Abstract
AIMS: Although previous twin studies have modeled the association between drug initiation and abuse, none has included the obvious risk factor of drug availability. Our aim is to determine whether the genetic and environmental risk factors for cannabis availability also generate variation in cannabis initiation and/or progression to DSM-IV symptoms of abuse. DESIGN: We used multi-stage modeling, also known as causal-common-contingent (CCC) analysis, to partition the genetic and environmental factors into common and stage-specific components. PARTICIPANTS: This report is based on data collected from 1772 adult males from the Mid Atlantic Twin Registry. MEASUREMENTS: The twins participated in two structured interviews which included clinical and non-clinical measures of cannabis abuse as well as retrospective assessments of perceived cannabis availability between ages 8 and 25 years. FINDINGS: Cannabis availability explained almost all the shared environmental risks in cannabis initiation and abuse. The influence of availability on the symptoms of abuse was indirect and mediated entirely by cannabis initiation. CONCLUSION: These findings have begun to elucidate the causal processes underlying the liability to drug use and abuse in terms of putative risk factors. Specifically, our results show that the latent shared environmental factors in cannabis initiation and abuse can be explained by measured aspects of the shared environment--those responsible for variation in cannabis availability.