The Missouri Adolescent Female Twin Study (MOAFTS; Heath et al., 2002; Knopik et al., 2005) is a population-based longitudinal study of female twin pairs born between July 1, 1975 and June 30, 1985 in Missouri to Missouri-resident parents The sample was demographically representative of the Missouri population at the time the twins were born, with nearly 15% of twins being African-American (AA) and the remainder being of European-American (EA) descent. A baseline interview was conducted with 3,258 twins beginning in 1995 (median age=15 years). All available twins were targeted for three waves of telephone interviews (Waves 1, 4, and 5, at median ages 15, 22, and 24 years, respectively). Between 2002 and 2005, all twins from the target cohort (excluding those who had withdrawn from the study or whose parents asked that the family not be re-contacted) were contacted for Wave 4 interviews. As all twins (N=3,787) were 18 years of age or older at the time of recruitment for Wave 4, sensitive questions regarding their illicit substance use was queried. Therefore, we limited the sample to MOAFTS participants who
