Actually, Steve Benson posted a very interesting and informative bit of the information on the subject a while back. Sorry, I don't have the link but someone here should be able to get it for you.
It's a hideous and fatal disease that causes victims to have very small heads and poor mental faculties. It's evident in Pakistan and runs rampant in the Warren Jeff's groups. The rest of the world is almost never touched by it.
The Kingston group has many children with club feet and other genetic disorders because they don't allow marriage to non blood relatives.
I heard that some of the fundie groups actually welcome intellectually challenged male babies because they are tractable and won't be marrying. The same source stated that the people are so inbred that the women often give birth to undefined lumps of protoplasm.