Go ahead, marry your cousin—it's not that bad for your future kids

Yaniv Erlich has a soft spot for genealogy. A data scientist at Columbia University and the chief science of officer of the DNA test company MyHeritage, he describes many things in the context of family. Columbia and MyHeritage are "mom and dad" and he's "got to make both happy." And his new study on a 13 million-member family tree is best measured in terms of his child's development. "My son was born when I started the study," Ehrlich says of the seven year project. "Now, he is in first grade."

The paper, published Thursday in the journal Science, looks at genetic data from millions of online genealogy profiles. Among other things, the researchers were able to determine at what point in history marrying your cousin went out of vogue, and the average degree of relation between married couples today. And hey, since we're on the subject: just how bad is it to have kids with your cousin?

While it's taboo today, cousins used to get hitched all the time. Franklin Delano Roosevelt's wife, Eleanor, was his fifth cousin once removed; she didn't even have to change her name. And scientific geniuses like Albert Einstein and Charles Darwin married their cousins, too. For much of human history, these unions weren't considered bad or gross. Oftentimes, there weren't many better options.

From 1650 to 1850, a given person was, on average, fourth cousins with their spouse, according to Erlich’s data. “Many people may have married their first cousin and many people married someone not at all related to them,” he says. But within a century, that had changed. By 1950, married couples were, on average, more like seventh cousins, according to Erlich.

One common sense explanation for this shift is that when transportation methods improved, bachelors and bachelorettes had access to potential partners they had once been denied by geography. This makes sense, given that before 1950, most people stayed in place and ended up marrying someone who lived with in a six-mile radius of where they were born.

Other factors could be at play, however. Erlich says that, according to his data, many continued to marry their cousins even after the Industrial Revolution dramatically improved mobility. While proximity may be one key to romance, it seems consolidating money or power played an important part in family marriages, too. Erlich believes it was changing social norms—and the advent of this cousin marriage taboo—that finally pushed people to look beyond their village and their family. Other factors, including the increasing autonomy of women and shrinking family sizes (which left fewer cousins to marry) could also have been involved.

Whatever the underlying cause, by the end of the Civil War, many states moved to outlaw cousin marriages. Today, 24 states ban marriage between first cousins, while 20 states allow it. The others allow first cousins to couple up, but only under certain circumstances. ("Certain circumstances" include: only if both are over 50, or 55, or 65, depending on the state; only if one or both are permanently infertile; and only if the couple has received genetic counseling.) And, of course, even in states where it is legal, the practice is taboo.

First cousins share 12.5 percent of their DNA. (Siblings, as well as parents and kids, share about 50 percent.) Any child that results from a first cousin union is, therefore, going to have a pretty substantial portion of similar-looking genes. And that can pose a problem.

In biology, genetic diversity is all the rage. If something goes wrong with the genetic material provided to you by your mom, you're more likely to shake it off if your dad's genetic material is very different. If dad's left you hanging when it comes to susceptibility to a certain disease, a mom from a radically different gene pool could confer the protection you require. If mom and dad are genetically similar, however, both versions of a gene are likely to shut down at the same time. It's estimated that 4 to 7 percent of children born to first cousins are likely to have birth defects, compared to 3 to 4 percent for children whose parents are more distantly related.

That's not nothing, but it's also not the end of the world—or the family tree. The real issue would arise if the next generation of kids also married their first cousins. Their offspring will have even more DNA in common—and an even greater chance for birth defects.

There are plenty of historical examples of this. Charles II, the last Hapsburg king, had so many intermarried ancestors that his genes seemed more like the product of a union between siblings than the reality of uncle marrying niece. But it's a contemporary problem, too. In nations with small populations like Iceland, which has just 330,000 people concentrated mostly in the capital city of Reykjavík, many people worry they'll accidentally marry a close relative. Instead of more traditional dating apps like Tinder, which matches potential partners based on physical attraction, locals use Íslendinga-App, which stops matches between people who have too much genetic material in common.

Ultimately, marrying your first cousin carries some risk. But the odds of healthy offspring dramatically improve with each new distance of relation. Second cousins share only 6.25 percent of their genes and third cousins share just over 3 percent. Seventh cousins—the average distance between modern American spouses—have no meaningful genetic relation at all.

Today, you’d be hard pressed to find an advocate of cousin marriages, and there are of course good reasons for that. But looking at Erlich’s family tree, it’s not an “ew” factor one feels, but an “aw” factor. The genetic data, branching off this way and that, reveals just how closely related we all already are. “[All] of us are something like 10th to 12th cousins of each other,” Erlich says softly. “When you think about wars and violence all over the world, it’s all within the family.”