The monozygotic (MZ)/dizygotic (DZ) concordance rates for schizophrenia and the relationship between schizophrenia and schizophrenic spectrum disorders were studied in a sample of 31 MZ and 28 DZ schizophrenic probands and their co-twins. All subjects were personally interviewed with structured diagnostic instruments and classified according to DSM-III-R criteria. The concordance rates of 48% for MZ twins and 4% for DZ twins indicate a genetic transmission of DSM-III-R schizophrenia. In addition to the schizophrenic co-twins, 3 MZ co-twins had a noneffective psychotic disorder, thus supporting the hypothesis that genes are involved in the development of Axis I schizophrenic spectrum disorders. Schizotypal and paranoid personality disorders were observed in both MZ and DZ co-twins. These disorders may be familially related to schizophrenia, but a genetic relationship was not confirmed for the Axis II spectrum disorders. A substantial number of MZ co-twins of schizophrenic probands had no mental disorder.
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Onstad et al. (1991) studied this question.
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