US States Marriage Map

[Rev. Rye]

What you might also not know is the cultural taboo against first cousin marriages is probably a very recent thing. 80% of all marriages throughout the history of Homo Sapeins have been between second cousins or closer:

Kinship-Marriage-Anthropological-Perspective-Anthropology

Also, the overall risk of birth defect in first cousin marriages is comparable to the risk of birth defects from women married to non-relatives, but giving birth at age 40. So if a woman is giving birth past age 40, she is doing a worse thing for her offsprings than marrying her first cousin.

So if marriage between first cousin is to be outlawed based on risks to the offspring, then women should go to jail for attempting to conceive after 40.

Finally, Charles Darwin of the Origins of Species married his first cousin Emma Wedgwood. Some of their offsprings turned out rather well even by the standards of their father.

The real danger comes when you go overboard. I recently discovered this guy.
Ladies and Gentlemen, meet Charles II, King of Spain from 1665-1700.



And here is his family tree, with many of the relationships noted for clarity.