Guide to dating your cousin
Though the church required dispensations for marriages between cousins of even distant degrees of separation, they were routinely given, as was obviously the case among Roman Catholic royal families like the Hapsburgs. Cavalli-Sforza documents very high levels of cousin marriage in Italy in the 19th century in Consanguinity, Inbreeding, and Genetic Drift in Italy.But once given the dispensation could be revoked, rendering the marriage null and void. But for much of European history the marriages of common folk were not of much concern to the church. The rates dropped rapidly with economic development, especially better transportation networks in mountainous regions.Why is it that cousins have a higher risk of this occurring?Because two cousins are much more likely than two random individuals to share the same distinct gene from a common ancestor, because their common ancestors are so much more recent.What a first cousin marriage does is change the odds.How you present these odds matters a great deal in how scary they sound.
3-4% for a non-consanguineous couple, it might not sound that bad.
More precisely the coefficient of kinship between two first cousins is 1/8.