Moral evelopment is a field of psychology and a field of education. Lawrence Kohlberg, Jean Piaget, and Elliot Turiel have cognitive-developmental approaches to moral development; to these theorists morality forms in a series of constructive stages or domains. Social psychologists such as Martin Hoffman and Jonathan Haidt emphasize social and emotional development based on biology, such as empathy. Moral identity theorists, such as William Damon and Mordechai Nisan, see moral commitment as arising from the development of a self-identity that is defined by moral purposes: this moral self-identity leads to a sense of responsibility to pursue such purposes. Of historical interest in psychology are the theories of psychoanalysts such as Sigmund Freud, who believe that moral development is the product of aspects of the super-ego as guilt-shame avoidance.