History of parental alcoholism was coded from parent self-report, parent ratings of coparent, and twin ratings of each parent. Parent interviews included self-report assessment of lifetime history of DSM-IV alcohol dependence (AD), with AD symptoms experienced by the twins’ biological coparent assessed using an adaptation of the Family History Assessment Module [25]. A probable diagnosis was coded for coparents without requiring 12-month clustering, as temporal clustering of coparent symptoms was not assessed. Twin ratings of parental alcoholism were drawn from Wave 4, when all twins were aged 18 or older. At Wave 4, twins were asked whether “drinking ever caused your biological (mother/father) to have problems with health, family, job or police, or other problems,” an item that originated in the Family History Research Diagnostic Criteria [26] assessment, and whether they ever felt that their biological parent was an “excessive drinker.” Endorsement of both problem and excessive drinking items was required to code a parent positive by twin report. Following earlier analyses [16,19], a parent was coded positive based on either self-report AD or any family history rating.
