Past-year diagnosis of major depression (MDD) was assessed at four separate time-points in each study, providing four non-overlapping one-year-long assessment windows. In Dunedin, MDD was evaluated when Study members were ages 18, 21, 26, and 32 years. In E-Risk, MDD was evaluated when study members were, on average, ages 33, 35, 38, and 40 years. In both studies, MDD diagnosis was established with the Diagnostic Interview Schedule (DIS; Robins et al., 1989; Robins et al., 1995) to obtain DSM-IV diagnosis of MDD (American Psychiatric Association, 1994). Functional impairment was required for diagnosis. We created six outcome variables. Persistent depression, the primary outcome variable, was operationally defined as the presence of past-year MDD at two or more of the four assessments. Single-episode depression (diagnosed on only one of the four occasions) was used for comparison to establish the specificity of the persistence hypothesis. Time 1 depression was a diagnosis of past-year MDD at the first assessment (age 18 in Dunedin and mean age 33 in E-Risk). Time 2 depression was past-year MDD at the second assessment (age 21 in Dunedin and
