Two studies have examined the relationship between COMT val158met SNP (rs4680) and discounting. In the first, a study of alcoholics and healthy controls (N = 19), individuals who were homozygous for the val variant exhibited significantly more impulsive discounting (Boettiger et al., 2007). In the second study, individuals with attention deficit/hyperactivity disorder (ADHD) were compared to control participants (N = 68) and discounting differed by COMT val158met genotype, but met-met homozygotes exhibited significantly greater discounting independent of ADHD diagnosis (Paloyelis, Asherson, Mehta, Faraone, & Kuntsi, 2010), a result that differs from Boettiger et al. (2007), where val-val homozygotes exhibited the greatest discounting. Paloyelis et al. also examined discounting according to genetic variation in the DAT1 (the dopamine transporter gene) 10/6 haplotype4 and DRD4 VNTR. Interestingly, diagnosis interacted with genotype, such that control participants with two copies of the DAT1 10/6 haplotype exhibited significantly more impulsive discounting than those with fewer than two copies and exhibited a profile of preferences that was equivalent to the ADHD participants. This is important because the presence of the 10/6 haplotype is robustly associated with
