Predictive and reactive mechanisms in smile reciprocity.

During face-to-face interactions, people reciprocate their conversation partners' genuine and polite smiles with matching smiles. In the research reported here, we demonstrated that predictive mechanisms play a role in this behavior. In natural interactions (Study 1), participants anticipated a substantial proportion of genuine smiles but almost no polite ones. We propose that reinforcement-learning mechanisms underpin this social prediction and that smile-reciprocity differences arise because genuine smiles are more rewarding than polite smiles. In Study 2, we tested this idea using a learning task in which correct responses were rewarded with genuine or polite smiles. We measured participants' smile reactions with electromyography (EMG). As in natural interactions, people mimicked polite smiles reactively, after seeing them appear. Interestingly, the EMG data showed predictive responding to genuine smiles only. These results demonstrate that anticipating social rewards drives predictive social responding and therefore represent a significant advance in understanding the mechanisms that underpin the neural control of real-world social behavior.