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 in molecular genetic studies of ADHD to date, population structure limitations (i.e. an ethnically/racially mixed population of inner-city individuals of lower SES) makes the results susceptible to population stratification effects. As noted, all results were analyzed with and without minority status as a covariate with no appreciable differences detected between the two sets of analyses. Lastly, it is important to acknowledge that Digit Span forward and backward are somewhat crude measures of working memory maintenance and manipulation, respectively. A comprehensive, multivariate approach to studying lower-order versus higher-order working memory might have been preferred. However, because of these limitations, this study focused on several polymorphisms for which there exists an expectation of association based on prior work. Nevertheless, replication in larger, independent samples of different racial/ethnic background is needed.
