Leon Festinger

Originated the theory of cognitive dissonance, the still-standard explanation for how people rationalize a gap between their beliefs and their actions, and developed social comparison theory alongside it. His work displaced the behaviorist assumption that human behavior could be explained by stimulus and response alone. He tested his ideas by joining a doomsday cult undercover to watch what happened to believers when the world failed to end on schedule, and was, after Skinner, Piaget, Freud, and Bandura, the fifth most-cited psychologist of the 20th century.