The phrase facial expression is not a neutral description of a class of behavior but an assumption. The assumption, with us since Homer and now part of our folklore, is that human faces express something, presumably from our inner selves, presumably emotions.

Put in more modern terms, certain facial movements are said to signal emotions. By the 1980s, psychologists had largely accepted as a “fundamental axiom of behavioral science” the link between faces and emotions. Emotion therefore explains facial behavior, and facial behavior is an objective index of emotion. On one major theory, facial expressions determine the emotion we feel. Encouraged by the writings of Charles Darwin, eminent researchers such as Carroll Izard and Paul Ekman had developed a closely related set of theories, methods, and evidence that together form a “Facial Expression Program.” This program stimulated a tremendous amount of valuable research on the face and revitalized the study of emotion in general. It provided the textbook account of facial behavior and guided the conduct of most available research on facial behavior.

In the last several years, there have appeared doubts about the program's fundamental assumptions, its methods, and its evidentiary base. Counterevidence has surfaced. Lack of evidence on some of its assumptions has become increasingly noticeable. As alternative views have appeared, advocates of the Facial Expression Program have revised some parts of it and vigorously defended others.