Schmidt, K.L., Ambadar, Z.,J.F. Cohn, and Reed, L.I. (2006).
Movement differences between deliberate and spontaneous facial expressions: Zygomaticus major action in smiling. Journal of Nonverbal Behavior 30 (1): 37-52.

Abstract
Previous research suggests differences in lip movement between deliberate and spontaneous facial expressions. We investigated within participant differences between deliberately posed and spontaneously occurring smiles during a directed facial action task. Using automated facial image analysis, we quantified lip corner movement during periods of visible Zygomaticus major activity. Onset and offset speed, amplitude of movement, and offset duration were greater in deliberate smiles. In contrast to previous results, however, lip corner movement asymmetry was not greater in deliberate smiles. Observed characteristics of deliberate and spontaneous smiling may be related to differences in the typical context and purpose of the facial signal.

Key words: emotion expression, nonverbal communication, voluntary movement.