Although studies have been made of large groups of criminals to discover the incidence of factors of heredity, physical condition, intellectual capacity, environmental conditions, educational and industrial training and mental status, an element often neglected is that of personality development, especially with relation to possibilities of social reconstruction. Mental defect is less apt than personality defect to make an individual anti-social. The field of personality offers a most productive field for study. Three viewpoints should be taken into consideration: the medical, studying the glands of internal secretion; the psychological, making studies of aptitudes and disabilities; that of psychiatry, studying the individual's mental status and applying to his conduct theories which have helped in understanding other individuals. There are three opportunities for studying personality traits of delinquents in penal institutions: the mental test, the social worker's history, the period of observation in the institution. A special study of two girls made at the New York State Reformatory at Bedford Hills is discussed in detail. The girls' history, conduct and mental status were very similar and the fact that they made widely differing adjustments to their environment after leaving the institution was felt to be largely due to differences in personality. From Psych Bulletin 19:05:00351. (PsycINFO Database Record (c) 2006 APA, all rights reserved)