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Leon Festinger (8 May 1919 – 11 February 1989) was an American social psychologist who originated the theory of cognitive dissonance and social comparison theory. The rejection of the previously dominant behaviorist view of social psychology by demonstrating the inadequacy of stimulus-response conditioning accounts of human behavior is largely attributed to his theories and research. Festinger is also credited with advancing the use of laboratory experimentation in social psychology, although he simultaneously stressed the importance of studying real-life situations, a principle he practiced when personally infiltrating a doomsday cult. He is also known in social network theory for the proximity effect (or propinquity).

Festinger studied psychology under Kurt Lewin, an important figure in modern social psychology, at the University of Iowa, graduating in 1941; however, he did not develop an interest in social psychology until after joining the faculty at Lewin's Research Center for Group Dynamics at the Massachusetts Institute of Technology in 1945. Despite his preeminence in social psychology, Festinger turned to visual perception research in 1964 and then archaeology, history, and the human evolutionary sciences in 1979 until his death in 1989. Following B. F. Skinner, Jean Piaget, Sigmund Freud, and Albert Bandura, Festinger was the fifth most cited psychologist of the 20th century.