Saturday, April 02, 2005

Genetic Sexual Attraction and Incest Avoidance

Most people find incest gross. But if you ask them about incest in general, they will be less grossed out than if you ask them having sex with one of their own relatives. This fact gets to the heart of incest avoidance--the tendency of animals (including people) from the same family to avoid sexual contact with each other.

There are many theories to explain this well documented behavior. Some are sociological. Some are biological. But no matter what theory you may have as to why incest avoidance may have proven beneficial (fewer birth defects, or lots of advantageous, alliance building marriages to members of other families and clans), you still need an actual mechanism that explains exactly what makes individuals from the same family avoid each other later as sexual partners.

One line of thinking has to do with the ability of mammals to smell each other. In particular, there is evidence that we can subconsciously smell each other's histocompatibility antigens. These antigens are chemical tags onto which invading pathogens (bacteria, viruses) can attack your cells.

A good evolutionary strategy is to have kids with someone whose antigens are different from your own. That way, you'll be spreading your children's risk with respect to germs. By giving them lots of different antigens (which you will do if you mate with someone whose antigens are very different from your own), you'll be making it harder for any one virus to come in and easily take over all your kid's cells. By contrast, if you mate with a near relative, your kid will have a very limited set of antigens.

So one line of thinking about incest avoidance is that we avoid our relatives because they smell like we do--that is they have antigens that are so similar to our own that we know they are kin because they smell like us. This explanation has significant attractiveness because it is known that animals (including female humans) do in fact prefer mates with histocompatibility antigens very different from their own.

Another line of thinking is that the actual mechanism for incest avoidance is simple contact. In particular, you don't want to fuck as an adult anyone you were in contact with as a child. For instance, if you were raised by your biological parents, then you'll avoid them later on for partners. But it also goes for step parents and adoptive parents. Whoever you were around--be they actual blood relatives or not--you'll want to avoid. Good evidence for this comes from the fact that children raised together en masses in group homes for children on Israeli Kibbutzes almost never married anyone who lived with them in the same dormitories growing up. It didn't matter whether they were siblings or not. What seemed to matter was simple proximity growing up.

A very interesting paper in the proceedings of the Royal Society (London) presents evidence in favor of the raised-together mechanism for incest avoidance, and thus against the antigens mechanism. The three authors find that the longer kids were raised together, the more grossed out they are later on by the thought of sex with each other. Of particular importance, this is true no matter whether the kids were in fact related to each other. That is, there is not so much incest avoidance as there is avoidance of having sex with anyone you grew up with (and by grew up with, I mean lived with in the same residence.)

This research is also very interesting in light of a phenomenon that has come to be called genetic sexual attraction (GSA). This term refers to the fact that siblings and parents separated by adoptions many years before often find themselves hugely sexually attracted to their relatives later on if there is a reunion (as between a young man and his biological mother whom he hasn't seen in 25 years because she gave him up for adoption when he was an infant.)

This appears to be a big problem and most of the big adoption websites now have a page or two devoted to warning people about it before they go through with a reunion. Here's an article from the London Guardian about GSA. And here and here are some links to adoptions websites' warnings/suggestions. What is fascinating here is that GSA seems to be totally consistent with the idea that unless you grow up with someone, you are not going to avoid them later on as a sexual partner. In fact, what seems to go on with GSA is that when reunited, people find that they have huge amount in common and lots of shared interests and look alike, etc etc. All things that are attractive. And given that they have no tendency to avoid each other (since they didn't grow up around each other), sex is an natural outcome. Also check out the very candid site run by Barbara Gonyo, another who experienced GSA.

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