Daniel Levinson was an American psychologist best known for his work on adult development and life stages. He proposed the Seasons of a Man's Life theory, which outlines the various phases individuals go through from youth to old age. His research emphasized the importance of transitions and how they shape personal growth. Levinson's influential book, The Seasons of a Man's Life, published in 1978, detailed his findings based on interviews with men aged 35 to 45. He identified key developmental tasks and challenges that people face during different life stages, contributing significantly to the field of human development.