He's probably lying but hooking up with a cousin is far more common than one would think. Also, there's very little chance of the baby having birth defects... only 1-2% more than normal.
Now if two consecutive generations do it, the chances of defects go way up.
Really, it's a strange myth that has stayed in our social consciousness for far too long. It was disproved over 30 years ago. Hell, even marrying a cousin is legal in half the US and pretty much all of Europe.
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