The individuals included in the present analyses were not representative of the general population; rather, they were selected on the basis of reporting cocaine use from a larger genome-wide association study (SAGE; Bierut et al., 2010) that was over-sampled for alcohol dependence cases. The total SAGE sample was drawn from three primary studies designed to study alcohol, nicotine, and cocaine dependence. The SAGE study design results in greater levels of comorbidity among cocaine, alcohol, tobacco, and marijuana dependence, which could be expected to result in a more substance-general genetic risk score. Despite the clinical heterogeneity within our sample due to three different ascertainment schemes and despite a bias towards general substance risk factors, we still observed specificity of the genetic risk score to cocaine dependence symptoms within the testing sample.
