Charles Darwin married his first cousin.

Few people in Darwin's time were as aware as he was of the potential dangers of limiting a gene pool through inbreeding. The famed naturalist had 10 kids with his first cousin Emma, and when they became sick, he would worry that inbreeding was the source.

Marrying one's cousin was more common and more acceptable in Darwin's time than it is now. In fact, for wealthy families, one's cousin was long regarded as the perfect mate since such a union kept wealth in the family.