Leon Festinger was an American social psychologist. He was born in New York City in 1919 to a Russian immigrant family. As a graduate student at the University of Iowa, Festinger was influenced by Kurt Lewin, a leading social psychologist. After graduating from there, he became a professor at the Massachusetts Institute of Technology in 1945. He later moved to Stanford University, where he continued his work in social psychology. His theory of social comparison earned him a good reputation. Festinger actively participated in international scholarly cooperation. In the late 1970s, he turned his interest to the field of history. He was one of the most cited psychologists of the twentieth century. Festinger’s theories still play an important role in psychology today.