Cousin Marriage OK by Science

By Brandon Keim

In an age of sexual liberation, marriage between cousins remains taboo, at least in the United States — and from a scientific perspective, laws against the unions are a socially legitimized form of genetic and sexual discrimination.

That argument, raised Monday in an editorial in Public Library of Science Biology, may turn the stomachs of people raised to disapprove of any form of incest. But dispassioned analysis suggests that cousin marriage is no more troubling than childbearing by middle-aged women.

"Women over the age of 40 are not prevented from childbearing, nor is anyone suggesting they should be, despite an equivalent risk of birth defects," write zoologists Hamish Spencer and Diane Paul. Bans against cousin marriage, they say, should be repealed, "because neither the scientific nor social assumptions that informed them are any longer defensible."

Thirty-one states outlaw marriage between first cousins, making the United States the only developed country in which the practice is regularly banned. Most were passed in the Civil War's aftermath — not, say Spencer and Paul, to reduce the chance of defects caused by combinations of deleterious genes, but as part of a radical expansion of government authority over private lives.

"Unlike the situation in Britain and much of Europe, cousin marriage in the U.S. was associated not with the aristocracy and upper middle class but with much easier targets: immigrants and the rural poor," they write.

But their argument is far from consensus: in Forbidden Relatives: The American Myth of Cousin Marriage, Kansas State University anthropologist Martin Ottenheimer argues that the bans were driven by now-discredited 19th century research on birth defects among children born to first cousins.

Whatever their motivations, the laws are not supported by science. According to the National Society of Genetic Counselors, birth defects are 2 to 3 percent more common in children born to first cousins than among the general population — a real risk, but not enough to justify the bans.

"It's a form of discrimination that nobody talks about. People worry about not getting health insurance — but saying that someone shouldn't marry based on how they're related, when there's no known harm, to me is a form of discrimination," said Robin Bennett, a University of Washington genetic counselor who led the NSGC study.

Precise statistics on cousin-union frequency in the United States are hard to come by, she said, but discrimination and ignorance have serious consequences.

"I'm aware of people who have been afraid to tell people that they're in love with their cousins, who've become pregnant and potentially terminated a pregnancy based on false information," said Bennett. "Or they didn't marry the person they loved because of their concerns."

"The laws against cousin marriage are archaic, outdated and counterproductive," said Ottenheimer.

Repealing these laws doesn't seem likely in the near future: Gay marriage remains a more pressing issue. But anyone who wants to fight for cousin marriage won't have to fight against science.

"Ultimately it's a political question about what you allow individuals to do, and what that says about the structure of society," said Spencer.

Citation: "'It’s OK, We’re Not Cousins by Blood': The Cousin Marriage Controversy in Historical Perspective." By Diane B. Paul and Hamish G. Spencer. Public Library of Science Biology: Vol. 6 Issue 12, Dec. 22, 2008.

Images: 1. Emma Darwin/Darwin Day Celebration  2. Charles Darwin/WikiMedia Commons (Charles married Emma, his first cousin, shortly after returning from his expedition aboard the Beagle.)  3. Map of U.S. cousin marriage bans/PLoS Biology