A 15th-century miniature of Basina of Thuringia (438 - 477) marrying Childeric I (437 - 481), a Frankish king. A new study has found that the Catholic church's edicts on marriage during the medieval period may have shaped the unique modern psychology found in the West.

People in the West are psychologically unlike the rest of the world. Global studies find that Western Europeans and their descendents tend to be more individualistic, less conformist, and more trusting of strangers.

But why? New research posits that the medieval Catholic Church , and its emphasis on monogamous marriage and the small family unit as the foundation of society, is responsible.

According to a study published today (Nov. 7) in the journal Science , countries and regions with a longer exposure to the Western Catholic Church are more likely to show the individualist, nonconformist psychology common to Western nations. The church may have inadvertently molded this psychology with medieval-era policies that ended cousin marriages and other tribe-like bonds, and created nuclear, monogamous households.

Related: 13 Facts on the History of Marriage

"Many decades of research has shown that the psychology of Westerners is different from the rest of the world in that it's more individualistic, analytic, and less conforming. However, until now, we didn't have a good explanation for how people in the West ended up having a psychology that was so unique," said Steven Heine, a professor of psychology at the University of British Columbia who was not involved in the current work. "This paper convincingly demonstrates that people's kinship networks are central to their psychology, and that the medieval Catholic Church instituted some policies regarding family structure that had far-reaching impact that continue to affect how people in the West think today, even if they aren't religious themselves."

The story of the new findings began in 2010, when anthropologist Joe Henrich of Harvard University, along with Heine and another colleague, published a study in the journal Behavioral and Brain Sciences pointing out that the vast majority of psychological research has been conducted on what they called "WEIRD" societies: Western, educated, industrialized, rich and democratic. Comparative research between WEIRD societies and non-WEIRD societies suggested that WEIRD research subjects were indeed weird — less conformist, more individualistic and more trusting of strangers than most of the rest of the world, to name a few differences.

"The findings suggest that members of WEIRD societies, including young children, are among the least representative populations one could find for generalizing about humans," Henrich and his colleagues wrote.

Naturally, these findings raised the questions of how WEIRD societies became so different from the rest of the world. Henrich pondered this question while studying kinship networks in Fiji (a non-WEIRD society) and while reading about the changes in family structure that occurred in Europe during the Middle Ages . He then learned that Jonathan Schulz, now an economist at George Mason University in Virginia, was working on a similar problem. Schulz had been conducting experiments on cooperation around the globe, and was beginning to suspect that how willing people are to cooperate is influenced by their family and kinship circles.

Henrich, Schulz and colleagues began to investigate a major driver of change in the kinship structure of Western nations: The medieval Catholic Church. The Western Catholic Church, starting in about A.D. 500, gradually began issuing edicts having to do with marriage and family. Cousin marriages were banned, along with polygamy , concubinage and many forms of interfamilial marriage that had traditionally strengthened ties within tribes and clans. In these arrangements, families were tied together by overlapping bonds of marriage and blood relationships. This led to what psychologists and anthropologists call “intensive kinship.” In intensive kinship societies, people tend to be highly loyal to their in-group and to distrust outsiders. They’re also more likely to value conformity, because survival in these societies means throwing one’s lot in with family and kin. In contrast, societies with less-intensive kinship require people to trust and cooperate with strangers for survival, and encourages individualism and noncomformity to the larger group. In these less-intensive societies, people marry outside of their blood relations and set up independent family lineages.

“What we know about kinship structure before the church entered the scene [in Europe], you see that it's not so much different from the rest of the world," Schulz told Live Science. People lived in tight clans, held together by close intermarriage. By about 1500, though, Europeans were largely living in monogamous nuclear households that were only weakly bound to other nuclear families.

The new study shows that these changes had psychological consequences. The researchers pulled together psychological data at a country level, at an individual level, and among second-generation immigrants who lived in one country but grew up influenced by the culture of another. They then calculated the length of time of exposure to the influence of the Western Catholic Church, both country-by-country as well as regionally within Europe. Exposure was measured by how many years the Western Church held sway in a region. For example, in A.D. 1054, when the Roman Catholic Church and the Eastern Orthodox Churches split, the Western Roman Catholic Church continued a comparatively more aggressive campaign of social engineering in Western Europe, but its edicts weren’t relevant in areas where the Eastern Churches had control.

The researchers found that there was a correlation between WEIRD psychology on a countrywide level and exposure to the Western Catholic Church. There wasn't a correlation between WEIRD psychology and the Eastern Church, which fits the hypothesis, the researchers wrote: The Eastern Church issued far fewer edicts involving marriage and family structure, and the analysis found that the length of time under the Western Church, but not the Eastern Church, was correlated to weaker kinship ties. The researchers also measured the intensity of kinship ties and found that the more intensive people's kinship networks, the less individualistic they were.

The researchers controlled for a number of factors that might have provided alternative explanations for the psychological shift, ranging from religiosity and strength of supernatural beliefs to the prosperity of a given region in medieval times. For instance, the researchers wondered if Roman institutions, rather than Catholic marriage policy , could be at the root of these shifts. But the research didn’t bear that out, Henrich told Live Science. The Eastern Roman Empire continued in the form of the Byzantine Empire until 1453. If Roman rule was the driver of kinship changes and psychological shifts, former Byzantine areas should have been the most affected by the new psychology. But they weren't.

The Catholic Church's connection explained differences in individualism not just country by country but also regionally within Europe. Regions that spent longer under the sway of the church show more individualism, less conformity and more trust and concern with fairness between strangers. The analysis of second-generation immigrants, born in Europe with parents who immigrated from elsewhere, also revealed the same links between exposure to the Catholic Church, kinship networks and psychology. Those whose mothers immigrated from places with more Catholic Church exposure and less intensive kinship were more individualistic, less conformist and more trusting than those whose mothers came from places less influenced by that Western Church and heavier in intense kinship ties.

It’s unclear how long it takes for people's psychology to change once their social environment does, Henrich said. The church's campaign on marriage and family took hundreds of years to enact. Typically, immigrants to a new nation take on their adopted culture's psychological profile in about three generations, Henrich said.

"We're hoping, in future projects, to try to pull data from written sources to see how psychology was changing," in Medieval Europe," he said.

Also unclear: Whether humanity is inadvertently doing anything today that might alter cultural psychology hundreds of years in the future. It's a tough question, Schulz said, but researchers are interested in the possible psychological effects of China's One-Child Policy . The One-Child Policy, which began in 1980 and persisted until 2015, prohibited most families in China from having more than one child, and changed family structures to be smaller and less sprawling. We don’t yet know what, if any, psychological consequences might result.

Catholic edicts on marriage aren't the whole story, but the findings suggest the importance of considering history in understanding psychology."Of course, there is also variation in kinship intensity around the world that doesn't stem from the Catholic Church," Schulz said.

Originally published on Live Science.