Medieval church regulations against cousin marriage and modern-day democracy in Europe

I was really interested in this job market paper by Jonathan Schulz (Harvard), the abstract of which ends with this sentence:

Twentieth-century cousin marriage rates explain more than 50 percent of variation in democracy across countries today.

That seems like an extraordinary claim. Do differences in cousin marriage rates really explain differences in democracy?

Essentially, the paper tests two hypotheses about kin-based networks:

First, anthropologist Jack Goody (1983) hypothesized that, motivated by financial gains, the medieval Catholic Church implemented marriage policies—most prominently, prohibitions on cousin marriage—that destroyed the existing European clan-based kin networks. This created an almost unique European family system where, still today, the nuclear family dominates and marriage among blood relatives is virtually absent. This contrasts with many parts of the world, where first- and second-cousin marriages are common... Second, several scholars have hypothesized that strong extended kin networks are detrimental to the formation of social cohesion and affect institutional outcomes...

Schulz tests these hypotheses in several steps. First, he establishes that the Church's Medieval prohibitions against cousin marriages explains the formation of communes (self-governing democratic towns or cities) before 1500 C.E. Using data on exposure to the Church (that is, how long a city was within 100 kilometres of a bishopric), he finds that:

...cities that experienced longer Catholic Church exposure were more likely to adopt inclusive institutions and become communes.

This is supported by analysis that shows that, when cousin marriage prohibitions were extended from second to sixth cousins, cities more exposed to the Church were even more likely to become communes.

In the next step, Schulz shows that exposure to the Church weakens kin-based social networks. To proxy for these networks, he uses cousin marriage rates (since marriage between cousins is an indicator of stronger kin-based networks), and differences in 'cousin terms' in different languages. In the case of the latter:

...in some societies the children of one’s parents’ same-sex siblings are called brothers and sisters — an indication of an incest taboo. Yet, the differently called children of one’s parents’ opposite-sex siblings are often preferred marriage partners. Cousin terms reflect historically distant cousin marriage practices...

Again making use of his measure of Church exposure, he finds that:

Western Church exposure of 100 years longer is associated with a decrease in the percentage of individuals speaking a language that differentiates cousin terms by about 7 to 9 percentage points. Similarly, Western Church exposure reduces the preference for cousin marriage by 0.05 points... and cousin marriages by 38%...

Exposure to the Eastern Church had somewhat lesser effects, although they were still statistically significant. In the third step, Schulz shows that:

...longer Church exposure and low cousin marriage rates are associated with higher contemporary civicness as proxied by voter turnout and self-reported trust in others. The association holds for regions within Italy, Spain and France that have been firmly within the sphere of the Catholic Church for at least half of a millennium but which differ in their previous experience of the Church’s medieval marriage regulations.

This suggests that breaking down the kin-based networks increased trust and civic-mindedness, both essential for the development of democratic norms. Note that these results relate the ancient Church exposure to modern-day effects. There is no possibility of reverse causation here (that is, democratic norms affecting cousin marriage rates). Finally, Schulz goes on to show that:

...countries that differentiate cousin terms have a 7.5 units lower democracy on a 21-item scale compared to countries that do not. At the same time, 20th-century cousin marriage rates account for more than 50% of the cross-country variation in democracy scores today.

So, there you have it. Prohibition of cousin marriage explains modern-day democracy in Europe. Ok, it's probably not that simple (nothing ever is). But certainly, this is a paper that tells an interesting story, backed up by some compelling data analysis.

[HT: Marginal Revolution, last November]