Genome-wide association studies (GWAS) have successfully discovered thousands of variants robustly associated with a variety of human traits including psychiatric outcomes long known to be heritable.1 Although addiction related GWAS typically have used thousands of individuals in their samples, there is compelling evidence across a variety of traits that even larger sample sizes will be required to discover robustly associated genetic variants.2,3 This leads to the phenomenon that as larger samples are collected and ascertained, maintaining homogeneity across multiple domains including phenotype, culture, or genetic ancestry becomes increasingly difficult.
