This study has limitations that could have influenced the results. First, despite considerable effort, it was not possible to follow up all individuals from the childhood sample and obtain their genetic information, although available data suggest that this subsample was representative of the original cohort (Halperin et al., 2008). Second, our modest sample size is potentially under-powered to detect genetic risk factors of small effect for an etiologically complex disorder like ADHD. Nevertheless, our ability to detect significance with our limited sample size suggests that the findings may represent robust associations. Conversely, the study lacks sufficient power to protect against making Type II errors. This could be relevant to the non-significant inverse relationship indicating that attention problems got worse even though digit span backward improved in minor allele carriers at rs4532 and rs265978, and relevant to other null findings as well. Third, this study did not have a non-ADHD comparison/control group that was followed from childhood; thus, it is not clear whether these results generalize beyond individuals with ADHD. Fourth, despite the fact that minority groups have been mostly underrepresented
