Inhibitory deficits in attention-deficit/hyperactivity disorder are independent of basic processing efficiency and IQ

Background. A substantial proportion of children with attention-deficit/hyperactivity disorder (ADHD) show deficits on inhibitory control tests. However, questions remain about (i) the extent of these deficits across different inhibitory domains, (ii) their relationship to deficits in non-executive processes and (iii) whether they extend into adolescence. Methods. Seventy-seven children and adolescents with ADHD and 50 non-ADHD controls completed three inhibitory tasks, a simple two choice RT task (2CR) and an IQ assessment. Results. ADHD was moderately associated with deficits on all tasks (effect sizes d=0.5-0.9). Deficits were equally marked in childhood and adolescence. Inhibitory deficits were not associated with IQ and, although reduced substantially, remained significant after performance on a simple reaction time task was controlled for statistically. Discussion. In highlighting the significant, but limited, role of inhibitory deficits in ADHD, these results are consistent with recent accounts that emphasize the neuropsychological heterogeneity of this condition.