For centuries European royals married only each other. It was believed to be the best way of consolidating power. But rampant royal inbreeding had increasingly negative consequences––including genetic abnormalities (like the protuberant “Habsburg Jaw”), the dying off of whole lines, and eventually serious geopolitical instability that culminated in World War I. Mo and Barnard College professor and bestselling author Caroline Weber discuss the practice that ended up being way more than just a family matter.

Portrait of Charles II of Spain, 1680-1683. Found in the collection of the Museu Nacional d'Art de Catalunya, Barcelona. (Photo by Fine Art Images/Heritage Images/Getty Images)