"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."