The DuPonts might stand out as an interesting case study because they had a good thing going with the company, the city, and the state, which was virtually a DuPont estate. There were clear incentives to keep things in the family. I suppose the same thing has happened many times over the years when aristocracies of land, wealth, or status start to get a little clannish. You don't have to go the full Deliverance route to see the effects. I don't know enough about Muslim societies to comment, but the thesis is intriguing.
For a good idea of the beneficial effects of introducing new blood into a family, look at Prince Charles versus his sons.
Diana wasn’t royalty, but she very obviously had good genes. Probably because she wasn’t royalty.
I can recall a passage in “Gone with the Wind” in which Scarlett’s father, I believe, extolls the virtue and success of Charleston families who retain their strength by marrying their cousins.
Along with retardation being multiplied through incest, talent can likewise be multiplied. The advocates of cousin-marrying likely think about the winners and ignore the losers.