We will focus on two types of genetic studies in this review, twin and association studies. Twin studies estimate the heritability of traits, whereas association studies examine specific polymorphisms in relation to a phenotype. We will not include family-based linkage studies of drug abuse or animal studies on genetic determinants of stimulant effects, both which have been carefully reviewed elsewhere (Phillips et al. 2008; Kreek et al. 2005; Uhl 2006). We will also not discuss the stimulant caffeine, because it acts by distinct neurochemical mechanisms and because we have recently reviewed the genetics of caffeine elsewhere (Yang et al. 2010). We will discuss two types of association studies: candidate gene studies and genome-wide association studies. Candidate gene studies draw on prior pharmacological knowledge of how stimulants affect the brain to select ‘candidate’ genes that are likely to be the source of genetic differences. In these studies, polymorphisms within or near the candidate gene are tested to see if they are statistically associated with relevant phenotypes. Phenotypes might include measures of acute response, patterns of drug use or therapeutic response in
