As described in Iacono et al. (2014), the sample is a subset of the larger sample in a recent family-based GWAS of substance abuse and related psychopathology conducted at the Minnesota Center for Twin and Family Research (MCTFR; McGue et al., 2013; M. B. Miller et al., 2012). Participants for the present investigation are from the older and younger cohorts and enrichment samples of the Minnesota Twin Family Study (MTFS; Iacono, Carlson, Taylor, Elkins, & McGue, 1999; Keyes et al., 2009; McGue et al., 2013). The cohort-sequential nature of the MTFS design is such that the two age cohorts of twins participate at partially overlapping assessment ages. The sample for this investigation was based on the age-17 laboratory assessment of twins and all parents who had completed an identical laboratory assessment. (See Iacono et al., 2014, for further details.) Participants in MCTFR studies gave written consent or assent, if under the age of 18, to participate in the initial study as well as to allow data used in GWASs to be placed in a public repository to be shared with other researchers.
