Marriage between first cousins has long been a taboo in the U.S., because of the assumption that marriage between such close relatives will produce children with birth defects. But new research shows the genetic risks of marriage between first cousins have been greatly overestimated.

Robin Bennett, a genetic counselor at the University of Washington, looked at data from six previous studies on cousin marriages that included millions of people. She found that children from first-cousin marriages are 1.7 to 2.8% more likely to have a serious birth defect than the children of unrelated couples. Although the risk is significant, Bennett says, “It is much lower than people have assumed.”
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