Modern proponents of an evolutionary theory maintain that emotions have rigidly fixed biological pathways that have been inherited from distant ancestors. Different emotions all have different physiological patterns that vary little from one member of the human species to another.
This theory, named after its creators William James and Carl Lange, puts forth the idea that emotions stem from basic biological responses to aversive stimuli. This concept was contrary to the notion of the 1880's, the decade that it was proposed, in that the common wisdom was that emotion provoked a biomechanical response.
Originally proposed by Walter B. Cannon, this theory has seen many revisions since its inception. Cannon first suggested in 1929 that the thalamus, an organ of the limbic system, played a central role in the creation and manipulation of emotions. Since that time, the theory has been expanded to include other portions of the limbic system, including the amygdala. The cerebral cortex is also recognized as playing a role in emotions under the revised theory.
Cognitive theories incorporate many of the key aspects of the theories listed above, and also put forth some critical concepts of their own. Cognitive-physiological theory, advocated by psychologist Stanley Schachter, maintains that emotional perception and expression is determined by an unconscious appraisal of three psychological inputs. Physiological changes, external stimuli, and internal memories are thought to play a decisive role in the formation of emotions. This theory has the most social implications of any of the theories mentioned here.