Researchers have developed several theories of how human emotions arise and are represented in the brain. Like the James–Lange and Cannon–Bard theories, the Schachter–Singer theory of emotion (also known as the two-factor theory) attempts to explain emotion as it relates to physiological arousal.
According to the Schacter–Singer theory, emotion results from the interaction between two factors: physiological arousal and cognition. More specifically, this theory claims that physiological arousal is cognitively interpreted within the context of each situation, which ultimately produces the emotional experience. These cognitive interpretations—how a person labels and understands what they are experiencing—are formed based on the person's past experiences.
The Schachter–Singer two-factor theory
The Schachter–Singer theory views emotion as resulting from the interaction of two factors: physiological arousal and cognition.
For example, if you were to see a venomous snake in your backyard, the Schachter–Singer theory argues that the snake would elicit sympathetic nervous system activation (physiological arousal) that would be cognitively labeled as fear (cognition) based on the context. What you would actually experience, then, would be the feeling of fear.
In their research, Singer and Schachter injected participants with adrenaline (epinephrine), which causes a number of physiological effects, such as increased blood flow to the muscles and increased heart rate. They found that injecting the drug did not lead participants to experience any given emotion. Contrary to the James–Lange theory, therefore, which asserts that emotions arise from physiological arousal, this theory argues that bodily changes can support conscious emotional experiences but do not necessarily cause them. Rather, the interpretation of a certain emotion depends on both the individual's physiological state as well as their circumstances, a relationship mediated by cognitive processing.