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.

UNSPECIFIED - CIRCA 1754: Philip IV (1605-1665) king of Spain from 1621. Full length portrait, 1628. On broad ribbon he wears orger of Golden Fleece. His long, pronounced Hapsburg jaw is evident in this portrait. Diego Veslasquez (1599-1660). Prado, Madrid (Photo by Universal History Archive/Getty Images)