Science says it's fine to marry your first cousin

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(RNN) – After studying the genealogy of 13 million relatives, scientists determined that only social norms are keeping people from marrying their first cousins and having children because genetically, it’s not that different from having children with a more distant relative.

Scientists from across the world answered several questions on family and relationships – for instance, genetics only determines about 16 percent of a person’s life span – by studying data from millions of online genealogy profiles.

Their paper, published last Thursday in Science, also shed light on when and why marrying your first cousin became taboo.

In the 1700s, people typically married a fourth cousin, but starting around 1850, they began marrying more distant relatives, according to Science.

About 15 years later, by the end of the Civil War, many states began outlawing cousin marriages, Popular Science reports. Today, these marriages are banned in 24 states and allowed in 20. The last six pose various restrictions on the practice.

Yaniv Erlich, a Columbia University data scientist who devised the study, believes this changing social norm pushed people to look outside their surroundings and families for partners.

By 1950, married couples were, on average, seventh cousins, Erlich says, and that has lasted to modern times.


But according to research, there’s not much of a reason for it to be that way – at least, genetically speaking.

First cousins share only about 12.5 percent of their DNA, according to genetic testing company 23andMe. Comparatively, siblings, as well as parents and children, share about 50 percent.

When it comes to babies, it’s best to have genetic diversity. The lower the percentage of DNA a couple has in common, the less likely their kids will be to have major birth defects.

It’s estimated that 4 to 7 percent of children born to first cousins are likely to have birth defects, Popular Science reports.

But even children whose parents are more distantly related have a 3 to 4 percent chance for birth defects.

With 24 states banning marriage between first cousins and it being thought of as taboo, it’s unlikely this research will change humans’ marital practices, but it does reveal how closely everyone is related.

"[All] of us are something like 10th to 12th cousins of each other,” Erlich told Popular Science. "When you think about wars and violence all over the world, it’s all within the family."