The impact of ADHD symptoms on intelligence test achievement and speed of performance (2023)


Attention Deficit Hyperactivity Disorder (ADHD) is one of the most commonly diagnosed psychiatric conditions in childhood (Kutcher et al., 2004). Prevalence of ADHD in the general population has recently been estimated at around 5% in childhood (Faraone, Biederman, & Mick, 2006) and it is now accepted that symptoms persist leading to clinically significant impairments in adulthood, with adult prevalence estimated at 1% (Asherson, Kuntsi, & Taylor, 2005). ADHD is defined by symptoms of inattention, impulsivity, and hyperactivity (American Psychiatric Association (APA), 2000), and symptomatic individuals can have difficulties with sustained attention, lack of impulse control or inhibition, over-activity, and following instructions (Barkley, 1998).

ADHD has further been documented as a common comorbidity among individuals with intellectual disability (Buckley et al., 2006). Intellectual measures which have been shown to discriminate ADHD in adults from normal controls include subtests of the Wechsler Scales: Digit-Symbol Coding, Arithmetic, Block Design, Digit Span (Hervey et al., 2004, Quinlan, 2001). These are all subtests that rely on speed of processing and/or working memory and which may be affected by the impulsivity and attention deficits characteristic of ADHD. Additionally, increased variability in reaction time has been a consistently reported deficit in children with ADHD (Castellanos and Tannock, 2002, Russell et al., 2006).

In the present study, Raven’s Standard Progressive Matrices (RSPM) (Raven, Raven, & Court, 1998) was used. This is a measure of both visuoperception and abstract reasoning, it is easy to administer, is reasonably culturally fair, and is a reliable and valid test of non-verbal intelligence (Lezak, 1995). It is a good measure of fluid (analytical) reasoning that allows people to solve novel problems and it taps into several working memory systems (Prabhakaran, Smith, Desmond, Glover, & Gabrieli, 1997). This test was therefore considered appropriate for the purpose of the present study, although there are available other suitable timed non-verbal tests, including some of the performance subtests of the Wechsler Adult Intelligence Scale (WAIS-R, WAIS-III and WAIS-IV).

Carpenter, Just, and Shell (1990) used computer modelling to analyse the processing involved by college students completing RSPM, and found that the “ability to induce abstract relations and the ability to dynamically manage a large set of problem-solving goals in working memory” (p. 404) are key processes that distinguish between individuals completing the matrices. These are likely to be some of the areas in which individuals with ADHD have deficits (Barkley, 1997), thus individuals with ADHD may be disadvantaged on tests that require these skills and scores obtained may not always reflect true deficits. This was evident, for example, in the case of Billy Joe Friend (Gudjonsson & Young, 2006). Prior to trial Billy Joe was reported to have an IQ score of 63, consistent with learning disability, however later testing at appeal indicated a borderline full-scale IQ of 79 and additional testing showed “significant residual problems diagnostic of ADHD in childhood” (p. 213). A neuropsychological expert concluded that Billy Joe’s intellectual deficits were secondary to his ADHD and, at time of the original IQ assessment, ADHD symptoms had prevented him from completing the test at his true intellectual capacity.

There seems to be no conclusive relationship between ADHD and intelligence at present, and research appears to be particularly lacking in forensic populations. Core ADHD symptoms of impulsivity, attention, and behavioural inhibition have been reported to affect performance-based tests in which these behavioural facets are challenged (e.g. Epstein et al., 2003) and an association between speed and accuracy in test situations has also been documented, for example on the Matching Familiar Figures Task ADHD individuals tend to respond quickly but incorrectly (Young & Gudjonsson, 2005). This suggests that ADHD adults may respond to test items impulsively, possibly without giving each one full consideration, thus performing tasks quickly but inaccurately. An important research question is therefore whether current ADHD symptoms are associated with impaired intellectual performance above that of the speed of performance? If this is the case then it suggests that ADHD symptoms adversely affect the efficacy of performance (i.e. abstract problem solving).

The aim of the present study was to examine the relationship between ADHD symptoms and performance on a non-verbal test of intelligence, namely the RSPM (Raven et al., 1998), in a forensic population. Associations of childhood and adulthood ADHD symptoms with completion time and total score on the RSPM were investigated, as well as the effect that adult symptoms have on test performance. It was expected that: (1) longer RSPM test completion times would be negatively correlated with adult ADHD symptoms (H1); (2) total RSPM score would be negatively correlated with adult ADHD symptoms (H2); and (3) longer test-completion times would be positively correlated with high test scores (H3). An exploratory analysis was also conducted in order to investigate whether, when controlling for test completion time, ADHD symptoms were significantly related to the total RSPM score obtained, demonstrating special variance beyond task completion speed.

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Copyright © 2011 Elsevier Ltd. All rights reserved.

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