Study Downplays Risk of Cousin Unions

Medically Reviewed by Gary D. Vogin, MD
From the WebMD Archives

April 4, 2002 -- Kissing cousins have long been considered taboo. In fact, first-cousin marriages are illegal in 30 states, and cousin relationships are often kept secret. But a new study shows that children born to first-cousin unions won't likely have birth defects -- at least, the risk is not as high as most people think. Those couples should get genetic counseling, however, the study authors recommend.

In fact, in many cultures around the world -- particularly in the Middle East, Asia, and Africa -- preferred marriages are between first cousins, writes lead author Robin I. Bennett, MS, a medical genetics researcher at the University of Washington in Seattle. Her study appears in the current issue of the Journal of Genetic Counseling.

"In some parts of the world, 20-60% of all marriages are between close biological relatives," she writes. "There are specific communities within the United States and Canada where [cousin marriages] are common."

Few studies have documented the actual risks of the offspring of these relationships, says Bennett. "This risks quoted for birth defects and mental retardation are often based on studies of non-Western populations where [cousin] unions are common." Those studies may not be applicable to unions in the U.S. and Canada.

In the current study, a group of genetics experts reviewed all studies published about these risks. They found that children of first-cousin unions may have a 2% to 3% increased risk for genetic disorders, especially disorders that affect metabolic processes. Children with those disorders have about a 5% higher risk of getting sick and dying in childhood, especially before their 10th birthday.

Researchers advise cousin couples to get genetic counseling, including a thorough medical history, before starting a family.