Major depressive disorder in a community-based twin sample - Are there different genetic and environmental contributions for men and women?

Background: Depression affects more women than men and often aggregates in families. Using a community-based sample of twins, we examined the contribu tions of genetic and environmental factors to the risk of developing major depressive disorder and the effect of sex and different definitions of depr ession on the relative contributions of genetic and environmental effects. Sex differences in genetic effects were also studied. Methods: A volunteer sample of Australian twins (2662 pairs) was interviewe d using an abbreviated version of the Semi-Structured Assessment for the Ge netics of Alcoholism, a semi-structured lay interview designed to assess ps ychiatric disorders. Depression was defined using 3 different criteria sets : DSM-III-R major depressive disorder, DSM-IV major depressive disorder, an d severe DSM-IV major depressive disorder. Genetic and environmental contri butions to the liability to develop depression were estimated using genetic model fitting. Results: Lifetime prevalences were 31% in women and 24% in men for DSM-III- R major depressive disorder, 22% in women and 16% in men for DSM-IV major d epressive disorder, and 9% in women and 3% in men for severe DSM-IV major d epressive disorder. In women, the simplest model to fit the data implicated genetic factors and environmental factors unique to the individual in the development of depression, with heritability estimates ranging from 36% to 44%. In men, depression was only modestly familial, and thus individual env ironmental factors played a larger role in the development of depression. F or DSM-III-R major depressive disorder, there were statistically different estimates for heritability for men vs women. For both sexes, the relative c ontributions of genetic and environmental factors were stable using differe nt definitions of depression. Conclusions: There was moderate familial aggregation of depression in women and this primarily was attributable to genetic factors. In men, there was only modest familial aggregation of depression. For both men and women, ind ividual environmental experiences played a large role in the development of depression. Major depressive disorder as defined by DSM-III-R was more her itable in women as compared with men. The relative contributions of genetic and environmental factors in the development of depression were similar fo r varying definitions of depression, from a broad definition to a narrow de finition.