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 prevalence rates of comorbidity that exceed those predicted from the rates of use, abuse, and dependence on individual substances when assuming independent liabilities. In the current study we examine the patterns of single and multiple drug involvement in both adolescence and young adulthood while making comparisons to expected rates based on a model of independent liabilities. By comparing the expected and observed rates of multiple substance use and SUDs we are able to test if these liabilities are independent. We hypothesized that the patterns of multiple substance use and abuse in adolescence and young adulthood would support a model of generalized risk. Additionally, given that young adulthood is a period during which drug initiation and progression to problematic use remains frequent (Wagner and Anthony, 2002), we also hypothesized that the prevalence of lifetime multiple substance use and abuse would increase from adolescence to young adulthood.
