It's OK for cousins to marry, as long as their children don't marry their cousins, according to a seven-year study by a Columbia University researcher.
The report, published Thursday in the journal Science, is based on an analysis of the family trees of 13 million people and examines the data from their genealogical profiles.
Yaniv Erlich, the Columbia researcher and chief science officer at the DNA-testing company, MyHeritage, found a 4 to 7 percent probability of birth defects in children of first cousins who marry, compared with 3 to 4 percent for distant relatives who marry.
First cousins share an estimated 12.5 percent of their DNA. Second cousins share 6.25 percent and third cousins share 3 percent.
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The percentage increases if the children of first cousins marry their own cousins, the study found. And such marriages make the children more susceptible to certain diseases handed down through their parents' DNA.
The study also revealed that from 1650 to 1850, the average living person was a fourth cousin of his or her spouse. By 1950, the average couples were seventh cousins; the estimate for today's couples is from 10th to 12th cousins.
Following the Civil War, several U.S. states banned marriages between cousins. Today, 24 states still ban marriage between first cousins, while 20 states allow it.
Follow Enrique Flor at @kikeflor