It is only some States in the US and China that have prohibited the marriage of 1st Cousins, other American States will allow 1st Cousin marriages providing they are incapable of bearing children. 1st Cousin marriages were commonplace throughout the world until scientists discovered the elevated risks of Recessive Degenerative Disorder through Cousin Marriages.
“British Pakistanis are 13 times more likely to have children with genetic disorders than the general population - they account for just over 3% of all births but have just under a third of all British children with such illnesses.
Indeed, Birmingham Primary Care Trust estimates that one in ten of all children born to first cousins in the city either dies in infancy or goes on to develop serious disability as a result of a Recessive Genetic Disorder. “
http://news.bbc.co.uk/2/hi/programmes/newsnight/4442010.stm
Charles Darwin himself, the originator of the concept of natural selection and Author of ‘The Origin of Species by Means of Natural Selection’ married his 1st Cousin Emma Wedgwood!
The practice of inter family marriages, has also now developed a trend of clusters of specific Recessive Degenerative Disorders and a typical example of this is the Thalassemia problem here in the UAE, Thalassemia are inherited disorders of haemoglobin synthesis. Their clinical severity varies widely, ranging from asymptomatic forms to severe or even fatal entities and is the primary reason for the mandatory blood tests all UAE nationals have to undergo before a marriage license can be issued.
http://www.thalassemia-dubai.com/en/home.aspx