Most epidemiological risk factor studies have focused upon only one stage of substance involvement (usually initiation [38]). Our findings in adolescents, together with those of several previous adult twin studies, indicate that initiation and progression of substance use are partially independent liabilities (particularly in the case of alcohol). Epidemiological studies providing greater insight into the extent to which risk factors influence initiation and progression stages of substance involvement separately, or in common, are important for prevention and intervention approaches.
