The Minnesota Twin Family Study (MTFS) [30,31] includes twins born in Minnesota and recruited to participate in longitudinal assessments. Two cohorts (a younger cohort who entered the study at age 11, and an older cohort who began at age 17) participated in repeated assessments approximately every 3–4 years around ages 11, 14, 17, 21, and 25. We identified all twins assessed for substance dependence symptoms through their 25-year-old follow-up visit. The MTFS sample (N = 2476) had a mean age of 25.00 (SD = 0.89, range = 22–29), was 52.5% female and primarily Caucasian, and was demographically representative of the Minnesota population. Through their age 25 assessment, 11.5% of MTFS participants met DSM-IV criteria for alcohol dependence at some point in their lifetime, 7.7% met criteria for marijuana dependence, and 1.6% met criteria for cocaine dependence.
