Jonah: I can offer this report from the Guardian newspaper, 11/16/05:
Birmingham [England] primary care trust estimates that one in 10 of all children born to first-cousin marriages in the city’s Pakistani community either dies in infancy or goes on to suffer serious disability as a result of recessive genetic disorders. Recessive genetic illness is one of the main reasons for admission to Birmingham’s children’s hospital.
The variant genes that cause recessive genetic illnesses tend to be rare. In the general population, the likelihood of a couple having the same variant gene is 100-1. In cousin marriages, if one partner has a variant gene, the risk that the other has it too is more likely to be one in eight.