Lawrence Kohlberg

Extended Piaget's account of children's moral development into a six-stage theory that reshaped how psychology understands ethical reasoning, arguing that people justify their choices through an ordered progression from self-interest to abstract principle. He built this framework while a professor at Chicago and then Harvard, and it took him five years to get his first paper on it published. A later citation study ranked him the 30th most eminent psychologist of the 20th century.