Martin Seligman

Developed the theory of learned helplessness, then pivoted his career toward studying what makes people flourish rather than what makes them suffer, co-founding the field of positive psychology while serving as president of the American Psychological Association in 1998. A 2002 survey ranked him the 31st most-cited psychologist of the 20th century; his books, including Learned Optimism and Flourish, helped make positive psychology a mainstream idea in both clinical practice and pop self-help.