• In a summary of the major twin studies of affective illness, there are significant differences between monozygotic (MZ) and dizygotic (DZ) concordance rates for both unipolar and bipolar illness, indicating the importance of genetic factors in the etiology of affective illness. However, since 28% of MZ twins are discordant for bipolar illness and 60% of MZ twins are discordant for unipolar illness, environmental factors are important as well. In addition, there is a significant difference between unipolar and bipolar concordance for MZ twins (although not for DZ twins). This supports other evidence that unipolar and bipolar illness are separate entities, and it suggests the possibility that genetic factors are more important in the occurrence of bipolar illness than in unipolar illness.
Allen MG. Twin Studies of Affective Illness. Arch Gen Psychiatry. 1976;33(12):1476–1478. doi:10.1001/archpsyc.1976.01770120080008