Dr. Bittles, who is working on an update of the 2002 study, and other researchers argue that laws against marriage between cousins were rooted in myth and moral objections, and that they amounted to genetic discrimination akin to eugenics or forced sterilization. People with severe disorders like Huntington’s disease, who have a 50 percent chance of passing it on to their offspring, are not barred from marrying because of the risk of genetic defects, he said, so cousins should not be, either.

Historically, marriage between cousins has been seen as desirable in many parts of the world, and even today, slightly more than 10 percent of marriages worldwide are between people who are second cousins or closer, Dr. Bittles said. In the United States, the percentage is thought to be much smaller, although it is difficult to estimate, since such marriages have long been an underground phenomenon, because of laws forbidding them and because of the lingering incest-related stigma.

Image FAMILY TIES Shane Winters and Kimberly Spring-Winters, first cousins, married this year despite her mother’s concerns. Credit... Suzanne DeChillo/The New York Times

Martin Ottenheimer, who wrote “Forbidden Relatives: The American Myth of Cousin Marriage,” a 1996 book that was the first detailed examination of the issue in the United States, compared marriage between cousins to same-sex marriage. “People say, ‘If we permit this, what are we permitting? We’re down the slippery slope toward chaos. Then we’ll permit people to marry dogs,’ ” he said in an interview. “The stigma has stuck for so long.”

But others who have revisited the 2002 study warn that potential risks should not be downplayed.

Diane B. Paul, a professor emerita of political science at the University of Massachusetts, Boston, and a research associate in zoology at Harvard, was an author of a paper published last year in the journal PLoS Biology that described the difficulty of generalizing about the potential for birth defects or increased mortality in the children of cousins. Each couple’s risk depends on the individuals’ particular genetic makeup, she said, which means “it’s very difficult to determine.” And even the small average risk of defects reported in the 2002 study, she added, represents nearly double the risk to children of unrelated parents.

That kind of uncertainty doesn’t sit well with many people, including some legislators. Despite the efforts of some in Minnesota and New Hampshire to overturn state laws against cousin marriage after the 2002 study was published, it remains illegal there. And as of 2005, it is against the law in Texas as well.

The Texas ban was part of a law targeting polygamy, and the state representative who proposed it, Harvey Hilderbran, a Republican, said he would not have introduced a bill simply to prohibit marriage between cousins. Still, he said in an interview: “Cousins don’t get married just like siblings don’t get married. And when it happens you have a bad result. It’s just not the accepted normal thing.”