Due to its strong association with addiction, availability of well established animal models, and relatively well elucidated neural substrates, DD can potentially serve as an intermediate biobehavioral phenotype that could help bridge the gap between genetic risk factors and complex psychiatric phenotypes such as substance use or externalizing disorders in which impulsivity is implicated as a core underlying dysfunction. An important requirement of such an intermediate phenotype is its heritability. However, little is currently known about the relative contribution of genetic and environmental factors to individual differences in DD in humans. Several previous studies have provided converging evidence for about 45% heritability for self-reported measures of impulsivity (Pedersen et al. 1988; Hur and Bouchard 1997; Seroczynski et al. 1999). However, these studies provide little information about genetic influences on DD because the self-report instruments used in these studies did not incorporate temporal discounting as a distinct component. Furthermore, correlations between DD measures and self-reported impulsivity have been fairly modest (Beck and Triplett 2009; Richards et al. 1999) or non-significant (Reynolds et al. 2006), suggesting that DD may represent a construct
