The title of the paper is “The Churches’ Bans on Consanguineous Marriages, Kin-Networks and Democracy” and the author is Jonathan F. Schulz, here is the 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.
I recall reading related ideas in the MR comments section from Steve Sailer and others. For the pointer I thank Alexander B.