Polymorphisms in several genes in the dopaminergic system, which targets the frontal and limbic brain structures that regulate impulsive behavior, have been identified as likely contributors to impulsivity (Kreek et al., 2005). For example, recent results indicate that genetic variation at the Val158Met polymorphism of the catechol-O-methyltransferase (COMT) gene influences decision-making behavior and underlying activity in brain activity associated with impulsive choice (Boettiger et al., 2007). The COMT enzyme plays a significant role in regulating frontal DA (Chen et al., 2004), and individuals homozygous for the enzymatically more active 158Val allele show an increased tendency to choose immediate over delayed rewards. Such genetically variation in COMT function may also contribute to other forms of impulsive behavior (Congdon and Canli, 2005; Cools and Robbins, 2004; Kreek et al., 2005). Studies linking COMT to alcoholism are equivocal (Ishiguro et al., 1999; Kauhanen et al., 2000; Kim et al., 2006; Samochowiec et al., 2006; Sery et al., 2006). However, some data suggest that alcoholic sub-phenotypes may associate with specific COMT diplotypes (Enoch et al., 2006; Kweon et al., 2005; Tiihonen et al., 1999;
