gathered and different measures used. A low or absent shared environmental influence, as estimated in the present study, has also been found in several previous studies (Kendler & Prescott, 1998; van der Bree et al. 1998; Kendler et al. 2006; Agrawal et al. 2007). Furthermore, multi-stage modeling has revealed that the influence of shared environmental influences on the symptoms of cannabis abuse was indirect and mediated entirely by cannabis initiation (Gillespie et al. 2009).
