According to the RBF (Resting Bitch Face) theory, if a woman is caught in contemplation (i.e., not smiling), people are more likely to think she’s angry than if a man shows the exact same facial expression. A woman who doesn’t smile is assumed to be in a bad mood because, so the theory goes, women are expected to smile at all times. Through cultural conditioning, women have learned that in order for people to like them, they have to wear a smile even if they don’t think anything is particularly funny. Men who look thoughtful are seen as serious; women with the same expression are perceived as unfriendly and unlikeable.
– Susan Krauss Whitbourne