What's wrong with marrying your cousin?

How you look at things.
April 10 2002 5:32 PM

The Love That Dare Not Speak Its Surname

What's wrong with marrying your cousin?

Illustration by Robert Neubecker

A new group is struggling for acceptance. The group is people who are married to their cousins. These people note that 20 percent of marriages around the world are between first cousins, that Albert Einstein and Charles Darwin married their first cousins, and that first-cousin marriage, while prohibited in half the United States, is legal in Canada and throughout Europe. Now a study by the National Society of Genetic Counselors says that having a child with your first cousin raises the risk of a significant birth defect from about 3-to-4 percent to about 4-to-7 percent. According to the authors, that difference isn't big enough to justify genetic testing of cousin couples, much less bans on cousin marriage. From this, the media have concluded that marrying your first cousin is "OK." Is it?

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

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As Frame Game has argued before, topics such as sex with animals, dog-eating, and sex with cousins are never as simple as they're made out to be. You can't just say the practice in question is icky. You have to state a principle and think through its implications. Often, you have to change your opinions on related issues in order to honor that principle, or you have to throw out the principle and change your mind about the original question.

Is cousin marriage icky? Why? You can't appeal to Victorian morality; Queen Victoria married her first cousin. You can't appeal to the Bible; in the Bible, God commands marriages between first cousins. Instead, advocates of laws against cousin marriage appeal to science. To let cousins marry, they argue, is "to play Russian roulette with genetics." Many genetic diseases are caused by recessive genes. To get the disease, you have to get the bad gene from both parents. The greater the genetic similarity between your parents, the greater your chance of getting two copies of the bad gene.

But if that's your reason for banning cousin marriage, you've drilled into a mother lode of problems. Many cousin couples can't pass on genetic diseases, since they're infertile. Are you going to ban them from marrying? If not, maybe the 24 states that ban cousin marriage should follow the lead of the five states that allow it if either party is sterile. And if procreation between first cousins is too dangerous, why stop there? Six states ban marriage between first cousins once removed, i.e., marrying the son or daughter of your first cousin. Theoretically, that's half as risky as marrying your first cousin, in terms of increasing the probability of passing on a genetic disease to your kids. How about marriage between second cousins? Theoretically, that's one-fourth as risky. No state bans such marriages. Should we change that?

If your purpose is to prevent people with dangerous genes from marrying each other, why use a crude standard such as kinship? Why not test everybody for bad genes, ban marriage between carriers, and let cousins without bad genes marry each other? Banning cousin marriage keeps these couples in the closet, deterring them from seeking genetic screening, which would help them decide whether they could safely have kids. And as the NSGC study notes, the crude assumption that children of cousins will turn out badly leads to unnecessary abortions.

If you're afraid of mandatory genetic testing and you'd prefer to ban marriage among people in high-risk categories, why not start with fertile women over 40? And what about ethnicity? Cousin couples compare laws against cousin marriage to laws against interracial marriage. They've got it backward. Sickle-cell anemia runs in blacks. Tay-Sachs runs in Jews. The best way to curtail such diseases would be to ban marriages within ethnic groups.

The New York Times, CNN, and their journalistic brothers in arms have spun the increased risk found by the NSGC study as no big deal. Why is an increase of 50 to 100 percent no big deal? At worst, said one of the study's authors, when cousins procreate, "'93 percent of the time, nothing is going to happen." What about the other 7 percent? One doctor calculated from the study's findings that "almost 10,000 children will be stillborn or born with birth defects this year in the United States from first-cousin marriages. Not marrying a cousin is a more potent remedy than many of the medications we prescribe for heart attacks." Perhaps states should at least require cousins to get genetic counseling before marrying, as Maine has done.

The study's authors and its trumpeters in the media suffer from the congenital liberal conceit that science solves all moral questions. The authors instruct genetic counselors to focus on "validating feelings" and helping cousin couples "normalize" their relationships by explaining how common cousin marriages are. In the Times, USA Today, and other publications, the authors declare that laws against cousin marriage are baseless. According to headlines and TV reports, "science" has proved that cousin marriages are "OK." No, it hasn't. Science has deflated the scientific objections to cousin marriages. Moral problems remain.

Start with the problem of double first cousins. Suppose your mom and my mom are sisters. If cousin marriage is legal, you and I can marry. What if, in addition, your dad and my dad are brothers? It isn't that hard to imagine: Boy meets girl, girl's sister likes boy's family, girl's sister gets interested in boy's brother, both couples end up getting married. The first couple produces me; the second couple produces you. North Carolina and West Virginia explicitly prohibit me from marrying you, but 20 other states don't. Is that OK? Because if it is, bear in mind that you and I have as many genes in common as an uncle and niece do. If you and I can marry, why can't an uncle and an adult niece? Science says there's no difference.

Would you rather restrict marriage to ordinary first cousins? That won't get you out of the woods. First cousins have as many genes in common as a man and his half-brother's child do. We're talking Roger and Chelsea Clinton. If first cousins can marry, why can't Roger and Chelsea? Science says there's no difference.

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