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.