Status of twin research in functional psychoses.

After briefly reviewing the main findings of twin research in functional psychoses, the author focuses the attention on neglected aspects of twin research, such as the study of co-twins of monozygotic (MZ) subjects with functional psychosis, and the study of life histories of discordantly affected twins. Through a thorough investigation of the nonpsychotic co-twins of MZ subjects, one might be able to identify vulnerability factors. By studying the life course of the twins in a pair, discovery of environmental etiological factors are possible. By including twins with both schizophrenic, manic-depressive and reactive psychotic symptomatology, one should be able to discover whether the environmental factors are of a specific kind.