"The practice is so reviled that 31 U.S. states ban marriage between first cousins or allow it only if the couple has undergone genetic counseling or at least one partner is sterile or no longer fertile because of age.

But those laws "seem ill-advised" and "should be repealed," a geneticist and medical historian write in today's PLoS Biology.
...
First cousins share about an eighth, or 12.5 percent, of their genes, according to a 2002 study in the Journal of Genetic Counseling. Because of that overlap, there's a 1.7 percent to 2.8 higher risk of intellectual disability and genetic disorders, including seizures and metabolic errors among children...
...

"It's not an issue because most people aren’t interested in their first cousin," Spencer admits. "But it does affect some individuals and it doesn’t seem particularly fair." "

http://www.sciam.com/blog/60-second-science/post.cfm?id=incest-may-not-be-best-but-marriage-2008-12-22

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