Clearly only a small number of molecular genetic studies have been conducted on humans and those that have include small to moderate sample sizes, often comprising both clinical and nonclinical populations. As such, it is challenging to make definitive conclusions, but one theme that is emerging is the implication of genetic variation that results in changes in dopamine neurotransmission and, in particular, reduced levels of dopamine neurotransmission. For example, in the case of the association between DRD2/ANKK1 Taq IA and interaction of DRD2/ANKK1 and DRD4 VNTR (Eisenberg et al., 2007), the alleles associated with greater impulsivity have both been associated with dopaminergic hypofunction (Jonsson et al., 1996; Jonsson et al., 1999; McGeary, 2009). Similarly, in the association between impulsive discounting and COMT val-val genotype, the val allele is associated with greater enzymatic metabolism of dopamine (i.e., more rapid degradation of dopamine, terminating the activity faster) (Savitz, Solms, & Ramesar, 2006), again implicating dopamine hypofunction. Finally, in the case of the DRD2 C957T polymorphism, the C allele is associated with reduced striatal D2 binding (Hirvonen et al., 2004), further supporting a role for reduced dopamine activity as a neurobiological mechanism for genetic influences on delay discounting.
