Despite this paper's narrow focus on cannabis, our findings are of broad, etiological significance; these findings have begun to elucidate the causal processes underlying the genetic and environmental factors on drug use and abuse in terms of particular measured risk factors. The previously estimated shared environmental factors in initiation and abuse are correlated and can be traced back to aspects of the shared environment responsible for variation in cannabis availability. Likewise, we have shown how proportions of the genetic liability to initiate and then progress to abuse can be explained, in part, by genetic effects in availability. We still do not know if the A and C components in drug availability are the causal components or whether, for instance, the C component is an index of other, broader aspects of the shared environment such as peer group deviancy or lack of parental monitoring. Nevertheless, it is a valuable step forward to have decomposed, into genetic and environmental components, the covariance between a known, yet understudied, risk factor on adverse substance use.
