The map above shows the distribution of consanguineous marriages. As you can see there’s a fair amount of cross-cultural variation. In the United States there’s a stereotype of cousin marriage being the practice of backward hillbillies or royalty. For typical middle class folk it’s relatively taboo, with different legal regimes by state. The history of cousin marriage in the West has been one of ups & downs. Marriage between close relatives was not unknown in antiquity. The pagan emperor Claudius married his niece Agrippina the Younger, while the Christian emperor Heraclius married his niece Martina. Marriage between cousins were presumably more common. With the rise in the West of the Roman Catholic Church marriages between cousins were officially more constrained. Adam Bellow argues in In Praise of Nepotism: A Natural History that there’s a material explanation for this: the Roman church used its power over the sacrament of marriage to control the aristocracy. 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. But once given the dispensation could be revoked, rendering the marriage null and void. A highly convenient power politically.