Although cultural wisdom warns 'don't judge a book by its cover,' we seem unable to inhibit this tendency even though it can produce inaccurate impressions of people's psychological traits and has significant social consequences. One explanation for this paradox is that first impressions of faces overgeneralize our adaptive impressions of categories of people that those faces resemble (including babies, familiar or unfamiliar people, unfit people, emotional people). Research testing these 'overgeneralization' hypotheses elucidates why we form first impressions from faces, what impressions we form, and what cues influence these impressions. This article focuses on commonalities in impressions across diverse perceivers. However, brief attention is given to individual differences in impressions and impression accuracy.