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OBJECTIVE: To evaluate the genetic contribution to schizophrenia using an adoption design that disentangles genetic and environmental factors. METHOD: Finnish hospital diagnoses of schizophrenic/paranoid psychosis in a nationwide sample of adopting-away women are compared with DSM-III-R research diagnoses for these mothers. DSM-III-R diagnoses of their index offspring are blindly compared with adopted-away offspring of epidemiologically unscreened control mothers. RESULTS: Primary sampling diagnoses of index mothers were confirmed using DSM-III-R criteria. Lifetime prevalence of typical schizophrenia in 164 index adoptees was 6.7% (age-corrected morbid risk 8.1%), significantly different from 2.0% prevalence (2.3% age-corrected morbid risk) in 197 control adoptees. When adoptees with diagnoses of schizoaffective disorder, schizophreniform disorder, schizotypal disorder and affective psychoses were added, the contrast between the index and control adoptees increased. CONCLUSION: The genetic liability to 'typical' DSM-III-R schizophrenia is decisively confirmed. Additionally, the liability also extends to a broad spectrum of other psychotic and non-psychotic disorders.
Tienari et al. (Thu,) studied this question.