Preference for Immediate Rewards in Attention-Deficit/Hyperactivity Disorder and Substance Use Disorder: A Shared Intermediate Phenotype?

This brief review explores to what extent ADHD and SUD are both associated with reward-related impulsivity, operationalized as steep delay discounting (DD). However, an integrated view on DD as a potentially shared intermediate phenotype for these frequently co-occurring conditions is lacking. Though mostly studied in separate conditions, some studies have investigated DD specifically in patients with co-occurring conditions of SUD and ADHD. In addition to reviewing literature that has documented the associations between ADHD and relatively steep DD and between SUD and relatively steep DD, we also reviewed more recent literature in which steep DD has been examined as a transdiagnostic mechanism. Overall, literature on DD in SUD suggests a dose-response relation between SUD severity and DD. The literature suggests that relatively steep DD is a mechanism associated to (ab)use of a range of substances. As for the link between DD and ADHD, several meta-analyses reported steeper DD in individuals with ADHD than controls (medium effect sizes). Because these bodies of literature are based on studies in which the influence of comorbidity was not assessed, it is hard to conclude that relatively steep DD is directly related to the ADHD symptoms per se, or to the SUD per se. Therefore, a close inspection of more recent literature in which comorbidity was taken into account, and DD was treated as a transdiagnostic mechanism, was included here. Based on this reviewed recent work, we propose a model about the nature of the co-occurrence of ADHD, SUD, and DD. Implications of the nature of co-occurring ADHD, SUD, and relatively steep DD for prevention and intervention strategies are discussed.