A theory on emotion by two independent psychologists William James and Lange in the year 1880s. The theory assumes that bodily changes occur when a stimulus is perceived and awareness of these bodily changes makes the emotion to be experienced (or felt).
Historically, James is the first scholar to formulate this theory. Independently, Lange postulated a very similar theory to that of James. Hence, this theory has been called as James-Lange theory.
This theory was the first to contradict the old popular notion wherein the perception of the stimulus produced the experience of emotion, which in turn, brought about the physiological changes.
Many studies then followed to expand the understanding on emotion. One of which is the study done by Walter Cannon, who formulated a theory as an argument to the theory of William James.
Compare: Hypothalamic theory