limits closer to 0 (or below) support independent liabilities and upper limits approaching 1 provide support for identical liabilities. This model therefore provides a means of testing directly the strength of association between the initiation and progression stages for a substance. It also allows the estimation of: (1) additive genetic effects (a2), (2) common environmental effects (c2) and (3) unique environmental effects (e2) on both initiation and progression of substance use. However, the genetic and environmental influences on progression are estimated after those on initiation have been taken into account. That is, the genetic and environmental influences on progression do not include those on initiation that also effect progression. The amount of variance in progression explained by those influences on initiation can be calculated by squaring the beta coefficient. The proportion of this variance that is explained by genetic factors is equivalent to the proportion of variance in initiation explained by genetic factors, and similarly for the environmental factors.
