later general susceptibility to develop a substance use disorder. The current data support further examination of early polysubstance initiation as a potential risk factor for later comorbid substance use problems. We emphasize, however, that strong causal conclusions cannot be drawn from the present findings, as (a) they concern cross-sectional data, (b) the association between early substance use initiation and disorder may be confounded by variables that differ within twin pairs and relate to both first use and problems, and (c) common measurement error related to the assessment of substance use initiation and disorder may contribute to their unique environmental correlation. Future research employing prospective data and measured confounds will help clarify the nature of the relation between early first substance use and later abuse/dependence. Longitudinal prevention studies evaluating the impact of delaying first substance use may be particularly informative.
