Affective Illness in Veteran Twins: A Diagnostic Review

The authors systematically reviewed diagnoses in the Veteran Twin Registry and found 62 pairs of twins (69 individuals) in which one or both had affective illness, a frequency of .22 percent (monozygotic [MZ] concordance = 33 percent, dizygotic [DZ] concordance = 0 percent, and MZ/DZ ratio ≥ 11.5). In 40 of the 62 pairs, one or both twins had unipolar depression (MZ concordance = 40 percent, DZ concordance = 0 percent, and MZ/DZ ratio ≥ 8). Bipolar depression was present in 22 pairs (MZ concordance = 20 percent, DZ concordance = 0 percent, MZ/DZ ratio ≥ 3.2). The data indicate that both environmental and genetic factors are important in the etiology of affective illness and present evidence that unipolar and bipolar illness are separate entities.