The face can communicate varied personal information including subjective emotion, communicative intent, and cognitive appraisal. Accurate interpretation by observer or computer interface depends on attention to dynamic properties of the expression, context, and knowledge of what is normative for a given individual. In two separate studies, we investigated individual differences in the base rate of positive facial expression and in specific facial action units over intervals from 4- to 12 months. Facial expression was measured using convergent measures, including facial EMG, automatic feature-point tracking, and manual FACS coding. Individual differences in facial expression were stable over time, comparable in magnitude to stability of self-reported emotion, and sufficiently strong that individuals were recognized on the basis of their facial behavior alone at rates comparable to that for a commercial face recognition system (FaceIt from Identix). Facial action units convey unique information about person identity that can inform interpretation of psychological states, person recognition, and design of individuated avatars.