have inferred a generalized liability to SUDs (Goldman and Bergen, 1998; Kendler et al., 2007, 2008; McGue and Iacono, 2008; Petraitis et al., 1995; Rhee et al., 2003, 2006; True et al., 1999; Vanyukov et al., 2003; Young et al., 2006). Other studies examining risk factors underlying substance involvement should not overlook the importance of data regarding the use of multiple drugs.