Doctoral candidate Tara Deliberto, who is training as a Cognitive-Behavior Therapist, wrote on her blog about consanguinamory, Genetic Sexual Attraction, the Westermarck Effect, and the Oedipal Complex.
Genetic sexual attraction occurs when genetic relatives meet for the first time in adulthood and an attraction develops. Although it is a rare occurrence, there has been an increase in the number of reported cases in recent years, typically as a consequence of adoption (according to Wikipedia).
GSA is not rare when it comes to situations of post-pubescent close genetic relatives meeting either for the first time or after being separated since at least one of them was under the age of 7. It happens in up to 50% of such situations; at least one of those people feels an overwhelming attraction to the other or one of the others. Sometimes, it is mutual.
But what about close relatives who have been around each other?
The Westermarck effect occurs for most people, but there might be a genetic reason why it doesn't kick in for some.
Most men are attracted to women and many of those men are averse to thinking about sex with other men. Some aren’t. And if there is a genetic reason why the Westermarck Effect isn’t dominant in someone, wouldn’t it make it more likely that a close genetic relative wouldn’t have that effect dominant in them, either?
So while there are probably genetic components coming into play when we see a lack of Westermarck effect, I'm guessing there might be some environmental components at work as well.
The old “nature vs. nurture” thing.
I've certainly come across research saying that people tend to pair off who are similar across many different areas. As you've probably experienced, bonding can occur when you have similar thoughts to someone else. Sure it would be boring if you were exactly the same, but I'm guessing you're probably more similar to the people you're close with than different.
Basically, if you're genetically similar to someone that you meet in adulthood, you could actually be very similar to them, being that genetics play such an important role in personality formation.
Good observation. It matches the experience of many people in GSA relationships.
With donated eggs/sperm/embryos and infant adoptions and with everything from one night stands that result in pregnancy to bitter divorces between parents, the number of people experiencing GSA is growing fast.
And while most people raised together apparently experience the Westermarck Effect, for whatever reason, it isn't present or isn't strong enough in some situations to overcome the physical attraction, convenience, and existing emotional bonds and trust that foster everything from experimentation to lifelong spousal-type relationships between close relatives.
We need more research into GSA and consanguinamory. Otherwise, therapists, counselors, social workers... you name it... are not going to be able to handle the ever-growing number of people who will be having problems related to these issues (many of which, by the way, will not be about the relationship or sex itself, but about the laws and prejudices intruding from outside of the relationship.)