Leon Festinger was an American social psychologist best known for his theory of cognitive dissonance, which he introduced in the 1950s. This theory suggests that individuals experience psychological discomfort when they hold conflicting beliefs or when their actions contradict their beliefs. To alleviate this discomfort, people often change their beliefs or rationalize their behaviors. Festinger also conducted influential research on social comparison theory, which posits that individuals determine their own social and personal worth based on how they stack up against others. His work has had a lasting impact on psychology, influencing fields such as behavioral science and marketing.