The Schachter-Singer Theory of Emotion
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The Schachter-Singer Theory of Emotion is a widely accepted social psychological theory of affective experience, which integrates the role of both physiological arousal and cognitive factors in determining emotion. The theory posits that the experience of particular emotions is dependent on cognitive labels exerting a “steering function” over general physiological arousal.
Preeminent American psychologist William James published the first widely influential theory of emotion in 1884. James’ theory, grown from “fragmentary introspective observations,” proposed that emotional experience is principally based in the proprioception of “bodily disturbances.” In James own words:
“Our natural way of thinking about these standard emotions is that the mental perception of some fact excites the mental affection called emotion, and that this later state of mind gives rise to bodily expression. My thesis on the contrary is that the bodily changes follow directly the perception of existing fact, and that our feeling of the same changes as they occur is the emotion.” 
That is to say when an individual is presented with an emotional object it first stimulates the appropriate sensory organs. These afferent signals are then sent to the cortex, triggering “variously combined” ordinary motor-sensorial “brain processes”. According to James, it is the afferent proprioceptive signals from the bodily changes, esp. fluctuation in visceral responses, mediated by signaling from these motor and sensory centers that account for the emotional experience.
In 1885 Danish physiologist Carl Lange independently presented a similar explanation of affective experience albeit slightly more elaborated. Lange had essentially “the same conception as James” however his theory focused primarily on the vasomotor center as being the “root cause of affections, however else they may be constituted.” 
Cannon’s Critique and the Cannon-Bard Theory:
The James-Lange theory went largely unchallenged until the 1920’s when Walter Cannon, an experimental physiologist at Harvard, and psychologist Philip Bard presented a competing theory. Considering the James-Lange theory in a 1927 review Cannon compiled key inadequacies with the previous theory. Firstly, Cannon noted that deafferentation of the viscera (in canines) produced no alteration of emotional behavior. Secondly, similar visceral changes and autonomic activation seem to occur across a spectrum of both emotional and non-emotional states and are thus “too uniform to offer a satisfactory means of distinguishing emotions… very different in subjective quality.” Cannon also observed that “in the nerves distributed to the viscera the afferent (sensory) fibers may be only one-tenth as numerous as the efferent.” In addition, changes in visceral organs were noted to “respond with relative sluggishness” to potential changes in emotional state. 
Taking into account the abovementioned critiques, Cannon and his associate Bard set out to provide an alternative theory in which “emotional expression results from action of the subcortical centers.”  Citing the nullifying effect of removing the thalamus in decorticated “sham rage” animal subjects and the proposed top-down mechanisms of alcohol and ether intoxication, Cannon presented “the optic thalamus as a region in which resides the neural organization for the different emotional expression.”  Thus in the Cannon-Bard theory, first an individual’s sensory organs take in the emotional stimulus. Next the stimulus is relayed to the cortex where “impulses… [are] associated with conditioned processes which determine the direction of the response… [which] therefore stimulate the thalamic processes”.  As the thalamic centers “discharge precipitately and intensely” they excite both viscera and “afferent paths to cortex” causing bodily fluctuation and emotional experience “almost simultaneously.” 
The Schachter- Singer Theory
In 1962, following suite with the tide of the “cognitive revolution” in the field of psychology, researchers Stanley Schachter and Jerome E. Singer devised a new theory of emotion that took into account the influence of cognitive factors. In their analysis Schachter and Singer noted that, with reference to the pervious physiological based theories, there remained the question of accounting for the fact that “the variety of emotion, mood and feeling states are by no means matched by an equal variety of visceral patterns.” This “rather ambiguous situation” led the them to conclude “that cognitive factors may be major determinants of emotional states.”  Considering this implication the researchers formulated what is known as the Schachter-Singer or Two Factor Theory of Emotion.
The theory thus presents a model of emotional experience based on cognitive labels in response to physiological excitation. In this theory the individual senses the particular emotional object of situation through the sense organs. An induced form of autonomic arousal then follows this perception. Accompanying this “general pattern of sympathetic excitation” is a specific cognitive label, which allows one to interpret “this stirred-up state in terms of the characteristics of the precipitating situation and one’s apperceptive mass.”  The theory also addresses the salience of feedback mechanisms, as “past experience provide the framework within which one understands and labels his (sic) feelings.” 
Following the implications of their theory Schachter and Singer also provided three important ancillary propositions:
(1) In the event that an individual has no causal explanation for an arousal state he or she will label arousal in terms of available cognitions.
(2) In the event that an individual has appropriate explanation for arousal alternative cognitive labeling will be unlikely.
(3) Under identical “cognitive circumstances” an individual will only respond with emotional experience to the degree that he or she is physiologically excited.
Schachter and Singer (1962)
In order to test their theory Schachter and Singer hypothesized that in the absence of an “appropriate explanation” for arousal participants could be manipulated into experiencing an emotion by manipulating aspects of the available “cognitive circumstance.”
The experiment was “cast in the framework of a study of the effects of Vitamin supplements on vision.”  Researchers told all 184 participants, all male college students, that they would be receiving injections of the vitamin compound “Suproxin.” In reality the injected compound was ½cc of either epinephrine or saline (placebo) solution, creating experimental and control conditions respectively.
Secondly, participants who received the epinephrine were assigned to one of three conditions “Informed”, “Ignorant”, and “Misinformed”. In the ”Informed” condition participants were made aware of the injection’s potential side effects (e.g. increased heart rate, shakiness, etc.), thus giving an “appropriate explanation” of arousal.” In the “Ignorant” condition participants were not given any information regarding potential side effects and thus no explanation for arousal. Lastly, in order to control for effects of introspective anxiety in the face of side effects, the “Misinformed” condition participants were made aware of fabricated side effects.
The final variable manipulated was the “cognitive circumstance.” Participants were left alone for 20 minutes with paired stooges (blind to participant condition) who were trained to act in either a “euphoric” or “angry” manner.
Emotional state was then experimentally measured via one-way mirror assessments (semiprivate index) of the participants’ behavior relative to the stooge. A Likert style scale self-report (public index) addressing participant mood and physical condition was then administered immediately after the experiment.
The results of the experiment confirmed Schachter and Singer’s original hypothesis. In both the “euphoric” and “angry” conditions participants in the “Ignorant” and “Misinformed” conditions consistently showed significantly higher scores on both activity indices and self report scales than those in the Informed and Placebo conditions. 
Dutton and Aron (1974)
Donald Dutton and Arthur Aron carried out a second highly influential study providing experimental support for the Schachter-Singer theory. In their study they hypothesized that through misattribution of arousal, or excitation transfer, induced feelings of arousal can be converted into sexual attraction.
The study consisted of male participants crossing either a sturdy slightly arousing low bridge or a precarious highly arousing suspended bridge. At the end of the bridge a male or female interviewer asked participants to take part in a survey for his or her psychology class. After completing the survey the interviewers gave the willing participants his or her name and number and” offered to explain the experiment in more detail when she had more time.” 
The number of willingly surveyed participants and subsequent number of phone calls received by the interviewer were assessed as measures of sexual attraction. In addition sexual imagery in survey responses was also measured. The results of the study largely confirmed the researchers’ hypothesis. Across measures the female interviewer elicited higher scores of sexual attraction from participants in the experimental precarious bridge condition (compared to the control condition and both conditions in the case of the male interviewer).
Criticism and Alternate theories
As with any psychological theory the Schachter-Singer Theory has evoked several criticisms. One major criticism of the theory is there is too much an emphasis on a “general pattern of excitation.” However Schachter and Singer did note that, “Whether or not there are physiological distinctions between various emotional states must be considered an open question.”  Additionally the Schachter-Singer Theory centers primarily on the autonomic nervous system and provides no account of the emotional process within the central nervous system aside from signaling the role of cognitive factors. This is important considering the heavy implication of certain brain centers in mitigating emotional experience (e.g, fear and the amygdala). 
Stemming from criticisms notable alternative theories have also arisen that both challenge and build on the Schachter-Singer Theory. One theory, Facial Feedback Hypothesis, proposes that feedback from changes within the facial skeletal musculature can amplify or even elicit certain emotions.  Another alternate theory proposed by Lisa Feldman Barrett examines emotional experience as a confluence of two dimensions, valence (positive or negative) and arousal (high or low). 
1. James, William (1884). What is an Emotion?. Mind, 9(34), 188-205.
2. Cannon, Walter B. (1927). The James-Lange Theory of Emotions: A Critical Examination and an Alternative Theory. The American Journal of Psychology, 39(1), 106-124.
3. Schachter, S. & Singer, J. E. (1962). Cognitive, Social, and Physiological Determinants of Emotional State. Psychological Review, 69(5), 379-399.
4. Dutton, D. G. & Aron, A. P. (1974). Some Evidence For Heightened Sexual Attraction Under Conditions of High Anxiety. Journal of Personality and Social Psychology, 30(4), 510-517.
5. LeDoux, J. E. (1995) Emotion: Clues from the brain. Annual Review of Psychology, 46, 209-235.
6. Martin, L. L., Stepper S. & Strack F. (1988). Inhibiting and facilitating Conditions of the Human Smile: A Nonobtrusive Test of the Facial Feedback Hypothesis. Journal of Personality and Social Psychology. 54(5), 768-777.
7. Feldman Barrett, L., Mesquita B., Ochsner K. N. & Gross J. J. (2007) The Experience of Emotion. Annual Review of Psychology, 58, 373-403.
James-Lange theory of emotion: 
Cannon's critique: 
The study by Marañon: 
Schachter and Singer's theory: 
The experiment by Dutton and Aron: