hypofunction as an underlying mechanism is not consistent with all the published findings (e.g., Koffarnus, Newman, Grundt, Rice, & Woods, 2011; Pine, Shiner, Seymour, & Dolan, 2010). Moreover, other neurotransmitter systems have also been implicated in delay discounting (Bevilacqua et al., 2010; Mobini, Chiang, Al-Ruwaitea, Ho, Bradshaw, & Szabadi, 2000; Mobini, Chiang, Ho, Bradshaw, & Szabadi, 2000), as have important interactions between neurotransmitter systems (Winstanley, Dalley, Theobald, & Robbins, 2003; Winstanley, Theobald, Dalley, & Robbins, 2005). Thus, although diverse neurotransmitters are likely involved, an emerging theme is that genetic variation that contributes to reduced corticomesolimbic dopamine tone is associated with more impulsive discounting. This hypothesized model is presented in Figure 4.
