In 1989, a psychologist named Robert Zajonc published one of the most significant studies on the emotional effect of producing a smile.
His subjects repeated vowel sounds that forced their faces into various expressions. To mimic some of the characteristics of a smile, they made the long "e" sound, which stretches the corners of the mouth outward. Other vowel sounds were also tested, including the long "u," which forces the mouth into a pouty expression.
Subjects reported feeling good after making the long "e" sound, and feeling bad after the long "u."
Other studies reported similar results. One had subjects make the positive and negative expressions by holding a pen in their mouths, either protruding outward for a pout or held lengthwise in the teeth to make a smile. In another, researchers had subjects mimic each physiological trait of a smile until their faces were in a full Duchenne expression.
In yet another experiment, one group of subjects was shown pictures of various facial expressions; another group made those facial expressions and a final group made those expressions while looking in the mirror.
The evidence all points toward smiling as a cause of happy feelings. Subjects were asked questions that pinpointed their emotional state before and after smiling, and they overwhelmingly scored happier after smiling. In the study involving the mirror, subjects who watched themselves smile saw an even more pronounced change in mood than those who smiled without the mirror, and the subjects who merely looked at pictures didn't experience that change at all.
Those researchers hypothesized that self-consciousness is a factor in the effect -- that introspective people might experience a greater smile-related mood lift than those who are less aware of their feelings. Thus the mirror-related boost. But what about the difference between those who looked at pictures and those who created the expressions? Why would the people who put their faces into a smile feel happier afterward?
Most other studies on the topic note the cause-and-effect relationship without having a definitive explanation for it. The reason why Dr. Zajonc's research is so significant in the field is because he proposes a detailed, physiology-based explanation for the cause-and-effect relationship. According to his hypothesis, the facial changes involved in smiling have direct effects on certain brain activities associated with happiness.