Zubin Mehta, famous conductor, was a Farsi. He admitted that first cousin marriages were bad but, he added, sometimes they produced genius, like himself! No conceit there. (sarcasm).
The British peerage used to do that as well. They don't now, mostly because the crown bankrupted the peerage.
My people the Jews would (sometimes) do these first cousin marriages when stranded on Shtetls (rural enclaves) in Russia, Ukraine, Lithuania Poland etc with few marriage possibilities. But we did not do it generation after generation within a clan. All first cousin marriage is repulsive to me but it gets exponentially worse when repeated within a clan