Our genetic and environmental estimates for initiation and abuse are consistent with previous findings (6, 9, 12, 16, 37–40). Our results are also consistent with results based on overlapping twin samples from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (3, 41). Portions of the genetic and environmental variation in abuse can be explained by genetic and environmental risks to initiating cannabis. Our modelling is an extension of the two-stage CCC design used by Maes et al (2004), Neale et al (2006), and Agrawal and colleagues (3). It illustrates that the familial risks in initiation and abuse can be directly and indirectly explained by the risk factors for availability.
