Why the church grew obsessed with incest is still unknown. Co-author Jonathan Beauchamp, assistant professor of economics at George Mason University, suggests that one possible reason may have been material gain. Religious leaders could benefit financially from shrinking family ties — without a tight extended network those without heirs often left their wealth to the church. Whatever the reasons, one thing seems clear: The Western Church’s crusade coincides with a significant loosening in Europe’s kin-based institutions.

Comparing exposure to the Western Church with their “kinship intensity index,” which includes data on cousin marriage rates, polygyny (where a man takes multiple wives), co-residence of extended families, and other historical anthropological measures, the team identified a direct connection between the religious ban and the growth of independent, monogamous marriages among nonrelatives. According to the study, each additional 500 years under the Western Church is associated with a 91 percent further reduction in marriage rates between cousins.

 

“Meanwhile in Iran, in Persia, Zoroastrianism was not only promoting cousin marriage but promoting marriage between siblings,” Henrich said. Although Islam outlawed polygyny extending beyond four wives, and the Eastern Orthodox Church adopted policies against incest, no institution came close to the strict, widespread policies of the Western Church.

Those policies first altered family structures and then the psychologies of members. Henrich and his colleagues think that individuals adapt cognition, emotions, perceptions, thinking styles, and motivations to fit their social networks. Kin-based institutions reward conformity, tradition, nepotism, and obedience to authority, traits that help protect assets — such as farms — from outsiders. But once familial barriers crumble, the team predicted that individualistic traits like independence, creativity, cooperation, and fairness with strangers would increase.

Using 24 psychological variables collected in surveys, experiments, and observations, they measured the global prevalence of traits that correspond or conflict with individualism. To test for willingness to help strangers, for example, they collected data on blood-donation rates across Italy, finding a correlation between high donation rates and low cousin-marriage rates. With their kinship intensity index, Schutz said, they can also predict which diplomats in New York City will or will not pay parking tickets: Those from countries with higher rates of cousin marriages are more likely to get a ticket and less likely to pay one.

And, although willingness to trust strangers, as opposed to family or neighbors, is associated with higher levels of innovation, greater national wealth, and faster economic growth, which factor causes which is not yet known.

“We’re not saying that less-intensive kin-based institutions are better,” said Beauchamp. “Far from it. There are trade-offs.” Tight families, for example, come with inborn financial safety nets.