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 of tobacco initiation, use, and dependence; Gillespie and colleagues (2009) found a strong relationship between the liabilities to use and abuse of cannabis. The current study examined variability in DSM-IV dependence symptoms amongst substance users; non-users were included in our analyses by assigning them a missing value.
