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.”

First-cousin marriages are common and even preferred in many parts of the world. There are 19 U.S. states that place no restrictions on them, yet it?s still a major taboo here. Bennett believes this may be rooted in farmers’ experience that extreme inbreeding produces unhealthy livestock.

Biologists say unfavorable mutations, which pop up at random, will be gradually weeded out of large groups by interbreeding. But in small populations, individuals may mate with those who are genetically similar to themselves. Among these groups, there will be a build-up of harmful changes. The high prevalence of Tay-Sachs disease among Ashkenazi Jews is an example of the dangers of inbreeding within small groups.

Small, isolated groups that intermarry for generations, such as the Mennonites, are often at a higher risk for genetic disorders than unrelated groups of people. But a single-generation first-cousin marriage is different than many close-relation marriages over many generations. Bennett says, “We hope that people will re-examine why such marriages are illegal.”

First cousins or not, you can still produce Indigo children. Learn how to cope with this new kind of kid from ?The Indigo Children? and ?An Indigo Celebration? by Jan Tober and Lee Carroll, click here.

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