We used PRSice to calculate polygenic risk scores for liability to a number of psychiatric traits, and evaluated their ability to predict heroin addiction (Table 4). The base data included GWAS for smoking and related behaviors and datasets for diseases known to be comorbid with addiction, for example, schizophrenia and ADHD. We found significant evidence that genetic risk of schizophrenia could predict heroin dependence, (p = 0.0007) at PT = 0.0085. This is likely in part explained by the relatively higher power of the schizophrenia GWAS, compared to the other psychiatric traits investigated, leading to improved identification of any shared genetic factors between heroin dependence and any generalized liability to psychiatric disorders [41]. We did not find evidence for a genetic overlap between heroin dependence and any of the other GWAS traits investigated here. The data from the Tobacco Addiction Genetics (TAG) Consortium may be the most relevant for the phenotype of heroin addiction; in a previous study [42], PRS for cigarettes per day had predicted the number of glasses of alcohol consumed per day and age at onset of
