Recent research (e.g., Baker, Maes, & Kendler, 2012; Samek, Keyes, Iacono, & McGue, 2013) has started to investigate specific variables, such as parental attitudes toward substance use and peer deviance, that may account for the shared environmental influence on early substance involvement. To date, the focus has primarily been general adolescent substance involvement or adolescent use of multiple drugs (e.g., Walden, McGue, Iacono, Burt, & Elkins, 2004). Notable exceptions are studies that have found parental separation (Waldron et al., 2014a; Waldron et al., 2014b) and childhood sexual abuse (Nelson et al., 2006; Sartor et al., 2013) predict early substance use. Research examining additional measured shared environmental contributions to the timing of initiation of multiple drugs will be necessary to (a) determine the most important proximal influences on substance use uptake and (b) clarify to what degree these factors exert common and specific influence across substances.
