When we look at measured IQ, individual children with ADHD fall across the full spectrum of IQ. However, research that uses samples drawn from clinics often report lower IQ. For example, Barkley (1991) reviewed evidence to indicate that students with ADHD were about 7 to 15 points below comparison samples on standardized IQ tests, including samples of siblings. Barkley (1995) concluded that hyperactive-impulsive behavior has an “inherent association” with diminished verbal IQ while also noting that these scores may reflect test-taking attentional difficulties. Barkley (1998) clarified his position by stating that students with ADHD can run the gamut from gifted to mental retardation but that cognitive problems were primary features of ADHD.

Others (e.g., Leung et al., 1996) have concluded that lower IQ and academic deficits may be a consequence of ADHD rather than a core feature. The fact that lowered IQ may be an outcome explains lower scores documented in later but not earlier grade levels (Leung & Connolly, 1998). A well-controlled study further demonstrated that the distribution of IQ for students with ADHD was not different from normals (Kaplan et al., 2000).

In sum, when differences in IQ are found, they are most likely due to the selection of more severe cases of children from clinical samples (Light & DeFries, 1995), with some evidence suggesting that the IQ of children with ADHD decreases as they age (Loney, 1974). When children with ADHD have been referred for special education services, it has been associated with poor classroom achievement but not typically with low IQ (Fischer, Barkley, Edelbrock, & Smallish, 1990). In contrast, non-ADHD students who received services had both lower achievement and lower IQ than students who were not receiving such services.

Because so many possible outcomes could result just from IQ differences, it is important to control IQ and examine the essential performance and behavioral differences of students with ADHD (Felton, Wood, Brown, & Campbell, 1987). In addition to statistically equating groups in IQ, students could be divided into subtypes using IQ and achievement to determine how these factors influence the expression of the disability. Bonafina, Newcorn, McKay, Koda, and Halperin (2000) have identified four subtypes to assess: (a) ADHD with average intelligence and reading ability, (b) ADHD with average intelligence but with reading disability, (c) ADHD with high intelligence, and (d) ADHD with low achievement and low intelligence.

Even when IQ is diminished in some students with ADHD, these differences in IQ do not define ADHD. Findings of normal and above-average IQ in any of these students indicates that IQ is an independent factor. Where decreased intelligence is observed, it represents only one type of contextual intelligence (i.e., failure to adapt to their current school environment) and their lack of control in being able to select a more optimal environment (Sternberg, 1985). When students with ADHD attempt to use the third type of contextual intelligence (i.e., to shape their present environment to better fit their skills, interests, and values [Sternberg, 1985]), it causes disruption. For example, when students with ADHD were placed in settings with delays, they more frequently “dived under the table that held the apparatus, danced while watching their reflection in the observation window, and twirled their chairs. Typical children, however, sat in their chairs waiting for each trial to begin” (Schweitzer & Sulzer-Azaroff, 1995, p. 682).