After accounting for the genetic and shared environmental overlap between substance use initiation and substance use disorder, we obtained significant unique environmental correlations among men and women. This indicates that familial factors could not entirely explain the association between the timing of substance use initiation and later disorder, and are suggestive of a “quasi-causal” relation (Turkheimer & Harden, 2014). Specifically, this suggests that the propensity to initiate substance use at a younger age may exert some direct – or phenotypic – influence on the general risk for disorder. As reviewed above, evidence for a causal impact of early substance use initiation on disorder is mixed. To our knowledge, however, no prior studies have addressed this question from a multivariate perspective. That is, they have not examined whether the general tendency for earlier initiation of substance use might convey risk for 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
