Are You Smiling, or Have I Seen You Before? Familiarity Makes Faces Look Happier.

It is clear that unreinforced repetition (familiarization) influences affective responses to social stimuli, but its effects on the perception of facial emotion are unknown. Reporting the results of two experiments, we show for the first time that repeated exposure enhances the perceived happiness of facial expressions. In Experiment 1, using a paradigm in which subjects' responses were orthogonal to happiness in order to avoid response biases, we found that faces of individuals who had previously been shown were deemed happier than novel faces. In Experiment 2, we replicated this effect with a rapid "happy or angry" categorization task. Using psychometric function fitting, we found that for subjects to classify a face as happy, they needed less actual happiness to be present in the face if the target was familiar than if it was novel. Critically, our results suggest that familiar faces appear happier than novel faces because familiarity selectively enhances the impact of positive stimulus features.