Does the proportion of the genetic and environmental contributions to depression differ for broadly and narrowly defined depression?
SUBJECTS Subjects were drawn from the Australian National Health and Medical Research Council Twin Registry, a volunteer sample of twins. The cohort first completed a mailed questionnaire in 1980-1982 when they were 18 years or older, and the twins were subsequently followed up with a second mailed questionnaire in 1988-1990.17-18 In 1992-1993, telephone interviews of twins were conducted, and this direct interview provided the data for analyses in this article.19 Data were obtained from both members of 2685 twin pairs. The mean age of the sample was 44 years (range, 28-89 years; SD, 12.35 years) in women and 42 years (range, 28-84 years; SD, 11.23 years) in men. Because this was a study of unipolar affective disorder, twin pairs were eliminated from analyses if one or both twins from a twinship screened positive for mania, which left 2662 twin pairs for the final analyses. Verbal informed consent was obtained from all subjects before the start of the interview. ASSESSMENT Subjects wer
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