The effort to characterize the behavioral effects of genetic polymorphisms has produced a massive web of ambiguous associations and linkages [1–3]. One strategy to clarify the genetic bases of behavior is the endophenotype approach [2, 4, 5], which seeks to elucidate genetic associations with phenotypes of interest, typically diseases, by examining intermediary phenotypes (i.e., endophenotypes) that are more closely related to the functional influence of genetic variants. By characterizing endophenotypes, or "upstream" phenotypes that do not always result in the "downstream" disorder, progress may be made in both deconstructing the etiologies of complex psychiatric disorders and understanding the genetic and evolutionary basis for variation in non-disordered individuals . In addition, endophenotypes are putatively more closely connected to genetic functionality, so larger magnitude genetic effects may be evident and thus more readily detectable in smaller samples [6, 7], cf. .
Impulsivity is a prototypic candidate for the endophenotype approach because it is a trait that varies considerably in the overall population [9–12] and is associated with an array of psychiatric disorders. These include alcohol and drug dependence [13–19], pathological gambling [20, 21], attention deficit-hyperactivity disorder (ADHD) [22, 23], borderline personality disorder  and antisocial personality disorder [25–27]. Moreover, there is evidence for the heritability of impulsive behavior in both humans and non-human animals . In terms of personality disorders, familial transmission of impulsive traits have been reported [24, 29]. In addition, twin studies using the Karolinska Scale of Personality (KSP), Multidimensional Personality Questionnaire (MPQ) and Barratt Impulsivity Scale, Version 11 (BIS) also found substantial heritable components to impulsivity [30–33]. Similarly, impulsivity has also been demonstrated to be heritable in vervet monkeys as assessed by the Intruder Challenge Test , and in mice assessed by a delay discounting test . However, impulsivity has also been found to vary with such factors as gender , age [37, 38], education [37, 39, 40], health , savings  and parent rearing styles , suggesting that other variables also have a meaningful influence. Although the relative contributions of genetic and environmental variables are unclear at this point, converging lines of evidence suggest genetic factors play an important role.
As a result, a number of studies have explored the molecular genetic basis for variation in impulsivity by examining the associations between genetic polymorphisms and measures of impulsivity. Focusing on the serotonergic system, Preuss et al.  reported an association between A alleles of the 5HT2A receptor – G-1438A polymorphism and increased impulsivity, but Patkar et al.  and Baca-Garceiro et al.  did not replicate that relationship. Within the dopamine system, Retz et al.  found an association between heterozygotes of the DRD3 single nucleotide polymorphism (SNP) and increased impulsivity, and Limosin et al.  found an association with the A2 alleles of the DRD2 TaqI A SNP and increased impulsivity in alcoholics, but both represent isolated reports. More broadly, in studies of the genetics of personality, impulsivity has been examined in the context of novelty seeking, a trait of which it is a cardinal feature . From this perspective, a number of studies have found associations between long alleles of the DRD4 48 bp Variable Number of Tandem Repeats (VNTR) polymorphism and novelty-seeking, but many have not. One meta-analysis has found no overall association between DRD4 48 bp and novelty seeking , another a small effect  and a third review reports a positive association . On balance, the current empirical literature is highly heterogeneous, in terms of the genes examined, phenotypic scales used and actual findings.
A limitation of the previous attempts to characterize genetic influences on impulsivity has been the prevailing reliance on self-report measures of impulsivity. There are a number of limitations to the self-report measures in general [51, 52] and these apply also in the case of impulsivity. For example, individuals may vary considerably in their semantic construal of impulsivity-related question content and they may also vary in their positive or negative attributions about the content of the questions, creating an implicit or explicit response bias. Moreover, there is considerable evidence that individuals' self-reports can be substantially at variance with their actual behavior [51, 52], suggesting that self-reported impulsivity may not always accurately reflect actual levels of impulsivity. This is further complicated by the fact that impulsivity is itself a multifaceted construct [28, 53, 54], including aspects of cognitive deliberation, reward valuation, behavioral inhibition and behavioral execution, among others. As such, it is unlikely that one genetic polymorphism would be pleiotropically responsible for all of these diverse facets, especially given that these different aspects are not always significantly associated with each other [e.g., [55, 56]]. Indeed, there is ongoing debate as to which represent essential features of impulsivity, and which are different constructs altogether [e.g., [28, 55, 56]].
These limitations may be addressed by an increased emphasis on behavioral assessments of impulsivity. A number of behavioral indices of impulsivity have been developed [e.g., [57, 58]] and these measures more objectively assess narrowly defined aspects of impulsive behavior and may reduce the bias of self-report. Moreover, in some cases, animal models and cognitive neuroscience approaches have illuminated the underlying neurobiology subserving behavioral performance on such measures [59–62], permitting more refined hypothesis testing of genetic variants that influence impulsivity. Although behavioral testing involves considerably greater experimental burden than self-report assessments, these measures may nonetheless substantially contribute to clarifying impulsivity as an endophenotype. These behavioral endophenotypes are expected to be more powerful than similar association studies which instead use broader psychological disorders as phenotypes.
The most widely studied behavioral measure of impulsivity is the delay discounting task (DDT). From a delay discounting perspective, impulsivity is defined as the relative preference for a smaller reward, sooner in time, compared to a larger reward, later in time ; that is, the amount a person discounts a reward based on its delay. Importantly, this measure of impulsivity has proven highly sensitive to increased impulsivity in psychiatric populations. More precipitous discounting (i.e., increased impulsivity) is associated with alcohol misuse [13, 64, 65], tobacco dependence [66, 67], opiate dependence [68, 69], stimulant dependence , pathological gambling [20, 55, 71], and antisocial personality disorder . In addition, the DDT has been demonstrated to be stable over time .
Versions of the delay discounting paradigm may also be used to study impulsivity in animal models [35, 59, 60, 73]. Neurobiologically, non-human research suggests that corticostriatal-mesolimbic substrates mediate delay discounting performance [59, 61] and that dopamine is the critical neurotransmitter involved [60, 73–75]. In addition, recent human neuroimaging findings indicate that preference for smaller immediate rewards is associated with greater mesolimbic activation, whereas preference for delayed rewards is associated with greater frontal-parietal activation . Taken together, these findings suggest that impulsive decision-making from a delay-discounting perspective reflects a dynamic balance of frontal versus limbic dopaminergic activation. Importantly, there is indirect evidence that impulsivity as measured by delay discounting is heritable in humans  and direct evidence of its heritability in mouse strains .
Given the limitations to the current literature on impulsivity as an endophenotype and the potential promise of using behavioral measures, in the current study we examined impulsivity as a potential endophenotype using two dopaminergic genetic polymorphisms as candidates for observed variation in impulsivity as measured by the DDT and three traditional measures of impulsivity. These three measures include the BIS, Eysenck Impulsivity Questionnaire (EIQ), and the Sensation Seeking Scale – Form A (SSS), all of which have undergone extensive psychometric validation [11, 12, 76]. The BIS and EIQ are highly correlated and theoretically related scales, however they are associated with different neural activation profiles in a behavioral inhibition task , suggesting that they assess distinct facets of impulsivity. As previously mentioned, the BIS has shown a strong heritable component in a twin study . Sensation seeking is a related construct to impulsivity, and has been shown to be both heritable and to potentially share genetically-mediated common biological mechanisms with impulsivity . Additionally, SSS subscores are inversely related to KPS monotony avoidance, which has also been shown to be heritable . In general, the empirical literature suggests performance on these measures is heritable, although this is clearly not definitive.
The two dopaminergic genetic polymorphisms we examined were the DRD2 TaqI A and DRD4 48 bp VNTR polymorphisms. Both have been associated with psychiatric disorders involving impulsivity, namely substance abuse and ADHD (for reviews, see [78, 79]). In addition, the two polymorphisms appear to functionally influence the dopamine D2 and D4 receptors, which are densely located in the corticostriatal-mesolimbic system [80–87], the apparent neurobiological substrate and neurotransmitter system underlying delay discounting [59–61, 73–75].
The DRD2 TaqI A site is a SNP with two possible alleles, the major A2, and minor A1. The A1+ genotype (heterozygous or homozygous A1) has been most strongly associated with substance abuse, particularly alcoholism , albeit with some controversy. The A1+ genotype has also been related to pathological gambling, novelty seeking, and sensation seeking . The DRD2 TaqI A site is 9.4 kb downstream from the coding region for the dopamine D2 receptor gene. It is not in any known regulatory region, and although the A1 allele is associated with a decrease in dopamine D2 binding and glucose metabolic rates in many brain regions [83, 89, 90], its mechanism for influencing DRD2 expression is unknown. The TaqI A polymorphism is also located in a nearby kinase gene, the Ankyrin Repeat and Kinase Domain Containing 1 (ANKK1) gene, where it causes a Glutamate→ Lysine substitution [91, 92]. The results of the amino acid substitution are not known, but could impact interactions of ANKK1 proteins with other proteins including the dopamine D2 receptor . No other polymorphism has been revealed in linkage disequilibrium with TaqI A that could easily account for these associations [91–93].
The DRD4 48-bp VNTR polymorphism is in exon 3 of the gene coding for the dopamine D4 receptor. The VNTR polymorphism varies between 2 and 11 repeats of a similar 48 bp coding region sequence, with a trimodal distribution of 2, 4 and 7 repeat alleles (2R, 4R and 7R) in most, but not all, populations . Although the functional significance of the DRD4 VNTR polymorphism has not been definitively characterized, long alleles (typically 7R as opposed to 4R) have been generally found to be functionally less reactive in in-vitro expression experiments [95–99], with some heterogeneity [100–104]. Additionally, in-vivo pharmacological treatments are also generally consistent with 7R alleles resulting in less responsive D4 receptors than 4R alleles [105–109].
We predicted that possession of at least one A1 allele for the DRD2 TaqI A and at least one long allele (7-repeats or longer) of the DRD4 VNTR genotype would be associated with greater impulsivity. Moreover, we predicted that the delay discounting task would be more sensitive than the self-report measures, as reflected in larger magnitude effects. However, we did not predict one polymorphism to be more likely to exhibit significant associations relative to the other. Finally, based on previous findings reporting interactions between D2 and D4 receptor genes [e.g., ], we also examined both potential interactive effects (i.e., quantitatively disproportionate effects based on a combination of polymorphisms of both genes) and potential additive effects (i.e., linearly increasing effects based on a combination of polymorphisms of both genes).