Initial inroads into understanding the genetic influences of addiction in humans relied on both genetic linkage mapping and candidate gene association studies, resulting in the identification of hundreds of potential genes contributing to the addiction process. Yet, few of these associations have been replicated in independent studies, potentially reflecting a number of false positives and/or genetic heterogeneity in which multiple genes contribute modest effects. The last decade, however, has seen a revolution in genetic technologies so that hundreds of thousands of genetic variants (or single nucleotide polymorphisms; SNPs) can be queried in thousands of individuals in a cost effective manner. This technology facilitates genome wide association studies (GWAS) that test for an association of genetic variants with an illness in order to discover genetic contributions to complex diseases. Complex diseases are caused by many genetic and environmental factors working together, and GWAS has permitted the discovery of hundreds of genetic variants that alter the risk of developing multiple complex diseases, including type 2 diabetes, Crohn's disease, and Parkinson's disease (Hindorff et al., 2010). More recently, the genetic tools of GWAS
