A pilot Swedish twin study of affective illness including hospital- and population-ascertained subsamples: results of model fitting.

We investigated the heritability of liability to affective illness (AI) in twins ascertained through psychiatric hospitalization for AI from the Swedish Psychiatric Twin Registry and from the general population Swedish Twin Registry. Lifetime diagnoses were assessed by mailed questionnaire containing, in self-report format, DSM-III-R criteria for mania and major depression (MD). Jointly analyzing both subsamples using Mx, and assuming a multifactorial threshold model, the best-fitting twin model using narrow diagnostic criteria suggested that the liability to AI could be explained by additive genetic effects, with an estimated heritability of liability of 64%, and individual-specific environment. Using broad criteria, results were similar except that the estimated broad heritability of liability was higher (83%) and due largely to dominance genetic effects. Fitting sex-dependent models suggested that the same genetic and environmental factors influenced liability to AI in men and women to the same degree, although women had a lower threshold of manifestation. These results suggested that in Sweden, AI is a highly heritable syndrome and family resemblance is due largely or entirely to genetic factors.