Objective: The authors report on a systematic twin study of index twins suffering from schizophrenia spectrum psychoses. Using different diagnostic systems, they examined twin concordance, family history, and the frequency and severity of the birth complications of 22 monozygotic and 23 dizygotic twin pairs.
Method: All twins in the region of Lower Franconia, Germany, born after 1930 and hospitalized for psychiatric disease were ascertained. The zygosity diagnoses were based on molecular genetic methodology and a zygosity questionnaire. Two psychiatrists, working independently, formulated diagnoses according to DSM-III-R criteria and Leonhard's nosology.
Results: There were substantially different concordance rates with regard to diagnostic subgroups, and monozygotic concordance was significantly higher than dizygotic concordance in only two of the following five subgroups (subgroups 1 and 3): 1) strict schizophrenia according to DSM-III-R: monozygotic, 85.7%, dizygotic, 25.0%; 2) schizophreniform, schizoaffective, and delusional (paranoid) disorders and psychotic disorder not otherwise specified according to DSM-III-R: monozygotic, 47.1%, dizygotic, 30.8%; 3) unsystematic schizophrenia according to Leonhard: monozygotic, 88.9%, dizygotic, 25.0%; 4) systematic schizophrenia according to Leonhard: monozygotic pairs lacking, dizygotic, 0%; 5) cycloid psychoses according to Leonhard: monozygotic, 38.5%, dizygotic, 36.4%. In the case of cycloid psychoses and conditions less prominent in DSM-III-R schizophreniform, schizoaffective, and delusional (paranoid) disorders and psychotic disorder not otherwise specified, the affected twins had suffered significantly more severe birth complications than their healthy partners. Not one of the 37 monozygotic twins was diagnosed as having systematic schizophrenia, whereas six of the 25 dizygotic index twins received this diagnosis.
Conclusions: The results of the study suggest that schizophrenia spectrum psychoses may consist of clinically and etiologically heterogeneous subgroups with different genetic backgrounds.