In a new study, people were more attracted to faces that resembled their own or that were preceded by a subliminal image of their opposite sex parent.
But rather than suggesting we all secretly want to have sex with our family members, the results instead point to the power of familiarity in shaping who we find attractive.
They also cast doubt on the idea that people have an innate repulsion toward incest. However, not all researchers are convinced the new results have such far-reaching implications.
Cultures around the world have prohibitions against incest, and for good reason. Inbreeding brings together rare mutations that can cause severe birth defects. In the 17th century, Charles II of Spain, the last of the Spanish Habsburgs — known for their inbreeding — was infertile and could not properly chew his food thanks to a congenital overbite.
For nearly 100 years, researchers have been split on the origin of incest taboos. In 1913, Viennese psychiatrist Sigmund Freud proposed they exist because we have incestuous urges that need to be repressed.
In contrast to Freud, the Finnish sociologist Edward Westermarck argued in 1891 that people have evolved a biological mechanism to avoid incest. According to Westermarck, people who grew up together would find each other unattractive.
For the past few decades, as Freud's influence has waned in psychology, researchers have tended to side with Westermarck. Some evolutionary psychologists have proposed that we subconsciously estimate the relatedness of other people, using cues such as whether we've played together and spent lots of time together. If the relatedness is too high, the thought of sex with the other person triggers "incest avoidance" mechanisms, better known as disgust.
Some evidence backed up the idea. People do express disgust at the thought of sex with a close family member. And a study of married couples in Taiwan found that couples who had grown up together in the same families were less attracted to one another and had fewer offspring than other married couples.
But other evidence indicated that people were actually attracted to mates who resembled their parents. In another study published in 2004 in the journal Proceedings of the Royal Society of London B, people shown images of women's adoptive fathers were able to guess the spouses of the women based solely on the spouse's appearance. In other words, the women married men who resembled their adoptive fathers.
"Westermarck's ideas suggested that certain factors, such as familial resemblance or growing up with someone, tend to inhibit attraction because they trigger evolved incest avoidance mechanisms," study researcher R. Chris Fraley of the University of Illinois at Urbana-Champaign told LiveScience. "However, a lot of data in social psychology suggest that these very same factors tend to facilitate attraction between people. We hoped to better understand why those two streams of thought were in opposition to one another."
Fraley and his colleague Michael Marks of New Mexico State University in Las Cruces conducted a series of experiments using college students. In the first experiment, students were asked to rate images of strangers for attractiveness, but unbeknownst to them, the images were preceded by subliminal snapshots of either the rater's opposite sex parent or someone unrelated to them. The students rated the images more attractive if they were shown an image of their opposite sex parent first.
In the second experiment, the researchers created the images by morphing different faces together. Some students were shown faces that were morphed with their own face by up to 45 percent. The more the face resembled their own, the more attractive they rated it.
In a final experiment, Fraley and Marks told some of the students they would be viewing images that were morphed with their own faces, which was a lie. The images were actually composites of other people. In that case, the students rated the images as less attractive than those not given that false bit of information.
The results argue against Westermarck's view, the researchers wrote in the July 20 issue of the journal Personality and Social Psychology Bulletin. If students were subconsciously evaluating the images for relatedness to themselves, the presence of their parent's face or elements of their own face should have triggered disgust.
Moreover, the students who were told they were looking at images of themselves should have rated those images more attractive than they did — because the images were actually unrelated to them.
But what about data such as those of the Taiwanese couples, who had grown up together and weren't so attracted to each other? Fraley and Marks wrote that familiarity may be most powerful when people don't know where it's coming from. When people are conscious of having spent lots of time together, as in a decades-long marriage, they may become habituated to each other. Or in other words, the passion dies.
Fraley said it's the mix of familiarity and novelty that seems to work best for attraction. "If a potential mate falls in between those two extremes," he said, "we are drawn to them."
Debra Lieberman, an evolutionary psychologist at the University of Miami who does research related to Westermarck's hypothesis, said she isn't so sure the studies are truly putting the hypothesis to the test. The Westermarck hypothesis deals with attraction to siblings, she said, so a better test would have been to use subliminal images of opposite sex siblings, not parents. "I don't think we are dealing with Westermarck at all," Lieberman said.
The results of the face-morphing experiments don't have to be explained in terms of attraction, she said. It could be that we use our kin to create a template of what constitutes a healthy face of the opposite sex, she said, and the experiment was testing that effect instead.
"It does not show we are attracted to family members," she said. "The Freudian stuff — it's hard to refute and it keeps coming back."