To the Editor.—
Gottesman and Bertelsen1 ask an important question regarding the risk of schizophrenia in the offspring of normal twins in monozygotic (MZ) twins discordant for schizophrenia. Unfortunately, the database they have selected to answer it, Fischer's Danish twin study,2 is markedly unsatisfactory for such a purpose.Gottesman and Bertelsen's conclusions are based on the offspring of four normal co-twins in pairs in which the sick twin was labeled as having "schizophrenia." However,1. The normal twin in two of the pairs (Nos. 6 and 43) was never examined by Fischer or any other researcher, for both had been born in 1878 and 1888 and died prior to the onset of the study in the 1960s. An assumption of their normality was based on reports of the son of one and the wife of the other. Gottesman and Bertelsen acknowledge the unsatisfactory nature of such diagnostic procedures
Torrey EF. Offspring of Twins With Schizophrenia. Arch Gen Psychiatry. 1990;47(10):976–977. doi:10.1001/archpsyc.1990.01810220092013
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