Participants were 7,285 monozygotic (MZ) and same-sex dizygotic (DZ) twins from the Australian Twin Registry Cohort II and Cohort III. These individuals were drawn from the larger sample of 7,334 MZ and same-sex DZ twins, and consisted of individuals who (a) reported having ever used tobacco, alcohol, and/or cannabis in their lifetime, or (b) were lifetime abstainers, but had a co-twin who reported having ever used tobacco, alcohol, and/or cannabis (for more information about participants, see Knopik et al., 2004; Hines et al., 2015; and Lynskey et al., 2012). Table 1 displays the sample characteristics.1
