It can trivially be seen to have little to do with islam in examples like the House of Habsburg, and the famously ignominous end of the Spanish line. Or orthodox jews (the endogamous set is wider than just a family, but you still see lots of rare genetic conditions crop up, and some jewish communities don't even stray outside of it e.g. Mashhadi jews).
Though it does look related to Arabization.
While first-degree cousin marriage has gotten quite a bit rarer used to be extremely common across the globe, and aside from the middle-east it also remains legal throughout eurasia (except for a few eastern european countries and notably china where it's entirely banned and india where it's dependent on culture and religion) and south america
> IIRC it's much less common in Hindu tradition
It's less common but hardly non-existent: according to Shrikant Kuntla, Srinivas Goli, T.V. Sekher, Riddhi Doshi via wikipedia[0] rate of consanguinous marriage is 25% of indian muslims, 15% for indian hindu, and under 10% for other indians. State / culture presents a much wider spread, from 1.2% in Himachal Pradesh to 38% in Tamil Nadu.
While British Pakistanis are responsible for 3% of all births, they account for 30% of British children born with a genetic illness.
Cousin marriages in the UK are 1/200 for White couples, but 2/3 for Pakistani couples.
The solution is to ban cousin marriages. It is unfair on the rest of society to have to shoulder the burden of excess healthcare costs, caused by the outdated and harmful cultural practices of immigrants.
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.)
Yes, India seems extremely split between the north and the south, northern cultures have disapproved of it or forbidden it forever while southern states much less so.
In Hindu families too it was not that uncommon at all, though this trend has declined in the current generation marriages.
But it's certainly a way of concentrating power in the family. And presumably, for a very long time, having lots of grandsons of undivided loyalty was important in conflicts with the neighbouring family/tribe... worth the individual fitness cost (and the extra miscarriages etc -- many of these strange illnesses would of course have killed the child).