Smiles when lying.

Subtle differences among forms of smiling distinguished when subjects were truthful and when they lied about experiencing pleasant feelings. Expressions that included muscular activity around the eyes in addition to the smiling lips occurred more often when people were actually enjoying themselves as compared with when enjoyment was feigned to conceal negative emotions. Smiles that included traces of muscular actions associated with disgust, fear, contempt, or sadness occurred more often when subjects were trying to mask negative emotions with a happy mask. When these differences among types of smiling were ignored and smiling was treated as a unitary phenomenon, there was no difference between truthful and deceptive behavior.