Reinforcing, Reassuring, and Roasting: The Forms and Functions of the Human Smile

What are facial expressions for? In social-functional accounts, they are efficient adaptations that are used flexibly to address the problems inherent to successful social living. Facial expressions both broadcast emotions and regulate the emotions of perceivers. Research from my laboratory focuses on the human smile and demonstrates how this very nuanced display varies in its physical form in order to solve three basic social challenges: rewarding others, signaling non-threat, and negotiating social hierarchies. We mathematically modeled the dynamic facial-expression patterns of reward, affiliation, and dominance smiles using a data-driven approach that combined a dynamic facial expression generator with methods of reverse correlation. The resulting models were validated using human-perceiver and Bayesian classifiers. Human smile stimuli were also developed and validated in studies in which distinct effects of the smiles on physiological and hormonal processes were observed. The social-function account is extended to the acoustic form of laughter and is used to address questions about cross-cultural differences in emotional expression.