For both cohorts, abortion history was included among questionnaire assessments of female reproductive heath, with male respondents instructed to skip this section. Respondents reporting a past abortion were coded positive for abortion history. Age of first or subsequent abortions was not assessed in either cohort. Zygosity was diagnosed based on twins’ responses to standard questions regarding similarity and the degree to which others confused them (Nichols & Bilbro, 1966).
