Psychological researchers generally recognize that facial expressions reflect emotional states. In fact, various emotional states give rise to certain patterns of electrical activity in the facial muscles and in the brain. The facial-feedback hypothesis argues, however, that the causal relationship between emotions and facial expressions can also work in the opposite direction. According to this hypothesis, signals from the facial muscles (“feedback”) are sent back to emotion centers of the brain, and so a person’s facial expression can influence that person’s emotional state. Consider Darwin’s words: “The free expression by outward signs of an emotion intensifies it. On the other hand, the repression, as far as possible, of all outward signs softens our emotions.” Can smiling give rise to feelings of good will, for example, and frowning to anger?
According to the passage, what did Darwin believe would happen to human emotions that were not expressed?