I’ve read where its downright rampant in the middle east
Lots of inbreeding among Hasidic Jews and lots of kids with disabilities.
Yes, it has been common practice for Muslims to marry their first cousins for over 14 centuries. As a result, mental retardation, physical deformities, and mental illness is much higher among Muslims than other people.
Cousin marriage there isn’t just the norm. It is incentivized by a dowry system that says you don’t have to pay one if you marry a paternal cousin. And she is incentivized to go along with it, because she wouldn’t have to cover in front of her father in law or brother in laws and he (as her uncle or cousin) can be her chaperone.
The Saudi Royals, apparently have long lists of DNA/inherited diseases/disorders.