Sex distribution and the severity of illness among samples of schizophrenic twins

Abstract

There is an excess of females over males in twin studies of schizophrenia which have been reported in the literature. Evidence suggests that there are about as many male as female twins in the adult population, and more males than females hospitalized for schizophrenia. Various data indicate that when sampling is based on consecutive admissions and twinship is ascertained from birth records, the excess of females does not occur. When ascertainment depends on delegated interviewers and when one samples from resident hospital populations, the excess of female schizophrenic patients appears. It is shown that excess of females is related to chronicity of the illness in the patient sample. When the former method of sampling is used, concordance between MZ twins with respect to severity of the illness is much less than when the latter method of sampling is used. The latter method produces data which suggest that hereditary factors account for almost all the variance with respect to the occurence of schizophrenia or its severity. The method of consecutive admissions, with twinship ascertained from birth records, yields information which suggests that nonhereditary factors are accounting for an appreciable amount of the total variance.