Cohort II twins were born between 1964 and 1971 and ascertained as children by their parents in response to flyers distributed throughout Australian schools during 1980–1982 (Heath, Howells et al., 2001). During the period 1990–92, 8536 twins were contacted and asked to complete a brief self-report questionnaire of similar format and item content to the 1988–89 questionnaire administered to Cohort I twins. Questionnaires were returned by 5058 individual twins (59% individual response rate), including 2270 pairs (53% pair-wise response rate). Excluding pairs that could not be located or where either twin was deceased or too impaired to give informed consent, 8020 twins (4010 pairs) were re-contacted between 1996–2000 for interview assessment. During this period, structured diagnostic interviews were administered to 6257 twins, including 2723 pairs (78% individual and 68% pair-wise response rates).
