A literature search was used to identify relevant autosomal dopamine-related genes. Genes were selected for inclusion if they were known to have definite and direct effects on the dopaminergic system; these were COMT, DBH, DDC, DRD1, DRD2, DRD3, DRD4, and SLC6A3. SNPs located within each of these genes that are available on the Illumina Human 1M Bead Chip were then identified for analysis and annotated for exact location and function using WGAViewer (Ge & Goldstein, 2007). A total of 273 SNPs were selected for inclusion in our analyses (identical to those identified in Derringer et al., 2010). Due to our modest sample size, we chose to be conservative in our inclusion of SNPs. With the inclusion of an increasing number of genomic regions, multiple testing becomes more of a concern and the likelihood of false positives increases as increasingly system-distal gene and gene products are included. Because we used narrow inclusion criteria for “dopaminergic” SNPs, this limited the extent to which spurious SNPs could be included in the risk score.
