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.This is interesting but I can think of reasons to be skeptical. First, family structure was quite different between northern and southern Europe both before and after Christianization. Old English didn't even have a word for "cousin," which is why we use the French. Europeans depended on family networks for a long time after they became Christian; this is a major part of the reason why early medieval states were so weak. Also, the notion that this depends on the church goes against certain other observations, for example that people in cities are more individualistic than peasants.
“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.
Not to mention that the data we use to show that Europeans are psychologically different from others only go back about 50 years.
But I wonder if this might be a part of the equation.