Well, royalty are special and weird, and a drop in the bucket, and prove little. My point was that it was a widespread pre-islamic custom, not one which arrived with the book; the geographical correlation with islam is because (as cstross says) it was largely stamped out in Europe a thousand years ago.

It's my understanding that cousin-marriage was rare among european Jews, but a bit more common elsewhere. (Not sure if as high as the surrounding peoples, not sure.) There are indeed rare and interesting genetic conditions in some Jewish groups, but that's another story.

(Am surprised by Tamil Nadu's 38%, thanks. But certainly north-indians take dowry pretty seriously, while avoiding cousins.)