We've often marveled at the incestuous nature of royal marriages in Europe, but the inbreeding really came to a head in 16th century Spain, when King Philip II married and had a son with his double first cousin, Maria Manuela of Portugal. Carlos, their baby boy, came into the world with significant disadvantages; his legs were different lengths and his spine curved abnormally, causing problems with his gait and posture.

These are issues to be compassionate about, but Carlos's behavior from infancy forward tended toward the violent and sadistic. He injured his wet nurses by biting them, and was known to torture animals and humans alike as a child and adolescent. It's unclear whether his behavioral issues might have been inherited as well; among his four great-grandparents (most people have eight) and six great-great-grandparents (most people have sixteen) was Juana I of Castile, better known to us as Juana the Mad.

While Carlos was clearly unfit to become a monarch, Philip II was in a bit of a bind because he had no other sons, and his wives - he would be married four times in total - kept dying on him. Carlos's mother, Philip's first wife, died from complications from delivering him, Mary I of England died four years into their childless marriage, Elisabeth of Valois - originally betrothed to Carlos - died after nine years of marriage and several daughters, and finally, Anna of Austria was able to produce a living heir before she died after ten years of marriage. In the meantime, Philip found himself going to extreme lengths to protect his kingdom from his son - perhaps even by murdering him.

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