Well, that was easy. Given that sexual reproduction evolved specifically to introduce some genetic diversity to future generations in order to remain ahead of the more quickly evolving parasites (Ridley, 1993), the suggestion that humans would also have some adaptations that predisposed them to breed with their immediate relatives seems misguided. Freud - I'm told - had suggested that children really did want to have sex with their parents, and it was only through imposition of a cultural taboo against incest that such drives were thwarted. It's just one of the many things he was wrong about.
"I don't always talk about your mother, but when I do....wait, never mind; I do always talk about your mother"
Might there have been something to that notion of Freud's though? No. Go read the introduction again if you're still confused on that point. However, there is at least one recent research paper in which the authors suggest that there may in fact be some forces at work that generate sexual attraction to closely related family members that a societal taboo is needed to stand in the way of. In a series of three experiments, Fraley & Marks (2010) attempt to demonstrate that possibility.
In the first experiment, subjects were either primed with a picture of their opposite sex parent, or were controls that were unrelated to that parent. Subjects were then asked to rate a few pictures of opposite sex strangers for their sexual attractiveness. The results showed a slight tendency for those who saw a picture of their parent to rate others as more attractive (a difference of about 0.2 on a scale of 1 to 7). The second study went a bit deeper. This time, participants had their own face morphed from 0 to 40% with those of opposite sex strangers and rated the new photos for attractiveness; the control group rated the same pictures, but were not the person being morphed into the photos. The results showed a similar pattern: there was a slight tendency for people who's faces had been morphed into the photos to rate them as more attractive (a difference of about a 0.4 on the same scale), relative to the controls. Finally, in the third experiment, the researchers lied to the participants about how much of their face had been morphed into the photos and mentioning the study was examining incestuous tendencies. This time, the effect reversed; participants rated the pictures with self-morphs as being slightly less attractive, relative to controls.
So where does that leave us?
Hiding in our closet, aka "The Shame Cave"?
Are we to admit Freud was onto something? No, and stop asking that silly question. Since I'm a big fan of theory, naturally my first question was: what was theory guiding this research? According to Fraley & Marks, the following findings need an explanation: (1) people tend to enter into relationships with others who are similar physically on a variety of traits, (2) that people tend to enter into relationships with those who live around them and are familiar, and (3) people find those who they are exposed to more frequently more attractive than those they're exposed to less frequently. However, those three findings do not a theory make; they need a theory to explain them, preferably one that doesn't cut again incest avoidance. Here's a simple and probable one that accounts for at least part of the picture here: sexual selection.
Take any species; since I like peacocks, I'll use them. When mating season roles around, the peacocks flaunt for the peahens, they have steamy bird sex, and soon after a new generation of birds are hatched into this world. The peacocks will inherit their father's sexy tails, and the peahens will inherit something else: their mother's preferences for those sexy tails. If those sexual preferences weren't inherited, mating in the next generation would be random with respect to the tails. Since it isn't, we can safely assume that, to at least some extent, the preferences are hereditary.
Just like the preference for hunting equipment is. I'm a shotgun man myself.
So let's return to the facts in need of an explanation. Picture your mother and father having sex to conceive you - make Freud proud. Whatever physical traits your parents had will be passed onto you. Additionally, whatever preferences your parents had for those traits that attracted them to each other will be passed on as well. That would seem to be able to explain (1) and the results of the photo manipulation study fairly well. By morphing in your own traits to the picture to some degree, you're morphing in those same traits that you're going to tend to have a preference towards. The result? You find those pictures slightly more attractive.
How about the first experiment that primed pictures of the parents? It seems at least plausible that if one truly found their opposite sex parent attractive, ratings of strangers would go down by comparison, not up. Concluding that one found strangers more sexually appealing because of that sexual aversion to their parents would be just as consistent with the data; at the very least, it can't be ruled out by the results found here. As for the third experiment, admitting a sexual attraction to one's own family can be quite socially damning, so it hardly seems surprising that people would avoid doing so.
"You look just like my sister and that is so hot! Would you mind wearing her clothes?"
Now I want to look at how the authors explain their results. Fraley & Marks (2010) suggest the following:
...the mechanisms that promote familiarity, bonding, and attraction are most likely to operate on inputs experienced in the early family environment. For example, if sexual imprinting really takes place in humans, then one’s early interactions with primary attachment figures can play an influential role in shaping the “ideal” for what kinds of people one will find attractive...A tempting suggestion for some, no doubt, until one asks some perfectly relevant questions, like: why would the sexual imprinting take place during early interactions in childhood? Why would the stimulus that the imprinting responds to be the caregivers in the house (especially them, given the costs of inbreeding), as opposed to the environment outside the family? Combining the two questions gives us the following: Why would anyone suppose evolution had designed our psychology to become sexually attracted (in the long term) to the physical traits of our close genetic relatives at a time that we are pre-reproductive? Frankly, I can't think of a reason we would expect that to happen, and one isn't suggested in the paper.
On the same token, Fraley & Marks (2010) go on to suggest that the aversion to incest is simply a matter of habituation - as opposed to the Westermarck effect - but again offer no reason as to why habituation would have this particular effect. At the same time, habituation would also seem to make people more attractive the more familiar they were, according to the author's interpretation of their work, and while Fraley & Marks (2010) note this contradiction, they don't do a good job of explaining it away. They try to draw on some kind of distinction between the conscious and unconscious recognition of the familiar, but I don't think they make a case for it.
On the whole, that is a very unsatisfying explanation, especially compared to other models of incest aversion. Point: Westermark. Freud is still wrong.
References: Fraley, R.C. & Marks, M.J. (2010). Westermarck, Freud, and the incest taboo: does familial resemblance activate sexual attraction? Personality and Social Psychology Bulletin, 36, 1202-1212
Ridley, M. (1993). The Red Queen: Sex and the Evolution of Human Nature. Harper: New York.