Beards are everywhere these days. From the urban lumberjacks of Brooklyn to the hirsute hackers of San Francisco, men’s faces have taken a turn for the hairy. But according to one theory of evolutionary population dynamics, the look is destined to die down because of its own popularity. And now an experiment involving 3 dozen bearded men lends credence to the prediction.
For most biological traits—stronger wings for catching prey, longer legs for evading predators—it doesn’t matter how rare or common the trait is in a population of organisms. But sometimes it’s bad to have a trait that’s too popular. A classic example is the coloration of guppies, in which rare color variants are less likely to be noticed by predators. This gives the nonconformist fish a slight fitness advantage because predators focus on the most common hues, allowing the unorthodox coloration to be passed on and spread in the population. But once the odd coloration gets too common, the advantage disappears—predators start to chow down on the formerly cryptic fish and the cycle begins anew. This mechanism, called negative frequency-dependent selection, is one explanation for how diversity is maintained in populations despite natural selection constantly filtering for the fittest set of traits.
Facial hair is a trickier trait to explain than wings or fins, because rather than being determined early in life by genes, it is determined by behavior. In the case of beards, it is the decision to shave or not to shave. But the same logic can apply if the behavior has an influence on the choice of potential mates.
A team led by Zinnia Janif, an evolutionary biologist at the University of New South Wales in Sydney, Australia, realized that it wasn’t enough to just measure the attractiveness of faces with or without beards. As any biologist will tell you, beards are indeed sexy . The question is, does the sexiness of beards depend on the hairiness of the rest of the males in the population? Or is the allure of a beard the same no matter what?
To find out, the researchers recruited 36 men who were willing to grow beards. Then they took photographs of the men’s faces under identical lighting conditions at intervals: clean-shaven, light stubble (5 days), heavy stubble (10 days), and full hipster beard (4 weeks). They showed these photographs to 1453 women and 213 men—all of the women self-identified as either bisexual or heterosexual, and all of the men self-identified as heterosexual. The subjects scored the faces on an attractiveness scale commonly used in psychology experiments.
But there was a catch: The frequency of beardiness varied in each set of photographs, ranging from rare to common. Some subjects viewed sets of photos in which most of the men were clean-shaven, while others saw mostly the heavily stubbled or bearded versions, and others saw intermediate ranges of stubble and beardiness. If frequency-dependent selection plays no role in facial hair trends, the context shouldn’t matter.
But the context did matter. When facial hair was rare among faces, beards and heavy stubble were rated about 20% more attractive.  And when beards were common, clean-shaven faces enjoyed a similar bump, the team reports online today in Biology Letters. The effect on judgment was the same for men and women.
“This study breaks new ground,” says Peter Frost, an anthropologist at the Interuniversity Centre for Aboriginal Studies and Research in Quebec City, Canada. Although previous studies have shown that people prefer novelty for certain traits, such as the color of clothing, this study shows “that the novelty effect applies not only to colors but also to other visible features [of the body],” he says. But hipsters shouldn’t let their beards get too gnarly. “There are certainly limits to this effect,” Frost says. “Something can be novel but also disgusting.”