The more beards there are, the less attractive they become - giving clean-shaven men a competitive advantage, say scientists in Sydney, Australia.
When "peak beard" frequency is reached, the pendulum swings back toward lesser-bristled chins - a trend we may be witnessing now, the scientists say.Their study has been published in the Royal Society journal Biology Letters.
In the experiment, women and men were asked to rate different faces with "four standard levels of beardedness".
Both beards and clean-shaven faces became more appealing when they were rare.
The pattern mirrors an evolutionary phenomenon - "negative frequency-dependent sexual selection", or to put it more simply "an advantage to rare traits".
The bright colours of male guppies vary by this force - which is driven by females' changing preferences.
Scientists at the University of New South Wales decided to test this hypothesis for men's facial hair - recruiting volunteers on their Facebook site, The Sex Lab.
Hirsute film stars George Clooney and Ben Affleck were said to have fuelled the beard boom "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," said Prof Rob Brooks, one of the study's authors.
"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 band wagon the value of being on the bandwagon diminishes, so that might be why we've hit 'peak beard'."
"Peak beard" was the climax of the trend for beards in professions not naturally associated with a bristly chin - bankers, film stars, and even footballers began sporting facial hair.
Some argue the Rubicon was crossed in January, when Jeremy Paxman, the BBC Newsnight presenter, shaved his beard off, saying "beards are SO 2013".
In this latest experiment, 1,453 women and 213 men were asked to rate the attractiveness of different samples of men's faces.
Jeremy Paxman's beard briefly trended on TwitterSome were shown mostly "full" beards. Others were shown mostly clean-shaven faces. A third group were shown an even mixture of all four varieties - clean-shaven, light stubble, heavy stubble and full beard.
Both women and men judged heavy stubble and full beards more attractive when they were rare than when they were common.
Likewise, clean-shaven faces were least attractive when common and more attractive when rare.
"Negative frequency-dependent preferences may therefore play a role in maintaining variation in men's beards and contributing to changing fashions," the researchers conclude.
They plan to continue their pogonophilic investigations and are looking for volunteers for their latest experiment testing how people like faces with varying levels of beardedness.
