Twin Studies and Genetic Models of Schizophrenia

To the Editor.—  The elegant and important data reported by Farmer and colleagues¹ raise several issues that deserve comment. A different interpretation of their data would be to exclude from analysis those twin pairs with one twin with an affective disorder and one schizophrenic twin, on the grounds that there is a reasonable possibility of a misdiagnosed affective psychosis in the schizophrenic twin. Reanalysis of the remainder of the sample lends support to the possibility of a single-gene dominant form of schizophrenia.The co-occurrence of schizophrenia and affective psychosis in the same identical twin pair is more likely (in any individual case) to be due to misdiagnosis of the "schizophrenic" twin than to a genetic relationship between schizophrenia and mood-incongruent affective disorders (as suggested by Farmer and associates). Patients with early-onset affective psychoses often receive schizophrenic diagnoses because they tend to have more bizarre and psychotic symptoms than patients