Spoiler: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.
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".
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.
"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 bandwagon 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.
Both women and men judged heavy stubble and full beards more attractive when they were rare than when they were common. And likewise for clean-shaven faces.
Negative frequency-dependent preferences may therefore contribute to changing beard fashions, Prof Brooks concluded.
"We know beards go through cyclical fashions. People used to speak about a 30-year timescale," he said.
"In the 1970s it was handlebar moustaches. In the 80s it was Magnum PI moustaches. In the 90s we saw a lot of clean shaven men, and now big bushy beards are back."
The recent boom may have its roots in the financial crisis of 2008, Prof Brooks suggests.
"I think one of the reasons beards have made a comeback now is that it's a difficult time.
"Young men are competing to attract someone when work is not easy to come by. So we might expect some aspects [of masculinity] to get turned up to eleven.
"Heavy stubble seemed to be the best in our last study. Maybe a 5-10 day growth. But those describe average tendencies," he said.
"Luckily in real life, we never mate with an average. We mate with an individual."
So beards are in - or are they out? Do you agree with the premise of the article, that facial hair tastes are guided by evolutionary forces? What do you think are the major forces this time around - the contradiction between wanting to look unique yet popular? Amping up the masculinity in tough economic times?
What do you think about fashion tastes in general? Do you think people spend too much time and thought on how they look? What do you think about our desire to be unique or speshul? Do we take it too far at times?
My own afterthoughts: the paradox in wanting to be unique yet popular is very interesting. I think it's been socially symbolized in the hipster subculture - the mainstreaming of the non-mainstream.
excerpts taken from: https://www.bbc.com/news/science-environment-27023992