No Genetic Reason to Discourage Cousin Marriage, Study Finds
Contrary to widely held beliefs and longstanding taboos in America, first cousins can safely have children together, without a great risk of birth defects or genetic disease, scientists are reporting today. They say there is no biological reason to discourage cousins from marrying.
First cousins are somewhat more likely than unrelated parents to have a child with a serious birth defect, but scientists say the risk is not large. In the general population, the risk that a child will be born with a major birth defect, like spina fida, is 3 to 4 percent; to that background risk, first cousins must add another 1.7 to 2.8 percentage points, the researchers said.
Although the increase represents almost a doubling of the risk, since the background risk is small to begin with, the result is still not considered large enough to discourage people from having children, geneticists say. And they point out that no one questions the right of other people with far higher levels of risk to have children. For example, people with Huntington's disease, a severe neurological disorder, have a 50 percent chance of passing the disease to their children.
The researchers, convened by the National Society of Genetic Counselors, based their conclusions on a review of six major studies conducted from 1965 to August 2000, involving many thousands of births.
For first cousins, "there is a slightly increased risk, but in terms of general risks in life it's not very high," said Dr. Arno Motulsky, a professor emeritus of medicine and genome sciences at the University of Washington, and the senior author of the report, being published today in The Journal of Genetic Counseling.
Dr. Motulsky said that medical geneticists had known for a long time that there was little or no harm in cousins' marrying and having children.
"Somehow, this hasn't become general knowledge," Dr. Motulsky said. "Among the public and physicians there's a feeling it's real bad and brings a lot of disease, and there's a lot of social and legal disapproval."
Thirty states have laws forbidding first cousins to marry, but no countries in Europe have such prohibitions, and in parts of the Middle East, Africa and Asia, marriages between cousins are considered preferable. "In some parts of the world," the report says, "20 to 60 percent of all marriages are between close biological relatives."
Dr. Motulsky said the American laws against cousin marriage should be abolished, because they are based in part on the mistaken belief that the children of such parents will suffer from terrible physical and mental illnesses.
"They are ancient laws in terms of thinking it's really bad," he said. "The data show it isn't that bad. There shouldn't be a law that you can't marry your cousin."
The article says, "Romantic relationships between cousins are not infrequent in the United States and Canada." But many cousins who marry or live together keep their family ties a secret because of the stigma, so the frequency of such unions is not known. Estimates of marriages between related people, which include first cousins and more distant ones, range from less than 0.1 percent of the general population to 1.5 percent. In the past, small studies have found much higher rates in some areas: a survey in 1942 found 18.7 percent in a small town in Kentucky, and a 1980 study found 33 percent in a Mennonite community in Kansas.
The report made a point of saying that the term "incest" should not be applied to cousins, but only to sexual relations between siblings or between parents and children. Babies who result from those unions are thought to be at significantly higher risk of genetic problems, the report said, but there is not enough data to be sure.
The new report says genetic counselors should advise cousins who want to have children together in much the same way they advise everybody else, and that no extra genetic tests are required before conception. The guidelines urge counselors to take a thorough family history and, as they do for all patients, look for any inherited diseases that might run in the family or in the patients' ethnic group, and order tests accordingly. During pregnancy, the woman should have the standard blood tests used to screen for certain neurological problems and other disorders, and an ultrasound exam.
As a newborn, the baby should be tested for deafness and certain metabolic diseases — tests already given to all newborns in some parts of the country — which are among the conditions that may be slightly more likely to occur in children whose parents are cousins. Some of the metabolic problems are treatable, and children with hearing losses do better if they get help early in life.
Dr. Motulsky said the panel of experts began working on the cousin question about two years ago after a survey of counselors found a lot of variability — and misinformation — in the advice given to people who wanted to know whether cousins could safely have children together.
The president-elect of the counselors' group, Robin L. Bennett, who is a co-author of the report and a genetic counselor at the University of Washington, said: "Just this week I saw a 23-year-old woman whose parents were cousins, and she was told to have a tubal ligation, which she did at the age 21, because of the risk to her children. And there's no risk to her children. People are getting this information from small-town doctors who may not know the risk, don't have access to this information and just assume it's a big risk."
The young woman hoped to have the operation reversed, Ms. Bennett said.
The geneticists' article includes a personal account from a woman who said she had lived with her cousin for six years, "and we are madly in love." When she became pregnant, her gynecologist warned that the child would be sickly, and urged her to have an abortion. A relative predicted that the baby would be ********. Describing herself as heartbroken, the woman had the abortion, which she called "the worst mistake of my life."
When she learned later that the increased risk of birth defects was actually quite small, "I cried and cried," she said.
The small increase in risk is thought to occur because related people may be carrying some of the same disease-causing genes, inherited from common ancestors. The problems arise from recessive genes, which have no effect on people who carry single copies, but can cause disease in a person who inherits two copies of the gene, one from each parent. When two carriers of a recessive gene have a child, the child has a 1 in 4 chance of inheriting two copies of that gene. When that happens, disease can result. Cystic fibrosis and Tay-Sachs disease, for example, are caused by recessive genes.
First cousins share 12.5 percent of their genes, and 6.25 percent of their children's genes will consist of pairs in which identical copies have been inherited from the two parents. Some of those identical pairs may be recessive genes that cause illness. Unrelated people share fewer genes, and so their risk of illness caused by recessive genes is a bit lower.
Keith T., a 30 year-old-man, married his cousin seven year ago. In 1998, frustrated by the lack of information for cousins who wanted to marry, he started a Web site, cousincouples.com. Thousands of people have visited the site, he said. It is full of postings from people who have married their cousins or want to, and it highlights famous people who married their first cousins, including Charles Darwin, who, with Emma Wedgwood, had 10 children, all healthy, some brilliant.
Mr. T. asked that his name not be used, because he does business in a small town and fears that he will lose customers if they find out that his wife is also his cousin.
"If someone told me when I was young that I'd marry my cousin I would have said they were crazy," he said. "I thought the idea of marrying your cousin was kind of icky."
But then, as a teenager, he got to know a cousin whom he had hardly seen in childhood, and they fell in love.
Their families, he said, were in "total shock." Relatives said, " `They'll throw you in jail, you'll have defective children,' " Mr. T. recalled. "Those were some of the nicer things they said." When he and his cousin married, they invited only three people to the wedding. They hope to make up for that someday by having a "huge wedding," he said.
Mr. T. said he had read everything he could find about cousin marriages, including an entire book on the subject, "Forbidden Relatives," by Martin Ottenheimer, a professor of anthropology at Kansas State University. The subtitle of the book is "The American Myth of Cousin Marriage," and Dr. Ottenheimer heaps criticism on the state marriage laws.
Mr. T. said he was delighted to learn years ago that cousins' risks of birth defects were only slightly higher than those of unrelated people. So the study being published today did not surprise him. But he said he hoped it would convince the rest of the world.
"You can find people in the general population who have a greater risk than first cousins," he said. "It's one of my pet peeves. State laws single out cousins. They shouldn't." He and his wife hope to have children, he said.
