This paper highlights the role of kin-networks for the functioning of modern societies: countries with strong extended families as characterized by a high level of cousin marriages exhibit a weak rule of law and are more likely autocratic. To assess causality, I exploit 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

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... ntal 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 of cousin marriages in Italy, as well as within variation of a 'societal marriage pressure' indicator for a larger set of countries support these results. These findings point to a causal effect of marriage patterns on the proper functioning of formal institutions and democracy. The study further suggests that the Churches' marriage rules -by destroying extended kin-groups -led Europe on its special path of institutional and democratic development.