Twin studies

Twin studies have come to the same conclusion. Gottesman,(l) who reviewed the literature, calculated the average probandwise concordance rate for broadly defined schizophrenia in monozygotic twins to be 46 per cent, compared with 14 per cent in dizygotic twins. This difference has been taken to be a reflection of the fact that while monozygotic twins share all their genes, dizygotic twins share, on average, only 50 per cent. Further evidence of the effect of heredity comes from the evidence that the concordance rate in 12 pairs of monozygotic twins who were reared apart was 58 per cent.(1)

The above twin studies preceded the introduction of operational definitions of schizophrenia. When studies with such definitions were carried out, the rates for both monozygotic and dizygotic twins were both lower but the disparity between the two remained. Thus, in the latest and largest study, which examined 108 consecutive pairs of twins seen at the Maudsley Hospital in London, Cardno et al.(6) reported probandwise concordance rates for DSM-IIIR schizophrenia of 42.6 per cent in monozygotic twins and 0 per cent in dizygotic twins. As can be seen in Table 1, concordance rates for schizophrenia using other operational definitions were not dissimilar.

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Table 1 Probandwise concordance rates in the Maudsley twins for three operational definitions of schizophrenia

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