Short answers, pulled from the story.
Leon Festinger was an American social psychologist who originated the theory of cognitive dissonance and social comparison theory. He was born on the 8th of May 1919 and died on the 11th of February 1989, and he ranked as the fifth most cited psychologist of the 20th century.
Leon Festinger and James M. Carlsmith published their classic cognitive dissonance experiment in 1959. Subjects performed dull tasks, then told the next person the work was enjoyable, with half paid $1 and half paid $20. Those paid $1 later rated the task as more enjoyable, because they reduced dissonance by changing their own opinion.
When Prophecy Fails, published in 1956 by Festinger, Henry Riecken, and Stanley Schachter, studied a small apocalyptic cult led by Dorothy Martin who predicted the world would end on the 21st of December 1954. When the prophecy failed, members held their beliefs more strongly and began proselytizing, providing the first experimental evidence for belief perseverance.
The proximity effect, also called propinquity, is Festinger's finding that physical proximity predicts friendship. Studying married student housing at MIT with Stanley Schachter and Kurt Back, he showed people tend to befriend their neighbors, and that functional distance, such as living near a stairway, also shapes social ties.
Leon Festinger left social psychology in 1964, saying he felt he was in a rut and needed intellectual stimulation from new sources to stay productive. He turned to visual perception research, then to archaeology, history, and human evolutionary science until his death in 1989.
Leon Festinger studied under Kurt Lewin, an important figure in modern social psychology, at the University of Iowa, where he earned his MA in 1940 and his PhD in 1942 in the field of child behavior. He later joined Lewin's Research Center for Group Dynamics at MIT in 1945.