A good way not to win friends in an immigrant community is to blame its high rate of birth defects on the practice of cousin marriages. That’s what British environment minister Phil Woolas did in February, blaming birth defects in children in the UK’s Pakistani community on marriages between first cousins. “If you have a child with your cousin, the likelihood is there will be a genetic problem,” he told the Sunday Times. (Calls by a Muslim activist group that Woolas be fired went for naught; he was promoted in October to immigration minister.) That belief is reflected in laws in 31 U.S. states that either bar cousin marriage entirely or permit it only if the couple undergoes genetic counseling or cannot have kids.

But in a paper in the journal PLoS Biology, Hamish Spencer of New Zealand’s University of Otago and Diane Paul of Harvard’s Museum of Comparative Zoology argue that the genetic risk to children born of cousin marriages is much less than widely believed.

Risk is in the eye of the beholder, of course. But in 2002 an expert panel convened by the National Society of Genetic Counselors 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 background (which is vanishingly low to begin with) for dying in childhood.

Whether 2% and 4% seem like a big extra risk or a piddling one probably depends on how much you want to marry your cousin, but 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.”

And what of the belief that humans have an incest-avoidance gene that keeps people from lusting after their cousins? None has ever been found. And if avoiding incest with a cousin is part of human nature, as some evolutionary psychologists contend, then an awful lot of humans haven’t noticed. In Turkey and Morocco, first-cousin marriages account for 22% of all marriages, and second-cousin marriages for another 29%, finds demographer Georges Reniers of the University of Ghent. Cousin marriages are similarly common among China’s majority Han ethnic group and in the Middle East and sub-Sahara Africa.