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In fact, while virtually every culture in recorded history has held sibling or parent-child couplings taboo, royalty have been exempted in many societies, including ancient Egypt, Inca Peru, and, at times, Central Africa, Mexico, and Thailand. And while royal families in Europe avoided sibling incest, many, including the Hohenzollerns of Prussia, the Bourbons of France, and the British royal family, often married cousins. The Spanish Habsburgs, who ruled for nearly 200 years, frequently married among close relatives. Their dynasty ended in 1700 with the death of Charles II, a king so riddled with health and development problems that he didn't talk until he was four or walk until he was eight. He also had trouble chewing food and couldn't sire a child.

The physical problems faced by Charles and the pharaoh Tutankhamun, the son of siblings, point to one possible explanation for the near-universal incest taboo: Overlapping genes can backfire. Siblings share half their genes on average, as do parents and offspring. First cousins' genomes overlap 12.5 percent. Matings between close relatives can raise the danger that harmful recessive genes, especially if combined repeatedly through generations, will match up in the offspring, leading to elevated chances of health or developmental problems—perhaps Tut's partially cleft palate and congenitally deformed foot or Charles's small stature and impotence.

If the royals knew of these potential downsides, they chose to ignore them. According to Stanford University classics professor Walter Scheidel, one reason is that "incest sets them apart." Royal incest occurs mainly in societies where rulers have tremendous power and no peers, except the gods. Since gods marry each other, so should royals.

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