one year. The study included several different tests which aimed to measure causal understanding and indices of mental health. The results of the study showed that students who participated in the experimental learning program demonstrated higher causal understanding and more positive mental health indices than those who did not. These findings supported Muuss' hypothesis for the study. Examination of Theories One aspect of Muuss’ work in psychology is his examination of developmental theories, with a particular focus on adolescence. Among others, Muuss reviewed the theories and work of Sigmund Freud, Erik Erikson, Jean Piaget, and Carol Gilligan. Muuss believed that theories of development could be categorised into "families", based on the different original ideas from which the theories developed. For example, Muuss stated that Erikson's work had a basis in the work of Freud, and that therefore their theories belong to the same family. Muss also believed that the theories of Piaget, Kohlberg, and Gilligan were related. Muuss credits Sigmund Freud with founding the psychoanalytic movement, and changing the foundation of psychiatry and psychology. He recognises Freud's influence in areas including literature, advertising, art, philosophy, sociology, medication, and education. For example, as Muuss has stated, Freud's assumption that "frustration of normal satisfaction may lead to the development of neurosis" implies that educators should not frustrate their students. Muuss believed that this was proof that educators should create environments that include leniency, approval, and attention. In regards to the work of Erikson, in particular his theory of identity development, Muuss recognised not only the similarities to Freud, but also the increased acknowledgement of societal influence on development. Muuss believed Erikson's theory to be the base of contemporary identity research. Muuss reviewed Piaget's cognitive theory of adolescence. While Muss recognised that Piaget's theory has been criticised and suffered a loss of popularity, he believed that Piaget was responsible for completely changing the approach to and knowledge of children's learning in the Western world. Muuss stated "Piaget's influence will remain an intellectual, philosophical, and theoretical force to reckon with during much of the twenty-first century". Muuss took interest in and reviewed Gilligan's theory of sex differences, particularly in regard to the development of moral reasoning during adolescence. He was interested in the contradiction Gilligan's theory posed to work by Lawrence Kohlberg that claimed to demonstrate a higher level of moral judgement and maturity in males than females. Gilligan stated that Kohlberg's research was