Elsevier

Biological Psychiatry

Volume 57, Issue 11, 1 June 2005, Pages 1336-1346
Biological Psychiatry

Advancing the neuroscience of ADHD
Validity of the Executive Function Theory of Attention-Deficit/Hyperactivity Disorder: A Meta-Analytic Review

Aspects of this work were presented at the conference “Advancing the Neuroscience of ADHD,” February 28, 2004 in Boston, Massachusetts. The conference was sponsored by the Society of Biological Psychiatry through an unrestricted educational grant from McNeil Consumer & Specialty Pharmaceuticals.
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One of the most prominent neuropsychologic theories of attention-deficit/hyperactivity disorder (ADHD) suggests that its symptoms arise from a primary deficit in executive functions (EF), defined as neurocognitive processes that maintain an appropriate problem-solving set to attain a later goal. To examine the validity of the EF theory, we conducted a meta-analysis of 83 studies that administered EF measures to groups with ADHD (total N = 3734) and without ADHD (N = 2969). Groups with ADHD exhibited significant impairment on all EF tasks. Effect sizes for all measures fell in the medium range (.46–.69), but the strongest and most consistent effects were obtained on measures of response inhibition, vigilance, working memory, and planning. Weaknesses in EF were significant in both clinic-referred and community samples and were not explained by group differences in intelligence, academic achievement, or symptoms of other disorders. ADHD is associated with significant weaknesses in several key EF domains. However, moderate effect sizes and lack of universality of EF deficits among individuals with ADHD suggest that EF weaknesses are neither necessary nor sufficient to cause all cases of ADHD. Difficulties with EF appear to be one important component of the complex neuropsychology of ADHD.

Section snippets

Criteria for a Primary Neurocognitive Deficit

This review examines the validity of the hypothesis that ADHD symptoms arise from a primary deficit in executive control that is necessary and sufficient to cause ADHD. At least four criteria must be met for EF weaknesses to be considered the primary deficit in ADHD.

  • 1

    Groups with ADHD must consistently exhibit weaknesses on EF measures. In addition, many argue that these weaknesses must remain significant after controlling for potential confounding variables such as age, language, general

Selection of EF Measures

Previous theorists have criticized the construct of EFs as weakly defined and overly broad (Pennington and Ozonoff 1996; Sergeant et al 2003). This assertion is supported by factor analyses of batteries of EF measures (Barkley et al 2001; Miyake et al 2000; Robbins et al 1998; Willcutt et al 2001, 2005b). These studies suggest that EF tasks comprise at least four factors: 1) response inhibition and execution, 2) working memory and updating, 3) set-shifting and task-switching, and 4)

Results of the Meta-Analysis

The meta-analysis found significant differences between groups with and without ADHD on all 13 EF tasks (Table 2). Significant group differences were obtained in 109 of 168 (65%) total comparisons; the weighted mean effect size across all comparisons was .54 (95% CI = .51–.57). Weighted mean effect sizes for all measures fell in the range considered a medium effect (d = .43–.69; Cohen 1988).

Significant group differences were obtained most consistently for stop-signal reaction time (SSRT; 82% of

Potential Moderators of the Relation Between EF and ADHD

The overall results of the meta-analysis indicate that ADHD is associated with weaknesses in several key EF domains; however, interpretation of these results is complicated by differences among the studies in sampling procedures and the diagnostic criteria used to define the groups, as well as the extent to which variables such as intelligence, reading ability, and symptoms of other disorders were controlled. In this section, we briefly examine whether any of these variables influence the

Implications for the EF Theory and Comprehensive Neuropsychologic Models of ADHD

Although ADHD is consistently associated with weaknesses in key EF domains, the magnitude of the group difference on EF measures (d = .4–.6) is much smaller than the group difference in ADHD symptoms (d = 2.5–4.0 in the studies included in the meta-analysis). Moreover, fewer than half of children with ADHD exhibit significant impairment on any specific EF task (Nigg et al 2005, this issue), and correlations between ADHD symptoms and scores on EF tasks are typically significant but small in

Evaluate the Psychometric Characteristics of EF Tasks

Relatively few studies have examined the reliability of EF tasks in children. Although some EF measures appear to have satisfactory psychometric characteristics (Delis et al 2001; Kuntsi et al 2001b), reliability estimates are only moderate (or unknown) for many of the EF measures included in the meta-analysis. Because the reliability of a measure constrains the amount of variance that the measure can explain, measures with weak psychometric characteristics will never yield large effect sizes.

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    We were supported in part during the preparation of this manuscript by National Institute of Health Grant Nos. MH 62120 (EGW), MH 63941 (EGW), MH 38820 (BFP), MH 04024 (BFP), MH 59105 (JTN), R21MH/NS66191 (SVF), and K08-MH-66072 (AED).

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