As a whole, SUD twin studies suggest a common set of genetic and environmental factors that are shared across drugs as well as genetic and environmental influences that are specific to a given substance. Studies of initiation report substantial shared environmental influences common to multiple substances (Fowler et al., 2007; Han, McGue, & Iacono, 1999; Koopmans, Slutske, Heath, Neale, & Boomsma, 1999). In contrast, studies of use and dependence reported significant additive genetic and unique environmental influences shared across SUD as well as genetic and environmental influences specific to a given drug (Agrawal & Lynskey, 2006; Baker, Maes, Larsson, Lichtenstein, & Kendler, 2011; Palmer et al., 2012; Palmer et al., 2009; Xian et al., 2008; Young, Rhee, Stallings, Corley, & Hewitt, 2006). Further, the influence of genetic and environmental factors shared between different forms of substance abuse are likely to remain significant across time (Palmer, et al., 2009).
