Pko (photographer) · CC BY-SA 4.0

A German-American psychologist often called the founder of social psychology, Lewin pioneered the study of group dynamics and helped invent action research — the idea that psychological theory should be tested and refined through real organizational change rather than the lab alone. His work shaped how businesses, schools, and communities think about leadership and group behavior, and a 2002 survey ranked him the 18th most-cited psychologist of the twentieth century. He taught at the University of Berlin, Cornell, MIT, and Iowa.