Smiles Signal Surprise in a Social Dilemma

This study examines spontaneous facial expressions in an iterated prisoner's dilemma with financial stakes. Our goal was to identify typical facial expressions associated with key events during the interaction (e.g., cooperation or exploitation) and contrast these reactions with alternative theories of the meaning of facial expressions. Specifically, we examined if expressions reflect individual self-interest (e.g., winning) or social motives (e.g., promoting fairness) and the extent to which surprise might moderate the intensity of facial displays. In contrast to predictions of scientific and folk theories of expression, smiles were the only expressions consistently elicited, regardless of the reward or fairness of outcomes. Further, these smiles serve as a reliable indicator of the surprisingness of the event, but not its pleasure (contradicting research on both the meaning of smiles and indicators of surprise). To our knowledge, this is the first study to indicate that smiles signal surprise.