Trent Eady: June 2009

As with many other realms of inquiry, relying on gut feelings or "ick factors" to decide moral verdicts is bound to be a mistake. It makes sense that our moral intuitions would lead us astray, since they evolved in a world radically unlike 21st century Earth. When you live in a small, nomadic band of hunter-gatherers, you have to play by different rules than when you live in a city of millions, in a world with biotechnology, globalization, nuclear weapons, and YouTube. Here's an example: I imagine it would be much easier for someone to kill ten people by pushing a button than by killing a single person by strangling or stabbing her to death. This is because when our moral minds were being shaped by natural selection, killing was something you did by brute, physical force; there were no lethal buttons to push or triggers to pull. Our brains make a vivid connection between strangling-to-death or stabbing-to-death and its moral implications, whereas with button-pushing or trigger-pulling the connection is remote and abstract.

It's always a bad idea rely on intuition alone to make a moral decision. While some moral intuitions may be sound, others (like the above example) are amazingly backward. We can never know which intuitions are the sound ones and which are the backward ones until we evaluate them rationally. We need to look under the hood so we try to "override" the gut instincts that would lead us astray with our rational minds. Many people have strong moral intuitions when it comes to sexual taboos like incest. It makes them squirm. The idea gives them a queasy, "icky" feeling. It feels wrong. They're likely to say call incest "totally disgusting" or something similar. (Explaining this sexual and moral disgust isn't too hard. It's probably a relic of our evolutionary heritage: nature selected for a strong psychological aversion to mating with relatives because mixing genes in this way tended to be more harmful to offspring than finding more distantly related mates.) But what is so bad about incest? We need to take a hard look at the incest taboo and we can't decide this verdict with our gut; we have to use thinking and evidence.

It's a tricky issue because of the number of scenarios to consider: siblings separated at birth who marry in adulthood, siblings raised together who are romantically attracted, teenage cousins who experiment with each other, a son who in adulthood has a consensual sexual relationship with his mother, etc. There are also the scenarios of rape, molestation, and abuse which we are rightly horrified by, but I don't think these need further consideration. It is the taboo against consensual incest that I want to examine. The distinction between genetic relatedness and "psychological relatedness" is critical. We'll start with the genetic side of things. I don't know how you could argue that a daughter nonreproductively sleeping with her biological father is somehow worse than sleeping with her adoptive father. Or if it helps to pump your intuitions, compare a daughter sleeping with her biological mother versus her adoptive mother. I can't think of any difference between the two scenarios. If there's no reproduction involved, genes don't mean a thing. In cases of (nonreproductive) incest where relatives are separated at birth and reunited obliviously, by accident in adulthood, what could it possibly matter that they have genes in common? The couple have some eerie similarities, but that's not necessarily going to prevent them from having a happy, healthy romantic relationship. (If they are reunited knowing they are related, a purported phenomenon called genetic sexual attraction may come into play.) Aside from the issue of reproduction, I think it's fair to say we should treat cases of purely "genetic incest" – where there is no psychological relatedness at all (unlike the example I just gave) – as we treat all other sexual relations and romantic relationships. Gay, lesbian, and infertile couples are off the hook entirely. But of course reproduction can't be ignored.

The only reason relatives having children might be morally wrong is through birth defects of genetic diseases that might come as a result of inbreeding. According to a Slate article by William Saletan, "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." (Saletan also observes that the great scientists Charles Darwin and Albert Einstein both married their first cousins.) The increase in risk is pretty small. Five U.S. states allow first cousins to marry, but only if one partner is sterile. To me, that seems excessive. Maine requires cousin couples to get genetic counselling before marrying. That's probably a better compromise. It's also a double standard, as Saletan observes, because we don't require genetic testing for people for high-risk categories like feritle women over 40, we don't ban people with genetic diseases from having children, and we allow blacks and Jews to marry within their ethnics groups even though it promotes sickle-cell anemia and Tay-Sachs disease.

The risk of a birth defect if you have a child with your sibling is much more substantial than in the case of coupled first cousins. We might be tempted to say sibling couples should need to be infertile or sterilized before they marry and only allowed to have adoptive children, but that takes us back to the double standard. What about people who have the same chance of passing on their genetic disease to their children as sibling couples have of producing a birth defect? No one is eager to advocate eugenics.

Things get a lot messier when you get into incest between people who are "psychologically" related, who fill a particular family role, whether or not they are genetically related. While I think the taboo against purely genetic incest is nonsense, I can imagine that consensual "psychological incest" might be harmful to those involved, depending on the scenario. When a parent uses her authority as a parent to convince her adult child to consent to sex, that's abuse. But what if it's the child's idea and both parties are equally enthusiastic? Is it still harmful then? I don't know. I don't know that it's a settled issue. On the flip side, there are instances of "psychological incest" (whether or not it is also genetic incest) that don't seem unhealthy to me and might even be beneficial, like the scenario of teenage cousins hooking up. Again, the psychological factors are much stickier than the genetic ones, so it's hard to say. It's important for us to recognize that there is nothing intrinsically wrong with psychological incest (take a moment to consider whether "intrinsic wrongness" is even a coherent idea) and that it can only be right or wrong based on its actual consequences. We should distinguish between different scenarios and consider factors that might make individual cases harmful or not. The real trouble with psychological incest is the overlap between different kinds of relationships. A romantic or sexual relationship will doubtlessly disrupt the familial relationship between two people. It will also create some tricky situations within the family.

This is Saletan's sagely warning:

This is the problem with sleeping with your cousin. You can move on from an ex-spouse or ex-lover, but there's no such thing as an ex-cousin. How are your parents and your ex's parents supposed to handle a nasty divorce or a breakup? How can they support their kids without antagonizing their siblings and their siblings' kids? You've wrecked your whole family. It isn't as bad as if you'd slept with a sibling, but it's a lot worse than if you'd slept with a friend or an officemate. We don't ban you from dating people at the office, but we don't tell you it's a great idea, either.

The issues are far from clear, but at least we can start the conversation.