“Results suggest that facial hair is worn to enhance a man’s marriage prospects by increasing physical attractiveness and perception of social status. Men shave their mustaches, possibly to convey an impression of trustworthiness, when the marriage market is weak and women might fear sexual exploitation and desertion.”
— A new study shows that facial hair “increas(es) physical attractiveness and perception of social status.” (via Andrew Sullivan)
I mean, they used a regression. What more evidence do you need?
Also, be sure to check out this diagram, which explains everything.