Twin study of illness history variables in psychosis.

BACKGROUND: Genetic factors are important in the aetiology of psychotic disorders, but it is unclear how far they influence aspects of illness history within psychoses. AIMS: To investigate the extent, and type of familial aggregation for a range of illness history variables in psychosis. METHODS: Two-hundred-and-twenty-four proband-wise twin pairs (106 monozygotic, 118 same-sex dizygotic), where probands had psychosis, were ascertained from the Maudsley Twin Register in London. We investigated the following illness history variables, rated using the OPCRIT checklist: age at onset; chronicity of course; mode of onset; psychotic/affective predominance; pre-morbid social adjustment; and the presence of a psychosocial precipitant. We used Mx statistical modelling software to analyse correlations of variables within pairs of monozygotic twins concordant for psychosis; and relationships between variables in probands and risk of psychosis in monozygotic and dizygotic co-twins. RESULTS: There was a high monozygotic within-pair correlation for age at onset (intra-class correlation=0.9); moderate correlations for chronicity of course (polychoric correlation=0.4) and psychotic/affective predominance (polychoric correlation=0.5); and lower non-significant correlations for other variables. No variables consistently predicted risk of psychosis in co-twins. CONCLUSIONS: Illness history variables in psychosis show a broad range of familial aggregation. It is likely that familial influences are predominantly modifying effects, independent of susceptibility factors for psychosis.