Lawrence Kohlberg developed the theory of moral development based on his doctoral research with children. He proposed that moral reasoning develops through six stages from childhood to adulthood. The stages progress from focusing on obedience, individual needs, and social norms, to reasoning based on social contracts, individual rights, and universal ethical principles. Kohlberg used moral dilemmas to study how people at different stages justify their moral judgments. His stage theory expanded on Piaget's work and influenced research on moral psychology and education.