The facial feedback hypothesis is the idea that your facial expressions can have an effect on your emotional experiences. This idea was first proposed by Charles Darwin, who suggested that physiological changes that were caused by emotions could also cause emotions.
As it developed as a hypothesis, two possible variations began to appear. The first, which was proposed by Darwin and later supported by a study by McCanne and Anderson, suggested a weak correlation between physiological facial movements and emotional responses. Rather than causing an emotion, physiological changes can only suppress or excite emotions that are already present. McCanne and Anderson's study involved instructing test subjects to vary their muscle tension while looking at unpleasant scenes. A slight emotional change due to facial expressions was verified.
The other variation of the theory is that facial feedback itself can create an emotion. This has not been verified to the same extent that the first variation has.
Most of the support for the facial feedback hypothesis comes from studies done in the late 1900s, including studies by Ekman, Levenson and Friesen in 1983 and Lundqvist and Dinberg in 1995. Both of these studies involved measuring subtle changes in expression and the subsequent changes in emotion.