An important question in biology is whether a particular function or ability is the result of evolution or an accidental byproduct of it. Some biologists believe that human perception falls into this second category because there has been little evidence that how we perceive things like faces affects our biological success in ways that are selected for or against.
But the evidence is mounting that evolution has conditioned our perception in subtle ways.
Claire Conway and colleagues at the University of Aberdeen in Scotland, UK, paired nearly identical photos of computer-generated faces, with smiling or disgusted expressions. The pair differed only in where the irises were pointed: straight at the viewer, or off to the side (see image top right).
Several hundred Aberdeen undergraduates, in the lab and online, rated the faces for sexual attractiveness, and for likeability, a sexually neutral quality. Both men and women found faces looking straight at them to be more attractive and more likeable, even if the faces looked disgusted though unsurprisingly, there was a greater preference for smiles.
But when the viewers were rating the faces for attractiveness, the preference for being gazed at directly by smiling eyes was much greater for faces of the opposite sex, especially when they were rated by men. There was no such sexual bias in the preference for a direct gaze when the students rated disgusted-looking faces, or when they were rating any faces for likeability.
The Aberdeen team says the sexual bias in subjects' perception of sexual attractiveness in a direct, smiling gaze is hard to explain as a functionless byproduct of perception. But it could have evolved to ease the effort of mating, by directing efforts towards people who are already expressing an interest.
The idea that evolution played a role in determining our facial preferences is backed up by other work, such as research showing that perceptions of attractiveness change depending on peer pressure, or even the time of the month.
What about cultures, common in Asia, where gazing directly at someone is rude? “The Asian participants [in the study] demonstrated preferences for direct over averted gaze,” Conway told New Scientist. But these are private preferences, she cautions. “Whether or not such preferences are also expressed in public situations we don't know.”
Journal reference: Proceedings of the Royal Society (DOI: 10.1098/rspb.2007.1073)