The binary lifetime DSM-IV diagnosis of MDD was derived from the respondent’s most recent assessment. The binary lifetime PTSD diagnosis was obtained from the Childhood Trauma Study interview. Individuals who did not report any lifetime trauma exposure were coded as missing for PTSD since the diagnosis of PTSD is contingent on prior trauma exposure. (See the article by Heath et al35 for the applicability of this approach to structural equation modeling with contingent phenotypes.) Because the nature of traumatic events experienced (eg, seasonal flooding vs a devastating tsunami), and how some events are perceived, can vary considerably between populations, traumatic events were classified empirically for the present study as either low or high risk based on the relative risk of PTSD associated with each specific event when nominated by respondents as most disturbing. Respondents were not categorized exclusively into low- vs high-risk trauma exposure groups; a given respondent could endorse both event types. The availability of these measures (other than the diagnosis of PTSD) in cohort II data enabled the same classification of low- and high-risk traumatic events to be
