Dorien Bangma

150 | CHAPTER 6 concluded, following their meta-analysis, that too few studies may have been carried out so far to reliably estimate the level of delay discounting in adults with ADHD. Another explanation might be that adults, compared to children, have better learned to cope with their propensity to discount future rewards. According to Mowinckel et al. (2015) the latter explanation seems, however, unlikely as adults with ADHD are believed to experience problems in reward-related learning (Mowinckel et al., 2017; Sethi et al., 2018), which seems to be in line with the findings of the present review. Based on the currently available literature, no clear conclusions can be drawn on the level of delay discounting in adults with ADHD. With regard to deliberative decision-making, reward-related decision-making, social decision-making, decision making in relation to driving, and financial decision-making in adults with ADHD, no clear conclusions can be drawn either on the basis of this review. These domains of decision making are largely underrepresented in the current evidence base. As the included studies do seem to indicate the existence of deficits in these domains of decision making, additional research into these domains is strongly recommended. The tasks used to assess these domains of decision making possibly provide a more valid model for daily life decision making in adults with ADHD. Comorbidities. There are major differences between the studies concerning the inclusion and exclusion of participants with comorbid disorders (Table 6.2). These differences may partly explain the inconsistency in the current evidence base regarding decision making in adults with ADHD. Adults with ADHD, more often than children and adolescents, present with psychiatric comorbidities, including anxiety-, mood-, substance use-, and personality disorders (Gentile et al., 2006; Katzman et al., 2017). It is therefore not surprising that the majority of studies included in this review show a high level of (psychiatric) comorbidity in the ADHD group. A downside to the inclusion of participants with diagnoses comorbid to ADHD is that this limits the extent of which to speak of pure ADHD-effects in relation to decision-making task performance. Diagnoses comorbid to ADHD, including (current) depressive disorders (e.g., Matthies et al., 2012), can have an (additive) negative impact on the outcome measures of the decision-making tasks (Jensen et al., 1997). Excluding participants with comorbid disorders, which was done in nine studies (Table 6.2), can, however, limit the generalizability and ecological validity of the study results (Abramovitch et al., 2015; Jensen et al., 1997). Seven studies did not report on the presence of comorbidities in the ADHD groups (Abouzari et al., 2015; Hurst et al., 2011; Malloy-Diniz et al., 2008; Miller et al., 2013; Mowinckel et al., 2017; Schäfer & Kraneburg, 2015; Tamm et al., 2013). Medication. Another explanatory factor for the inconsistency in the evidence base is the treatment of ADHD symptoms through stimulant medication. Although stimulants, including methylphenidate and amphetamines, are often prescribed for the treatment of ADHD (Safer, 2016), only five studies examined the acute effects of stimulants on task performance. Three studies found that the use of stimulants had a positive effect on decision-making task performance in the ADHD group. The other two studies found no effect of methylphenidate on task performance. As Groen et al. (2013) also concluded in their study on risky decision making in individuals with ADHD, no conclusions can be drawn on the effectiveness of stimulants in