Incest also protects royal assets. Marrying family members ensures that a king will share riches, privilege, and power only with people already his relatives. In dominant, centralized societies such as ancient Egypt or Inca Peru, this can mean limiting the mating circle to immediate family. In societies with overlapping cultures, as in second-millennium Europe, it can mean marrying extended family members from other regimes to forge alliances while keeping power among kin.
And the hazards, while real, are not absolute. Even the high rates of genetic overlap generated in the offspring of sibling unions, for instance, can create more healthy children than sick ones. And royal wealth can help offset some medical conditions; Charles II lived far better (and probably longer, dying at age 38) than he would have were he a peasant.
A king or a pharaoh can also hedge the risk of his incestuous bets by placing wagers elsewhere. He can mate, as Stanford classicist Josiah Ober notes, "with pretty much anybody he wants to." Inca ruler Huayna Capac (1493-1527), for instance, passed power not only to his son Huáscar, whose mother was Capac's wife and sister, but also to his son Atahualpa, whose mother was apparently a consort. And King Rama V of Thailand (1873-1910) sired more than 70 children—some from marriages to half sisters but most with dozens of consorts and concubines. Such a ruler could opt to funnel wealth, security, education, and even political power to many of his children, regardless of the status of the mother. A geneticist would say he was offering his genes many paths to the future.
It can all seem rather mercenary. Yet affection sometimes drives these bonds. Bingham learned that even after King Kamehameha III of Hawaii accepted Christian rule, he slept for several years with his sister, Princess Nahi'ena'ena—pleasing their elders but disturbing the missionaries. They did it, says historian Carando, because they loved each other. —David Dobbs