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Questions: Bruce, Is Incest Always Wrong?


Every year or two, I ask readers to submit questions they want me to answer. That time has arrived once again. Any question. Any subject. Please leave your questions in the comment section or send them to me via email. I will try to answer them in the order received.

I look forward to reading and answering your questions.

A reader asked:

Bruce, is incest always wrong?

Ask people if sex between family members is wrong, and most people will say yes. Typically, this is a gut response to a difficult question. Sex between family members seems “icky” so it must be, right? The Bible says incest is a sin, so it must be, right? Lost on Christians is the fact that, according to the Bible, incest played a central role in the propagation of the human race. Adam and Eve had three sons, Cain, Abel, and Seth. These men all had children. Who did they have sex with? Either their mother, Eve, or their sisters (who are not mentioned in the Biblical text). So, if incest is wrong/immoral/sinful, why was it permissible in the book of Genesis? Good luck with this one, Evangelicals.

I oppose incest for scientific reasons; that sex (and pregnancy) between closely related people can and does lead to birth defects. The Amish have a problem with birth defects because they almost always marry within their group. Healthy population groups require outside DNA. Some Jewish sects have a similar problem.

If two closely related people want to marry and have sex without having children, I have no reason to say no. I may find such a thing icky, but I can’t think of one rational, scientific reason to object. My mother married her first cousin, Robert Slayman. Mom couldn’t have children, so she was free to marry him. Icky? Maybe, but ickiness is no reason to forbid such relationships.

The same thing can be said for adult step-children marrying each other. Icky? Perhaps, but there’s no scientific (or moral) reason for objecting to such marriages. We all agree that biologically connected parents, children, grandparents, aunts, and uncles should not marry each other. What about cousins?

A 2018 Popular Science article says:

The paper, published in the journal [by Paul Erlich]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 cousins went out of vogue, and the average degree of relation between married couples today. And hey, since we’re on the subject: Is it wrong to marry your cousin (for a survival perspective)?

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 from first-cousin marriages have birth defects, compared to 3 to 4 percent for children born from distantly related marriages.

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.

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.”

In 1975, a psychology textbook said one out of a million births were due to incest. Thanks to DNA testing, a recent study revealed that this number is actually one out of seven thousand.

The permissibility of incest is a good example of an issue that humanists and other non-Christians must wrestle with when determining morality, Ickiness is never a reason to forbid something. I can’t wait to read the headlines on Evangelical blogs that say, “Atheist Bruce Gerencser Approves of Incest.” 🙂

Bruce Gerencser, 66, lives in rural Northwest Ohio with his wife of 45 years. He and his wife have six grown children and thirteen grandchildren. Bruce pastored Evangelical churches for twenty-five years in Ohio, Texas, and Michigan. Bruce left the ministry in 2005, and in 2008 he left Christianity. Bruce is now a humanist and an atheist.

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    My mom admitted to me that she had a crush on her first cousin when she was a young teen, and I thought that was disgusting.

    My husband and his brothers and cousins used to discuss whether it would be ok or not to date their aunt’s stepdaughter – she wasn’t related by blood, so they thought it was probably OK. One of them actually had a one night stand with her.

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