Major depression and associated impairment: same or different genetic and environmental risk factors?

OBJECTIVE Impairment was added as a diagnostic criterion for many psychiatric disorders in DSM-IV. Does the addition of impairment influence only prevalence rates, or does it also introduce new etiological factors into psychiatric diagnoses? METHOD A lifetime history of major depression and associated functional impairment was assessed by personal interview with 3,669 female and 4,377 male twins from the population-based Virginia Twin Registry. Structural equation modeling was used to estimate the correlation between risk factors for major depression and associated functional impairment. RESULTS While the risk factors for major depression and associated functional impairment are substantially correlated, they are not identical. The most parsimonious model suggests that over a quarter of the variance in associated functional impairment is due to factors unrelated to risk for major depression. Of the variance unique to associated functional impairment, approximately one-third is familial. The relationship between associated functional impairment and major depression did not differ significantly between men and women. CONCLUSIONS Risk factors for major depression and associated functional impairment are substantially but imperfectly correlated. The addition of associated functional impairment as a criterion for the diagnosis of major depression not only lowers prevalence estimates but also introduces a small set of new etiological factors into the diagnosis of major depression.