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 ADHD (Asherson et al., 2007) and it suggests that 10/6 haplotype may be responsible for differences in discounting, but that this effect may be obscured in individuals with the clinical condition who exhibit a generally high level of discounting. In other words, because clinical populations are comprised of individuals who have a convergence of risk factors and by their nature exhibit non-normative levels of performance, it may be that nonclinical populations will be especially important for successfully detecting genetic effects. Notably, no DRD4 VNTR genotype main effect was present in the Paloyelis et al. study, converging with Eisenberg et al. (2007) and also a recent study by Garcia et al. (2010). It also is worth noting that these associations were present only for a decision-making discounting task, not an experiential discounting task using actual delays, a point discussed in greater detail below.
