King Tut—the offspring of a brother-sister marriage—had a partially cleft palate and a deformed foot. Charles II of Spain—the result of a long history of close cousin marriages—fared even worse: He was developmentally delayed, had trouble chewing and was impotent. So, given the rather obvious downsides, why are royal families so prone to keeping it in the family? David Dobbs tackles that question in a short piece at National Geographic. Essentially, it boils down to risk vs. reward. Sure, somebody might end up with a funky foot, Dobbs writes, but there are also benefits to the practice—and ways to hedge your genetic bets.