Here's a disturbing thought: humans are sexually attracted to -- guess who! -- themselves. At least that's one of the conclusions you could draw from a 2010 study conducted by psychologist R. Chris Fraley at the University of Illinois. Fraley told Wired magazine, "People appear to be drawn to others who resemble their kin or themselves. It is possible, therefore, as Freud suggested, that incest taboos exist to counter this primitive tendency."
See, Sigmund Freud thought that people's aversions to mating with close family is a social -- not biological -- construct. By his train of thought, we're all innately drawn to ma, pa and siblings, and the only reason we don't act on those impulses are generations and generations of adhering to social rules against incest.
Fraley thinks that Freud might've had a good point about incest taboos since a series of experiments indicated that we might be subconsciously attracted to ourselves. In one, he showed participants a series of photos and asked them to rate the sexual attractiveness of the photographed subjects. What half the participants didn't know was that they were being exposed to images morphed with their own faces. Funnily enough, they consistently rated the photos that most closely resembled themselves as more sexually attractive. Yet when Farley told another group of participants that they were looking at photos morphed with their own faces, they downgraded the attractiveness rating.
In other words, we find ourselves irresistibly attractive -- until we realize that we're looking at ourselves. Once that recognition kicks in, some sort of incest taboo, whether social, biological, or both, suppresses that urge.
Things get creepier, too, when we begin investigating whether we're subconsciously attracted to our parents. An uncomfortable number of studies show that we just might be. Intrigued (and terrified)? For more on this fascinating topic, give a listen to "Do we marry our parents?" on Stuff Mom Never Told You.