Beards may have once served an evolutionary purpose, providing men warmth and protection against the elements, but today they're mostly a fashion choice. Right now, they're a pretty popular fashion choice. And evolutionary biologists at the University of New South Wales think they know why. Beard trends follow an evolutionary principle known as "negative frequency-dependent sexual selection," whereby rare traits become more attractive to the opposite sex. The basic principle: the fewer beards, the more attractive they're perceived. And a trend is born -- at least until beards become commonplace. "Big thick beards are back with an absolute vengeance and so we thought underlying this fashion, one of the dynamics that might be important is this idea of negative frequency dependence," Professor Rob Brooks, a co-author of the new study, told BBC News. "The idea is that perhaps people start copying the George Clooneys and the Joaquin Phoenixs and start wearing those beards, but then when more and more people get onto the bandwagon the value of being on the bandwagon diminishes, so that might be why we've hit 'peak beard'." According to the new study, which was published this week in the Royal Society journal Biology Letters, so-called "peak beard" -- or beard equilibrium -- may be approaching. In other words, clean-shaven could be the new go-to look. Evolution isn't just about survival; it's also about sex, and attracting a mate can be difficult. One way to do it, nature has shown, is to stand out. The many bright colors of male guppies, one of the world's most common tropical fish, vary wildly, as males evolve to appease females' changing preferences for vivid pigment. If too many guppies take on orange and turquoise, for example, that combination can become passé. As reproductive competition heats up, guppies will evolve toward a new wardrobe in order to gain advantage. And the cycle continues. In a recent social experiment, researchers found that facial hair trends mimic the same evolutionary pattern. When asked to judge the attractiveness of faces, surveyed men and women found heavy stubble and full beards more attractive when they were rare, and less so when common. The authors concluded: "Negative frequency-dependent preferences may therefore play a role in maintaining variation in men's beards and contributing to changing fashions."