Proverbs 15:13 "When people are happy, they smile, but when they are sad, they look depressed." GNTDI love reading Proverbs, but I don't know if the physicality of a smile is that simple. Smile=Happy, Sad=Frown. People tend to be very "poker-faced" these days and in this society. And it's true that people smile for all sorts of reasons, only one of which is to signal happiness. Ekman describes many kinds of smiles, from the "felt" or true smile to the fear smile, the contempt smile, the dampened smile, the miserable smile, and a number of others.
Daniel McNeill, author of The Face: A Natural History, says, Whatever their origin or motivation, smiles have a powerful effect on us humans. As McNeill points out, "Though courtroom judges are equally likely to find smilers and nonsmilers guilty, they give smilers lighter penalties, a phenomenon called the 'smile-leniency effect.'"