From SSRN:

The Churches’ Bans on Consanguineous Marriages, Kin-Networks and Democracy

58 Pages Posted: 16 Dec 2016 Last revised: 20 Jan 2017

Jonathan F. Schulz

Yale University, Department of Psychology Abstract This paper tests the hypothesis that extended kin-groups, as characterized by a high level of cousin marriages, impact the proper functioning of formal institutions. Consistent with this hypothesis I find that countries with high cousin marriage rates exhibit a weak rule of law and are more likely autocratic. Further evidence comes from a quasi-natural experiment. In the early medieval ages the Church started to prohibit kin-marriages. Using the variation in the duration and extent of the Eastern and Western Churches’ bans on consanguineous marriages as instrumental variables, reveals highly significant point estimates of the percentage of cousin marriage on an index of democracy. An additional novel instrument, cousin-terms, strengthens this point: the estimates are very similar and do not rest on the European experience alone. Exploiting within country variation support these results. These findings point to the importance of marriage patterns for the proper functioning of formal institutions and democracy.

Schultz gives the example of variation within Italy:

A prominent example of within country variation in institutional quality is Italy where institutions function less well in the south. Cousin marriage rates (around 1960) are considerably higher in the south, where also the duration of the Western Church’s cousin marriage ban was shorter. Regression results reveal that cousin marriage rates at the provincial level are highly significantly correlated with mafia activity (as a proxy for institutional failure). This does not simply reflect a North South divide. The relation also holds within the North and the South.

Southern Italy is more mountainous than the Po Valley of the north, so it was harder for people to get around when courting. Cavalli-Sforza did a study decades ago that showed that people in Italian villages started marrying people from further away after bus service was introduced.

I hadn’t been aware that St. Augustine had made my basic point around 400 AD: