Stanley Schachter was an American social psychologist best known for his development of the two factor theory of emotion in 1962 along with Jerome E. Singer. In his theory he states that emotions have two ingredients: physiological arousal and a cognitive label. A person's experience of an emotion stems from the mental awareness of the body's physical arousal and the explanation one attaches to this arousal. Schachter also studied and published many works on the subjects of obesity, group dynamics, birth order and smoking. A Review of General Psychology survey, published in 2002, ranked Schachter as the seventh most cited psychologist of the 20th century.

Emotions are physical and mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is no scientific consensus on a definition. Emotions are often intertwined with mood, temperament, personality, disposition, or creativity.