Why Have So Many World Leaders Married Their Cousins?

Britannica has an article on cousin marriage:

A crown. A ring. Your cousin. What else do you need for a royal wedding?

At this point, the fact that so many royals through history wed relatives in what are now termed consanguineous marriages is something of a history joke. But why did they do it in the first place?

Before we begin, an acknowledgement: it wasn’t only royals who wanted to “keep it in the family.” The economist and patriarch of a major manufacturing family, Pierre-Samuel du Pont, wasn’t afraid to admit his plan for his lineage, writing in 1810: “The marriages that I should prefer for our colony would be between the cousins. In that way we should be sure of honesty of soul and purity of blood.” Other magnates were more discrete, but they often shared the same opinion; by refusing to allow his female descendants to inherit his wealth, banking great Mayer Amschel Rothschild guaranteed that for his daughters and granddaughters to find wealthy, suitable husbands, they would have to look among their cousins. (And so they did: four pairs of Rothschild cousins, as well as one uncle-niece pairing, were wed.)