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The existence of a nation-wide twin register and central psychiatric register has made possible a catamnestic investigation of an unselected and representative sample of twins with manic-depressive disorders.
From a total population of 11,288 same-sexed twin pairs born 1870–1920 in Denmark 126 probands from no pairs were ascertained. Among the co-twins of 69 monozygotic probands there were found 46 with manic-depressive disorders, and a further 14 had presented other psychoses or marked affective personality disorders or had committed suicide, yielding a proband rate of strict concordance, C1 = 0·67 and of broad, partial concordance, C2 = 0·87. The corresponding direct pairwise concordance rates were 32/55 = 0·58 and 46/55 = 0·84 respectively. For the dizygotic twins the proband concordance rate of C1 was 11/54 = 0·20 and of C2 20/54 = 0·37, and the direct pairwise rates were 9/52 = 0·17 and 18/52 = 0·35 respectively. The differences between the pairwise rates for the monozygotic and dizygotic twins are significant (P < 0·001 at χ2 analysis). This finding is in accordance with previous twin studies of manic-depressive disorders and confirms the evidence of a strong genetic factor. The concordance with respect to unipolar and bipolar forms was not in contradiction to recent evidence of a genetic difference between the bipolar and unipolar form, the latter probably related to the female sex.
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