Our findings must be interpreted in the context of three potential limitations. First, our data were drawn from white Virginian males. Males have a higher prevalence of drug use (87–90) and although previous analyses using the same data suggest it is broadly representative of US males and does not differ from the general population in rates of psychopathologic conditions, including illicit substance use, abuse and dependence (33), our results cannot be extrapolated to females. Second, w did not model cannabis initiation. However because there is converging evidence showing how initiation, regular use and progression to abuse and dependence can be explained by common genetic and environmental processes (29, 45) we would expect to see a similar pattern of results using binary measures of initiation. Finally, the latent PGD factor scores were assumed to take a uni-dimensional factor structure. The first three eigen values at 15 to 17 years (6.95, 1.12, 0.86), 18 to 21 years (5.56, 1.35, 0.80), and 22 to 25 years (5.85, 1.31, 0.73) followed by a comparison of the 1 and 2 factor solutions, suggested that a
