Marrying Cousins Is Not Nearly As Dangerous As Once Thought

“Cursed be he that lieth with his sister, the daughter of his father, or the daughter of his mother.” —Deuteronomy 27:22, King James Version

In A Nutshell

It is a popularly believed myth that marrying your cousins is wrong because your offspring will certainly come out deformed or be otherwise negatively affected. But recent research has shown that if there are any effects they are very minimal, and the risk of defects increases by no more then a couple of percentage points—or equivalent to the dangers to the baby if carried by a woman over 40.

The Whole Bushel

The importance of not sharing genetic material with a cousin is one of the most widely believed myths out there. Many US states specifically ban marriages to cousins, and if you asked most people they would probably tell you it is an immoral act. To some it seems disgusting or somehow against nature. Unlike many taboos, though, most people could point out immediately why it shouldn’t be done. The popular idea is that marrying your cousin gives you a much higher chance of having a child with birth defects. However, the body of research that has been conducted so far makes the whole thing look a lot less important than was previously believed.

There are multiple things that can affect birth defect rates (such as the age of the mother and her habits), but evidence on incest-related birth defects shows an interesting picture. It turns out that those children whose parents are related only had a slightly higher chance of birth defects and a mortality rate only a couple percentage points higher. One researcher working on the subject has suggested that banning marriage between cousins is unfair. He points out that Huntington’s disease has a 50 percent chance of passing on, but people with that condition are still allowed to breed.

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The researcher is not alone in feeling that way about the taboo. While some people are starting to become more outward about their cousin-love, there is still quite a broom closet for the behavior. Most Western societies frown on such relationships as incestuous and weird, so people tend to keep it quiet. Despite the taboo, marriage among cousins is actually quite popular around the world.

In fact, even in the United States where it is more of a taboo, nearly half of the states allow it. In Europe and the Middle East, it is far more common. In fact, the chance of defects or complications also has to do with the genetic problems currently within the family, but that could be a problem with unrelated people as well. Two closely related people may be more likely to pass on a family disease, but someone they aren’t related to could also pass down something negative and hereditary to a child.

There is still more to learn about the matter, and science has a lot to discover in regards to genetics. But at this point, the myth doesn’t stand up to the hype. This doesn’t mean that everyone should go marry their cousin, but that genetics are more complicated than we might imagine. Someday though, with proper testing, scientists may be able to better prevent the spread of hereditary conditions, and that can only be a good thing.

Show Me The Proof

NY Times: Shaking Off the Shame
Huffington Post: Why Ban Cousin Marriages?
The Straight Dope: What’s wrong with cousins marrying?