Facial Feedback Hypothesis

The hypothesis that facial expressions provide feedback to the responder that is necessary or sufficient to affect significantly his or her emotional experience and behavior.

Reference:
Buck, R. (1980). Nonverbal behavior and the theory of emotion: the facial feedback hypothesis. Journal of Personality and Social Psychology, 38(5), 811-824. doi: 10.1037/0022-3514.38.5.811.