Dr Adequate writes:
I'm English, I've never heard of this.
I have. It's not Muslims as such, but people from some areas of Pakistan (who happen to be Muslims) that have produced a lot of migrants. Here (link below), it says that the birth defect rate is thirteen times the average, which doesn't seem to fit the U.S. research for random cousin marriages Theodoric linked to above.
However, it may be a cumulative effect in communities that constantly do it (you're marrying your first cousin, second cousin and aunt/uncle once removed all at the same time!!!!).
BBC NEWS | Programmes | Newsnight | The risks of cousin marriage
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