These days beards are in. There will come a day when beards are out. And a day when beards are back in again. That’s how it seems to go.
Turns out there is a name for this kind of thing: negative frequency-dependent selection. Essentially, the less common it is, the more useful a trait it is. As opposed to positive frequency-dependent selection which, as you can imagine, is the reverse.
According to this study, for example, when there are fewer men with beards in a population, they are judged as more attractive than when there are more of them. Hence they become gradually more popular until the effect starts to wear off and the clean-shaven look starts to appeal again. And round the cycle we go.