Kohlberg was an American psychologist known for his theory of moral development which consists of six stages across three levels - preconventional, conventional, and postconventional. The preconventional level includes the punishment and obedience orientation and instrumental purpose stages. The conventional level comprises the good boy-good girl and social order maintaining stages. Finally, the postconventional level contains the social contract orientation and universal ethical principle stages. Kohlberg believed that moral reasoning progresses through these stages as individuals develop their understanding of justice.