THE GENETIC EPIDEMIOLOGY OF SCHIZOPHRENIA IN A FINNISH TWIN COHORT - A POPULATION-BASED MODELING STUDY

Background: The magnitude of heritability of schizophrenia remains con troversial, due in part to limitations of estimates derived from index twin pairs exclusively. We applied structural equation modeling in a total population of twins to determine the significance and magnitudes of the genetic and environmental contributions to schizophrenia. Meth ods: All monozygotic (1180 male and 1315 female pairs) and same-sex di zygotic (2765 male and 2613 female pairs) twins born from 1940 to 1957 in Finland were screened for nonorganic psychotic disorder diagnoses as recorded on an inpatient or outpatient basis or from an eligibility review for a disability pension. Results: The lifetime prevalence of schizophrenia was 2.0%, with a marginally higher prevalence in men (2. 2%) than women (1.8%). Model fitting indicated that 83% of the varianc e in liability was due to additive genetic factors, and the remaining 17% was due to unique environmental factors. Sex-limitation modeling r evealed no evidence of sex-specific genetic effects and no sex differe nce in the magnitude of heritability. A multiple threshold model incor porating affective and other psychoses as a phenotype intermediate bet ween schizophrenia and no diagnosis was rejected. Conclusions: In a po pulation-based twin study of schizophrenia, heritability was estimated at 83%, with the remaining variance in liability attributed to enviro nmental factors not shared in common among co-twins. Despite the notab le limitation of using diagnoses ascertained through treatment contact s, the heritability estimate in this study is almost identical to thos e reported in recent studies of index pairs using standardized applica tions of DSM-III or later criteria.