Lawrence Kohlberg built upon Piaget's theory of moral development by using stories involving moral dilemmas to study people's reasoning. Kohlberg proposed six stages of moral development across three levels - pre-conventional, conventional, and post-conventional. At the pre-conventional level, reasoning is based on obedience and avoiding punishment. At the conventional level, reasoning is based on social norms and approval. At the post-conventional level, reasoning is based on self-chosen principles of justice and rights. Kohlberg's methods have been criticized for using artificial dilemmas and not accounting for gender differences in moral reasoning.