In studies of monozygotic (MZ) and dizygotic (DZ) twins reared together, liability can be divided into additive genetic (A), common environment (C) and random environment (E) variance components. With multivariate data it is possible to decompose the association between availability, initiation, and abuse into these same components. Based on the multi-stage methods for CCC pathway analyses (17, 34) we fitted four models to test specific hypotheses about the nature of the association between the three measures. The key issue is whether any of the genetic and environmental variance in drug availability is associated with variation in drug initiation and subsequent lability to abuse.
