Data for this study are drawn from the Missouri Adolescent Female Twin Study (MOAFTS). MOAFTS consists of a cohort of female same-sex twin pairs born between July 1st 1975 – June 30th 1985 who were identified from birth records (39). Further details regarding the initial sample recruitment and the baseline interview (conducted 1994–1999), response rates and characteristics of the baseline data, which are not utilized in the current study, as they do not contain detailed assessments of substance use, are given elsewhere (40;41). Subsequently, the twins were invited to complete a mailed questionnaire and participate in the five-year follow-up telephone interview. The questionnaire was mailed one year (1996–1999) after the baseline interviews and included a series of questions on peer substance involvement. 2788 twins, aged 14–24 years, returned completed questionnaires. The five-year follow-up interview was conducted from 2002–2004 and included 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
