Data suggest that children with reading disability (RD) and non-RD children with attention-deficit/hyperactivity disorder (ADHD) may comprise distinct subgroups. Research has been hampered by variance in definitional criteria, which results in the study of different subgroups of children. Using cluster analysis, this study empirically divided children with ADHD (N = 54), based on their Full Scale IQ (FSIQ) and reading ability. Four distinct subgroups emerged in which cognitive, behavioral, and neurochemical function was compared. Cluster 1 was of average FSIQ and reading scores; Cluster 2 was of average FSIQ but showed impairment in reading; Cluster 3 had high FSIQ and reading scores; and Cluster 4 had low scores in both domains. The groups had different patterns of cognitive, behavioral, and neurochemical function, as determined by discrepancies in Verbal-Performance IQ, academic achievement scores, parent aggression ratings, and a measure of noradrenergic function. These distinctions are discussed with regard to etiology, treatment, and long-term outcome.
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