Dorien Bangma

152 | CHAPTER 6 individuals with ADHD is questioned by the finding that 45% of the studies reported no significant group differences. Limitations and future directions for research. The results of this literature search must be interpreted with care, and several limitations need to be acknowledged. Firstly, the diagnostic status of the participants with ADHD has not been verified in all of the included studies. Although most studies made use of a DSM interview to confirm diagnostic status prior to assessment, some studies adopted a different approach. For example, some studies made use of a (childhood) follow-up sample, leaving it unclear whether participants still met the diagnostic criteria for ADHD at the time of testing (Miller et al., 2013; Tamm et al., 2013). In a number of other studies, participants were asked whether they had received an ADHD diagnosis in the past. Such self-report methods can be seen as less reliable methods for group assignment (e.g., Hurst et al., 2011; Schäfer & Kraneburg, 2015). These differences in diagnosis verification may limit the extent of which to speak of (pure) ADHD effects, and can therefore be seen as a limitation of this review. A second limitation concerns the ecological validity of the present findings. While this review indicates the existence of an association between ADHD in adulthood and the performance on decision-making tasks, little can be said about the relationship between task performance and decision making in everyday life. The dynamic factors that contribute to daily- life decision-making, including social context, emotional arousal and the extent of gains and losses of a particular decision, do not easily compare to the artificial context of experimental performance-based tasks (Pollak et al., 2018). More research is therefore needed to firmly establish the relationship between decision-making task performance and decision making in everyday life (Groen et al., 2013). Moreover, future research should aim to improve the ecological validity of decision-making tasks. This can be achieved, e.g., by introducing social and emotional aspects to the task environment (Pollak et al., 2018), and by attaching long-term consequences to the task decisions made. Further, future research could focus on the use of tasks that focus on more specific domains of decision making, such as financial decision making or decision making in relation to driving, which may better reflect everyday decision-making situations. A third limitation, which manifests at the review level, is that despite the growth in the evidence base on adults with ADHD (see Lange et al., 2010), only a relatively small number of studies could be included in the present systematic review ( k = 31). Thus far, the findings of these studies are rather inconsistent. More research is therefore needed into decision making in adults with ADHD (for a recent meta-analysis, see Mowinckel et al., 2015). As discussed, this recommendation especially applies to the domains of decision making that are currently not sufficiently studied (e.g., reward-related decision-making, social decision-making, etc.). Future research can in this way fill the gaps in the current evidence base, and substantialize the conclusions offered by the present review. A final limitation of this review is that no risk of bias evaluation has been carried out. To the best of the authors knowledge, there is no valid and reliable measuring instrument available to date that can be used for cross-sectional studies, as have been included in this systematic review. This limitation should be taken into account when interpreting the results.