An independent analysis of the Copenhagen sample of the Danish adoption study of schizophrenia. IV. The relationship between major depressive disorder and schizophrenia.

Genetic investigations offer one approach at evaluating the validity of Kraepelin's division of the functional psychoses into two major groups, schizophrenia and affective illness. In this study, DSM-III criteria for major depressive disorder (MDD) were blindly applied to the interviews with relatives from the Copenhagen sample of the Danish Adoption Study of Schizophrenia. No significant difference was found in the prevalence of MDD in the biological relatives of the control adoptees and the biological relatives of all the schizophrenic or only the chronic schizophrenic adoptees. Furthermore, no difference in the prevalence of MDD was found in the adoptive relatives of the schizophrenics and control subjects. These results support neither a genetic nor a familial-environmental link between schizophrenia and MDD and support the validity of the diagnostic division between them established by Kraepelin.