The substance abuse literature documents an ongoing interest in understanding the risk factors underlying substance use and related disorders. While substance-specific factors have been supported, the high rates of comorbidity among nicotine, alcohol, and illicit substances have led many researchers to concentrate on risk factors that may be common across multiple substances. For example, Rhee and colleagues (2003) utilized simulated family data to discriminate between 13 different models of comorbidity defined by Neale and Kendler (1995), and then applied this methodology in a clinically ascertained adolescent sample (Rhee et al., 2006). Results suggested that two models were equally likely to explain the patterns of comorbidity observed. The first model, referred to as the correlated liability model, hypothesizes that each substance has its own set of risk factors (i.e., liability) and that these factors are correlated, accounting for poly-substance use and comorbid SUDs. The second plausible model, the alternate forms model, hypothesizes that comorbidity is driven by a single risk factor which manifests itself as an array of deviant behaviors, including substance use and SUDs. Both models are consistent with observed
