a detailed assessment of substance involvement. All eligible twins, including those who may not have completed a baseline assessment, were invited to participate in the follow-up, provided that they, or their parents, had not previously indicated an unwillingness to participate in future studies. A total of 3,787 twins, aged 18–29 years, completed follow-up interviews, however of those interviewed, only 2,484 twins also had data on PSI from the questionnaire. Of these subjects with interview and questionnaire data, we retained those who reported European-American ancestry and had a valid zygosity assignment as the member of a monozygotic or dizygotic twin pair – this final sample consisted of 2176 European-American women, from 574 MZ and 439 DZ pairs as well as 150 twins whose co-twin did not participate.
