Lawrence Kohlberg's stages of moral development constitute an adaptation of a psychological theory originally conceived by the Swiss psychologist Jean Piaget. Kohlberg began work on this topic as a psychology graduate student at the University of Chicago in 1958 and expanded upon the theory throughout his life. The theory holds that moral reasoning, a necessary (but not sufficient) condition for ethical behavior, has six developmental stages, each more adequate at responding to moral dilemmas than its predecessor. Kohlberg followed the development of moral judgment far beyond the ages studied earlier by Piaget, who also claimed that logic and morality develop through constructive stages. Expanding on Piaget's work, Kohlberg determined that the process of moral development was principally concerned with justice and that it continued throughout the individual's life, a notion that led to dialogue on the philosophical implications of such research. The six stages of moral development occur in phases of pre-conventional, conventional and post-conventional morality. For his studies, Kohlberg relied on stories such as the Heinz dilemma and was interested in how individuals would justify their actions if placed in similar moral dilemmas. He analyzed the form of moral reasoning displayed, rather than its conclusion and classified it into one of six stages. There have been critiques of the theory from several perspectives. Arguments have been made that it emphasizes justice to the exclusion of other moral values, such as caring; that there is such an overlap between stages that they should more properly be regarded as domains or that evaluations of the reasons for moral choices are mostly post hoc rationalizations (by both decision makers and psychologists) of intuitive decisions. A new field within psychology was created by Kohlberg's theory, and according to Haggbloom et al.'s study of the most eminent psychologists of the 20th century, Kohlberg was the 16th most frequently cited in introductory psychology textbooks throughout the century, as well as the 30th most eminent. Kohlberg's scale is about how people justify behaviors and his stages are not a method of ranking how moral someone's behavior is; there should be a correlation between how someone scores on the scale and how they behave. The general hypothesis is that moral behaviour is more responsible, consistent and predictable from people at higher levels. Stages Kohlberg's six stages can be more generally grouped into three levels of two stages each: pre-conventional, conventional and post-conventional. Following Piaget's constructivist requirements for