Courtesy of the Nathan Yau’s Flowing Data blog, here is a fascinating look at the prevalence of different types of facial hair from 1840 through the early 1970s:
This is so interesting! I naively assumed that male facial hair fashions came and went through the decades, but here you can plainly see that whiskers have been on a secular decline since a peak that occurred in about 1885. What happened? Was it an improvement of razor technology? Indoor plumbing? It’s incredible that almost 100% of men had facial hair at the turn of the century.
Other mysteries beckon. What’s going on with mustaches in the 19-teens? They shoot up to a peak and then decline almost as quickly. Does this have anything to do with Charlie Chaplin?
And how were these data collected? The only way I can think of is by analyzing old photographs. But wouldn’t that introduce a selection bias? Also, are these data from the U.S.? Europe? What would a worldwide whisker time series look like?
This graph comes from a scholarly article, and I’m sure it sheds some light on these questions. But I have not read it. As interesting as it must be, I just don’t have time to read journal articles about facial hair.
Finally, there’s the question about what has happened since the 1970s when this was published. We all know that beards are enjoying a bit of a fashion renaissance (although I do suspect that their popularity has reached an inflection point, if not a local maximum). But do they approach the lofty highs of the 1890s?
Maybe in Portland:
geovisualist 
As with predictions of peak oil, such conclusions are fraught with peril. Whiskers may be a random walk that only an expanded time frame can capture. Include middle eastern and Indian stats and everything will shift around. But for my money, there is no likely resurgence of whiskers in the future.
My hypothesis is that facial hair declines as access to warmth increases. An interesting way to test this would be to look at geospatial prevalence of facial hair over time. If I’m right, then I bet facial hair in Alaska and North Dakota have a higher occurrence than Louisiana or Alabama.
That seems like a good theory to me. Is that duck dynasty guy the exception that proves the rule?