During the last decade, there has been a growing interest in the advantages that MZ twin pairs offer for studying the effect of environmental factors on the development of emotional disturbances. Much of this research has been aimed at investigating the relationship between childhood differences and later discordance with regard to schizophrenia in MZ twin pairs. A few studies have used similar methodology to investigate the development of neurotic reactions. There has been very little work dealing with the causes of discordance in the normal personality development of MZ twin pairs. The paper presents some of the results from a larger twin project which show the relationship between childhood differences and differences in adulthood. The study examined differences in oral, obsessive, and hysterical personality traits, as well as differences in phobic fears, general neurotic symptomatology, occupational and marriage adjustment in 50 relatively unselected MZ twin pairs. The results show that many of the same childhood differences which have been reported to be associated with the discordance found in regard to schizophrenia and neuroses, are also related to differences in personality structure, emotional and social adjustment in less disturbed MZ twin pairs. Other childhood differences, however, seem to be more specifically related either to neurotic or to normal personality development, pointing to the importance of studying the relationship between specific intrapair differences in childhood and differences in various areas of later personality development.