Two-Factor Theory of Emotion
The notion that emotion is based on two factors: physiological arousal and cognitive label, such that when an emotion is felt, a physiological arousal occurs, and the person uses the immediate environment to search for emotional cues to label the physiological arousal.
Origin
Stanley Schachter and Jerome Singer proposed the theory in a 1962 paper in Psychological Review at Columbia University. Their experiment — injecting participants with epinephrine under false pretenses and varying whether they knew the source of their arousal — showed the same physiological state could produce either euphoria or anger depending on social context. The work synthesised the competing James-Lange (1884) and Cannon-Bard (1927) theories that had dominated emotion research for decades.