& Dishion, 1992) tend to highlight the importance of parenting and deviant peer groups, both twin studies (e.g., Slutske et al., 1998) and molecular genetics studies (e.g., Dick, 2007) suggest that common genetic influences are also contributing factors. However, there is evidence from twin studies suggesting that there is more complexity to the comorbidity among different forms of substance dependence (and other externalizing behavior problems) than would be expected on the basis of a single spectrum of externalizing behavior. For example, best-fitting multivariate models of the genetic architecture of common genetic factors among abuse of alcohol, nicotine, cannabis, caffeine, and cocaine suggest at least two distinguishable (although very highly correlated) common genetic factors (one principally associated with licit substances and one principally associated with illicit substances) as well as substance-specific genetic factors, especially for nicotine and caffeine (Kendler, Meyers, & Prescott, 2007). These findings were in contrast to a similar study of the genetic architecture of illicit substance dependence which found evidence only for common genetic influences on cannabis, cocaine, hallucinogen, sedative, stimulant, and opiate dependence (Kendler, Jacobson, et al., 2003): individual genetic paths to risk for abuse of each drug separately were not strong in this study. Collectively these
