Lawrence Kohlberg was an American psychologist known for his theory of moral development. He proposed that moral reasoning develops through six distinct stages - pre-conventional, conventional, and post-conventional. Kohlberg studied how children reason through moral dilemmas to determine their stage of moral development. He found that moral reasoning progresses sequentially through the stages and is influenced by cognitive development and social experiences that create uncertainty. Kohlberg's theory helped explain how morality develops over a lifetime through increasingly complex levels of reasoning.