It is one of the biggest taboos left in modern Western society. Mention cousins who have sex at a dinner party and you’ll likely see a dozen people suddenly become very interested in their salad. People have visceral reactions to it. Some people think it’s a one-way ticket to genetic disaster. Others think it’s just plain "gross." But honestly, when you look at the data, the reality of cousin relationships is a lot more complicated than the stereotypes from deliverance or Game of Thrones.
Most people don't realize that for the vast majority of human history, and still in many parts of the world today, marrying or having a relationship with a cousin was the norm. It wasn't about being "weird." It was about keeping wealth in the family and knowing exactly who your neighbors were.
The Genetic Reality vs. The Myth
Let's talk about the big elephant in the room: birth defects. Everyone "knows" that cousins who have sex and eventually have children are asking for trouble. Right? Well, sorta, but the risk is way lower than the urban legends suggest.
Professor Hamish Spencer from the University of Otago has done extensive work on this. He and other geneticists point out that the risk of a significant birth defect in a child born to unrelated parents is about 2% to 3%. For first cousins, that risk only bumps up to about 4% to 6%. It’s an increase, sure. It’s a doubling of risk. But it’s still a 94% chance that the baby will be perfectly healthy. To put that in perspective, a woman over the age of 40 has a similar or even higher risk of certain genetic issues in her children, yet we don’t treat that with the same level of social horror.
Why is there a risk at all? It comes down to "recessive" genes. We all carry some "bad" genes that don't do anything because they are paired with a "good" version from the other parent. But because cousins share about 12.5% of their DNA, there is a higher chance they both carry the same "bad" recessive gene. If they both pass it on, the child doesn't have a "good" version to balance it out.
But here is the catch: this only becomes a massive problem if it happens generation after generation. This is called "inbreeding depression." If it's a one-off thing in a family tree that is otherwise diverse, the genetic impact is usually pretty negligible.
✨ Don't miss: Bragg Organic Raw Apple Cider Vinegar: Why That Cloudy Stuff in the Bottle Actually Matters
The Law is a Mess
If you think the science is confusing, try looking at the legal system. In the United States, the laws regarding cousins who have sex or get married are a total patchwork. It makes no sense.
In some states, it's totally legal to marry your first cousin. Think New York, California, or Florida. In other states, like Texas or Ohio, it’s a criminal offense. Then you have the "middle" states. Take Arizona, for example. You can marry your cousin there, but only if you're both over 65 or if one of you is infertile. It’s basically the law saying, "We don't like this, but if you can't have kids, we guess it's fine."
Internationally, the vibe is even more different. In the United Kingdom, it’s completely legal. In much of the Middle East, North Africa, and South Asia, cousin marriage is not only legal but often preferred. In Pakistan, for example, research suggests that more than 50% of marriages are between first or second cousins. This isn't because of a lack of options; it's a social structure designed to strengthen family ties and ensure compatibility.
Famous Examples You Probably Forgot
We tend to look down on this now, but history is littered with famous couples who were cousins. It wasn't just "royalty" either, though they were the masters of it (looking at you, Queen Victoria and Prince Albert).
- Charles Darwin. The guy who literally figured out how biology works married his first cousin, Emma Wedgwood. They had ten kids. Darwin actually worried a lot about the health of his children, thinking maybe his theories on "self-fertilization" in plants applied to his own family.
- Albert Einstein. The smartest man in history? He married his second cousin, Elsa Löwenthal. Actually, they were even more related than that—their mothers were sisters AND their fathers were cousins.
- Edgar Allan Poe. He married his first cousin, Virginia Clemm. She was only 13 at the time, which is a whole other level of "yikes" by modern standards, but the cousin aspect was barely a footnote then.
Why the Taboo is Growing (or Not)
Social scientists like Martin Ottenheimer have argued that the American obsession with banning cousin marriage was less about science and more about "civilizing" the population. In the mid-19th century, as the US was trying to build a national identity, reformers wanted people to look outward for partners rather than staying in their small, insulated family groups. They wanted a mobile, interconnected society. Making cousin relationships "gross" was a tool for social engineering.
🔗 Read more: Beard transplant before and after photos: Why they don't always tell the whole story
Interestingly, we are seeing a weird split in modern culture. On one hand, the internet makes it easier for people to find out they are related via sites like 23andMe, which has led to some awkward "oops, we're related" stories. On the other hand, the globalized world is making these relationships rarer in the West because our social circles are so much wider than they used to be. You're not stuck in a village of 50 people anymore.
Real-World Complications
Let's be real: even if the science says the risk is low and the law says it's legal in your state, cousins who have sex face massive social hurdles.
If the relationship ends, Thanksgiving is going to be a nightmare. There is no such thing as a "clean break" when your ex’s mom is also your aunt. The potential for family drama is astronomical. Families often split into factions, and the pressure on the couple to stay together—or break up—is ten times higher than in a normal relationship.
There's also the psychological element. Some psychologists argue about "GSA" or Genetic Sexual Attraction. This is a controversial theory suggesting that relatives who meet for the first time as adults can feel a powerful, almost magnetic attraction to one another because of their shared genetics. While not everyone buys into this theory, it’s a recurring theme in support groups for people in these situations.
Navigating the Fallout
If you find yourself in this situation, or you're just curious about how people handle it, there are a few things to keep in mind. It's not just about the "ick factor."
💡 You might also like: Anal sex and farts: Why it happens and how to handle the awkwardness
First, genetic counseling is a real thing. If a couple is worried, they can get screened. This isn't the 1800s; we can actually see if both people carry the same rare mutation for cystic fibrosis or something similar.
Second, legal protection matters. Because laws change at state lines, a couple married in New York might find their marriage isn't recognized if they move to a state where it's banned. This affects inheritance, medical power of attorney, and taxes.
Third, social isolation is a very real risk. Many couples in these relationships end up "closeted," never telling their friends or coworkers the full truth about how they met. That kind of secrecy can take a massive toll on a person's mental health.
Actionable Steps for Understanding
If you're looking into this because it's affecting your life or someone you know, stop relying on Reddit threads and look at the actual mechanics of the situation.
- Check the local statutes: Don't assume. Laws vary wildly. Look up "consanguinity laws" for your specific jurisdiction.
- Consult a geneticist: If children are a concern, a clinical geneticist can provide a much more accurate risk profile than a general practitioner. They look at your specific family history, not just general statistics.
- Evaluate the family dynamic: Ask the hard questions about what happens if the relationship fails. Is the family support system strong enough to survive it?
- Find specialized support: There are private forums and groups (like those often discussed by the National Society of Genetic Counselors) where people discuss the social and ethical navigations of consanguineous relationships without the immediate "shame" factor.
The reality is that cousins who have sex are navigating a world that isn't built for them anymore. While the biological risks are often exaggerated, the social and legal risks are very, very real. Understanding the difference between a "gut feeling" of disgust and actual scientific fact is the first step toward a rational conversation about a topic that usually triggers nothing but emotional outbursts.