The final sample included 225 MZ male, 204 MZ female, 105 DZ male, and 121 DZ female twin pairs at intake. At the first follow-up, 189 MZ male, 200 MZ female, 87 DZ male, and 100 DZ female twin pairs were included (88% of the original individuals providing data) and there were 147 MZ male, 160 MZ female, 71 DZ male, and 74 DZ female twin pairs at the second follow-up (69% providing data). Because inclusion in the present sample required an in-person assessment to establish the substance use phenotype and up to approximately 20% of MTFS participants complete their follow-up assessment by telephone, participation rates in the current study are lower than those reported in other MTFS studies (greater than 90%) (19). Additionally, participants who either skipped items or endorsed fake items (“bleomycins” and “cadrines”) were excluded from analysis (see Figure 1). Twins in the final sample are identified by the target age for their intended assessment: 11-year-olds (M=11.70, SD=0.43) at intake, 14-year-olds (M=14.77, SD=0.51) at the first follow-up, and 17-year-olds (M=17.99, SD=0.60) at the second follow-up. Because some
