and twin studies report higher estimates of environmental influences for measures related to substance initiation and use compared to measures of drug problems (Hopfer et al., 2003). One approach to overcoming this problem is the implementation of conditional/multi-stage models that conditionally include information of individuals that have not met threshold criteria for each level of involvement via structured missingness (i.e., individuals are included in the analysis as having missing values if they do not meet a certain threshold at each stage of the analysis; for example, only repeat users can report having substance-related problems) (Neale et al., 2006). This conditional approach allows the estimation of influences specific to each level of drug involvement. Several studies have assessed whether the etiology of access to a substance is correlated with those for persistence or dependence (Heath et al., 2002, Heiman et al., 2008; Kendler et al., 1999; Maes et al., 2004)) and have found that there is evidence for common or partially shared genetic and environmental factors for substance initiation/use and problems. For instance, Eaves and Eysenck (1980) suggested a single dimension of risk for initiation and persistent tobacco use. Similarly, Maes and colleagues (2004) found a significant overlap in the liabilities
