The older cohort, Cohort I, were recruited as adults and first surveyed by mailed questionnaire over the period 1980–1981 (Heath et al., 1997). During 1980–81, 5,967 adult twin pairs born between 1893 and 1964 were contacted and asked to complete a brief self-report questionnaire. Questionnaires were returned by 8196 individual twins (68% individual response rate), including 3,810 pairs (64% pair-wise response rate). During 1988–89, pairs for whom both twins returned the 1980–81 questionnaire were asked to complete a follow-up questionnaire. The follow-up questionnaire replicated most of the original items, with additional assessment of female reproductive health. Follow-up questionnaire data were obtained from both members of 6234 individual twins, including 2995 pairs (82% individual and 79% pair-wise response rates). In 1992, all twins from the 1988–89 sample were asked to complete a structured diagnostic interview. 206 twin pairs from an earlier laboratory study (Martin, Oakeshott, Gibson, Starmer, Perl, & Wilks, 1985; Martin, Perl, Oakeshott, Gibson, Starmer, & Wilks, 1985) were also invited to participate. In all, 5963 individual twins age 27–90 were interviewed, including 2722 pairs (89% individual and 85% pair-wise response rates).
