Is it unethical to marry your cousin? Obviously the answer must depend on the risk of birth defects amongst your children. And according to a New Zealand scientist Hamish Spencer, writing in PloS Biology, the risk is not very high. In 2002 an expert panel convened by the National Society of Genetic Counselors (in the US) found that the risks of a first-cousin marriage are about 1.7% to 2% above the background risk for congenital defects and 4.4% above the background for dying in childhood.

Spencer concludes that “neither the scientific nor social assumptions behind [anti-cousin-marriage laws] stand up to close scrutiny. Women over the age of 40 have a similar risk of having children with birth defects and no one is suggesting they should be prevented from reproducing. People with Huntington’s disease or other autosomal dominant disorders have a 50 per cent risk of transmitting the underlying genes to offspring and they are not barred either.”

However, in 31 American states, cousin marriage is banned or permit it only where the couple obtains genetic counseling, is beyond reproductive age, or if one partner is sterile.

This study has political implications, as people from Middle Eastern backgrounds tend to marry cousins. As many as 50% of all marriages in Turkey and Morocco, for instance, are between first and second cousins. When communities from these countries migrate, they bring their customs with them, much to the consternation of politicians.

The authors of the paper call for the abolition of cousin marriage laws in the US: “These laws reflect once-prevailing prejudices about immigrants and the rural poor and oversimplified views of heredity, and they are inconsistent with our acceptance of reproductive behaviors that are much riskier to offspring. They should be repealed, not because their intent was eugenic, but because neither the scientific nor social assumptions that informed them are any longer defensible.” ~ Newsweek, Dec 30; PloS Biology, Dec 23