Helen of Troy might have had "the face that launched a thousand ships," but that visage might not float anyone's vessel in Fiji.
It turns out that a person's bias toward highly feminine or masculine facial features in the opposite sex might have more to do with how many people live close by than with anything else, according to a study published this week in Proceedings of the National Academy of Sciences.
If these hypotheses are true, researchers surmised, then preference for caricatured features ought to be greater in less economically developed societies, which are under greater stress and privation. And everyone across cultures ought to associate masculine faces with aggression, the researchers reasoned.
And while the association between aggression and masculinity of features was detected across groups, it was strongest in the wealthier urban samples, the study found. Urbanization, in fact, was the variable that best predicted the strength of that association, according to the study.
Preference for strongly male or female features not only appears to be less "universal," according to the study, it may be new to human evolution - a product of densely populated settlement and the social demands it entails.
At the very least, the study suggests that some scientists trying to read the evolutionary meaning behind facial variety might just be staring at a mirror of their own culture.
Still, what happens on paper may not be what happens in bed.
Sheehan is interested in a different set of questions over a longer period of time: how human faces came to be so varied in the first place. A genetic study he published in Nature Communications last week suggests that difference is long-lived in our genome.
"The traits on the face should be less correlated with each other than they are for other parts of the body," Sheehan said. "If you tell me how widely spaced your eyes are, I cannot predict at all how long your nose is going to be. They are not related to each other." But width of a hand can reliably predict its length, he noted.
The researchers searched the human genome for clues about whether this high variability was a byproduct of other changes over evolutionary time, or selected by evolution. They found evidence of selective pressure.
Stretches of DNA associated with facial features were more more diverse than the overall genome or than stretches associated with other physical features. Mutations that popped up around these areas also showed unusual diversity.
Many studies have shown that humans developed a brain with unrivaled social cognition - an ability to distinguish identity and glean information from faces. The reciprocal effect may be just as profound, according to the study. Social evolution may have changed the very shape of our faces.
Scott's work, in fact, suggests sexual selection for facial features is unlikely to have been the same across cultures and over time.