According to Paula Niedenthal, a psychologist who heads up the Niedenthal Emotions Lab at the University of Wisconsin–Madison and has studied facial expression extensively, there are three types of smiles: those that express pleasure at a reward or surprise, like when you get to see your friends in person after a prolonged separation (soon, please); those that convey a desire to be friendly, or at least non-threatening, which she calls smiles of affiliation; and those that show dominance, like the one Dirty Harry gives when he asks a certain punk if he feels lucky.
Office of Strategic Communication