We assume that environmental influences are equivalent for each sibling type, which has generally been upheld (Loehlin & Nichols, 1976; Neiderhiser et al., 2004; Reiss et al., 2000). We assume that individuals do not systematically choose their mates based on genetically influenced characteristics (assortative mating), although there is moderate evidence of this for antisocial behavior (du Fort, Boothroyd, Bland, Newman, & Kakuma, 2002). Assortative mating may inflate shared environmental influences on adolescent externalizing at the expense of genetic influences, potentially increasing the likelihood of finding passive rGE in the ECOT model. The inclusion of genetically-unrelated siblings in the child-based design attenuates this bias (Marceau et al., 2013). We assume there are no systematic differences in genetic and environmental influences on parenting or on adolescent externalizing across the two samples, equivalency of measurement error across phenotypes (i.e., ε1 = ε2), and that the mechanisms underlying the association under examination do not differ in the Swedish and US populations represented in TOSS and NEAD (Marceau et al., 2013; Narusyte et al., 2011; Narusyte et al., 2008).
