5. He Looks Insta-Masculine
Sounds like a cliche, but there's legitimate research behind this: a study published in Evolution and Human Behavior proved that the more facial hair a man has, the more masculine both men and women perceive him to be. So if you're into the burly, testosterone-driven type, he should skip the razor for 10 days — that's the exact length of time the study found that women find men the most attractive.
[Photo via @fabiannordstrom]