and what we look at in our paper is, I believe, a precursor for such a civic society,” says co-author Jonathan Schulz, an economist at George Mason University in Fairfax, Virginia. “It’s individualistic people who work together and cooperate across family boundaries.”

Ara Norenzayan, a psychologist at the University of British Columbia in Vancouver, Canada, who collaborated with Henrich on studies of WEIRD societies but who wasn’t involved with this study, says the work is convincing. “When you start to erode family ties, you have to evolve to find other ways to keep your society structured,” he says. “It does make sense.”

Although the model is “incredible in its scope,” says Michele Gelfand, a cultural psychologist at the University of Maryland in College Park, it still hasn’t shown that the church actually caused the development of WEIRD psychology. Other institutions also helped shape the Western world, she says. “There are a lot of open questions.”

*Correction, 8 November, 11:15 a.m.: An earlier version of this article incorrectly stated that Michele Gelfand suggested educational systems as another potential causative factor for WEIRD societies.