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.

Festinger was born in Brooklyn New York on May 8, 1919 to Russian-Jewish immigrants Alex Festinger and Sara Solomon Festinger. His father, an embroidery manufacturer, had "left Russia a radical and atheist and remained faithful to these views throughout his life." Festinger attended Boys' High School in Brooklyn, and received his BS degree in psychology from the City College of New York in 1939.

He proceeded to study under Kurt Lewin at the University of Iowa, where Festinger received his MA in 1940 and PhD in 1942 in the field of child behavior. By his own admission, he was not interested in social psychology when he arrived at Iowa, and did not take a single course in social psychology during his entire time there; instead, he was interested in Lewin's earlier work on tension systems, but Lewin's focus had shifted to social psychology by the time Festinger arrived at Iowa. However, Festinger continued to pursue his original interests, studying level of aspiration, working on statistics, developing a quantitative model of decision making, and even publishing a laboratory study on rats. Explaining his lack of interest in social psychology at the time, Festinger stated, "The looser methodology of the social psychology studies, and the vagueness of relation of the data to Lewinian concepts and theories, all seemed unappealing to me in my youthful penchant for rigor." Festinger considered himself to be a freethinker and an atheist.

After graduating, Festinger worked as a research associate at Iowa from 1941 to 1943, and then as a statistician for the Committee on Selection and Training of Aircraft Pilots at the University of Rochester from 1943 to 1945 during World War II. In 1943, Festinger married Mary Oliver Ballou, a pianist, with whom he had three children, Catherine, Richard, and Kurt. Festinger and Ballou were later divorced, and Festinger married Trudy Bradley, currently a professor of social work emeritus at New York University, in 1968.

In 1945, Festinger joined Lewin's newly formed Research Center for Group Dynamics at the Massachusetts Institute of Technology as an assistant professor. It was at MIT that Festinger, in his own words, "became, by fiat, a social psychologist, and immersed myself in the field with all its difficulties, vaguenesses, and challenges." It was also at MIT that Festinger began his foray into social communication and pressures in groups that marked a turning point in his own research. As Festinger himself recalls, "the years at M.I.T. [sic] seemed to us all to be momentous, ground breaking, the new beginning of something important." Indeed, Stanley Schachter, Festinger's student and research assistant at the time, states, "I was lucky enough to work with Festinger at this time, and I think of it as one of the high points of my scientific life."

Yet, this endeavor "started as almost an accident" while Festinger was conducting a study on the impact of architectural and ecological factors on student housing satisfaction for the university. Although the proximity effect (or propinquity) was an important direct finding from the study, Festinger and his collaborators also noticed correlations between the degree of friendship within a group of residents and the similarity of opinions within the group, thus raising unexpected questions regarding communication within social groups and the development of group standards of attitudes and behaviors. Indeed, Festinger's seminal 1950 paper on informal social communication as a function of pressures toward attitude uniformity within a group cites findings from this seemingly unrelated housing satisfaction study multiple times.

After Lewin's death in 1947, Festinger moved with the research center to the University of Michigan in 1948. He then moved to the University of Minnesota in 1951, and then on to Stanford University in 1955. During this time, Festinger published his highly influential paper on social comparison theory, extending his prior theory regarding the evaluation of attitudes in social groups to the evaluation of abilities in social groups. Following this, in 1957, Festinger published his theory of cognitive dissonance, arguably his most famous and influential contribution to the field of social psychology. Some also view this as an extension of Festinger's prior work on group pressures toward resolving discrepancies in attitudes and abilities within social groups to how the individual resolves discrepancies at the cognitive level. Festinger also received considerable recognition during this time for his work, both from within the field, being awarded the Distinguished Scientific Contribution Award by the American Psychological Association in 1959, and outside of the field, being named as one of America's ten most promising scientists by Fortune magazine shortly after publishing social comparison theory.