Media playback is unsupported on your device Media caption Have we reached 'peak beard'?

The ebb and flow of men's beard fashions may be guided by Darwinian selection, according to a new study.

The more beards there are, the less attractive they become - giving clean-shaven men a competitive advantage, say scientists in Sydney, Australia.

This might be why we've hit 'peak beard' Prof Rob Brooks, University of New South Wales

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.

Image copyright Getty Images Image caption 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.

Luckily in real life, we never mate with an average. We mate with an individual Prof Rob Brooks, University of New South Wales

"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.

Some say the Rubicon was crossed in January when Jeremy Paxman, the BBC Newsnight presenter, shaved his beard off, saying "beards are SO 2013".

Paxman's beard - which briefly trended on Twitter - sparked a debate about pogonophobia - the fear of beards.

Media playback is unsupported on your device Media caption Jeremy Paxman's beard briefly trended on Twitter

In this latest experiment, 1,453 women and 213 men were asked to rate the attractiveness of different samples of men's faces.

When Greece's economy tanked - did beards take off? That's something we're going to look at Prof Rob Brooks, University of New South Wales

Some 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. 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.

"There is a wonderful paper studying photographs of men from 1871 to 1972 in the Illustrated London News. Sideburns moved on to moustaches, then full beards.

"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."

Image copyright UNSW

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.

With female hair colour, there is speculation that red, brown and blonde spread via their novelty Prof Rob Brooks, University of New South Wales

"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.

"After the Wall Street Crash in the 1920s there is some circumstantial evidence that beards got big again. So maybe economic conditions have set the stage for the recent comeback in beardedness.

"When Greece's economy tanked - did beards take off? That's something we're going to look at."

One of the paradoxes of evolution is that genetically strong traits favoured by one sex do not simply become fixed in the other - a level of diversity is often maintained.

Though beard styles are of course not spread via genes, there may be other visible human traits which are.

"With female hair colour, there has been speculation that red, brown and blonde spread via their novelty - but the evidence is very ambiguous," Dr Brooks told BBC News.

His team 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.

"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."