Participants were adolescent male and female twins taking part in the ongoing Minnesota Twin Family Study (MTFS). The MTFS is an epidemiological-longitudinal study of the families of same-sex twin pairs born in Minnesota that was designed to investigate the etiology of substance use disorders and related conditions. A comprehensive description of the design and methods of the MTFS has been provided elsewhere41,42. Briefly, the MTFS includes a younger cohort first assessed at age 11, and an older cohort who began the study at age 17. Twin participants are then offered the opportunity to return for follow-up assessments every 3-4 years. Families were initially located using publicly available birth records and databases, targeting the birth years from 1972 to 1984. Over 90% of eligible twin families were successfully located for each target birth year. Families are representative of the Minnesota population for the target birth years in terms of parental occupational status, educational attainment, and history of mental health treatment. Consistent with the demographics of those born in Minnesota during the target years, 96% of the participants were Caucasian. All twins
