http://news.bbc.co.uk/2/hi/health/7404730.stm

Professor Alan Bittles, director for the centre for human genetics in Perth, Australia has collated data on infant mortality in children born within first-cousin marriages from around the world and found that the extra increased risk of death is 1.2%.

In terms of birth defects, he says, the risks rise from about 2% in the general population to 4% when the parents are closely related.