In Figure 2, Model 1 specifies that the genetic and environmental factors for drug availability, i.e., A1, C1 and E1, directly generate variation in both drug initiation (via a2,1, c2,1 and e2,1) and directly for drug abuse (via the a3,1, c3,1 and e3,1). The model also predicts that A1, C1 and E1 can act indirectly on drug abuse via drug initiation along the β2 pathway. The genetic and environmental factors for initiation (A2, C2 and E2) may also generate variation in drug abuse indirectly via β2.
