Noting the psychological similarity of identical twins, it has been suggested that study of their life histories might delineate the relative importance of environmental and hereditary factors in human development. Psychiatric studies of identical twins have emphasized their psychological alikeness, but they also have important emotional differences, including dissimilar symptoms in response to similar intrapsychic conflicts. That twins differ psychologically is also shown by the fact that one twin is likely to assume the dominant role early in life, and that mothers tend to treat them differently.

Studies of schizophrenic twins suggest that the probability of becoming schizophrenic increases in direct proportion to closeness of blood relationship to a schizophrenic patient. The authors present a detailed case history of identical