Since initiation is not contingent upon reported availability, direct pathways from A1, C1 and E1 can be estimated. The reason there are no direct pathways from A2, C2 and E2 to abuse in this model is because initiation is strictly binary. Therefore, there is no information on the cross-trait association between measures within individuals, which would allow a full decomposition of the association between initiation and abuse into genetic, common and specific environmental effects (34). The contingent nature of abuse on initiation means that individuals who do not initiate cannot, by definition, develop abuse, and so diagnostic data will be missing. However, if we restrict the genetic and environmental covariance between initiation and the outcome measures into a common pathway it is possible to estimate the regression of outcome on initiation.
