Personality and Individual Differences 50 (2011) 1273–1277

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Personality and Individual Differences

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The impact of ADHD symptoms on intelligence test achievementand speed of performance

Emily Goodwin a, Gisli H. Gudjonsson a, Jon Fridrik Sigurdsson b, Susan Young a,⇑a King’s College London, Institute of Psychiatry, De Crespigny Park, London SE5 8AF, UKb Landspitali – The National University Hospital of Iceland and University of Iceland, Iceland

a r t i c l e i n f o

Article history:Received 12 November 2010Received in revised form 11 February 2011Accepted 23 February 2011Available online 17 March 2011

Keywords:ADHD, Speed of performanceIntellectual performance

0191-8869/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.paid.2011.02.023

⇑ Corresponding author. Address: Department of Fo(PO 23), Institute of Psychiatry PO 23, De Crespigny Pa+44 207 848 5280; fax: +44 207 848 0921.

E-mail addresses: Susan.Young@kcl.ac.uk, SuzyYou

a b s t r a c t

There is evidence of an association between Attention Deficit Hyperactivity Disorder (ADHD) andimpaired performance on neuropsychological tests. Nevertheless, there is a dearth of research on thistopic, particularly among forensic populations where rates of ADHD are notably high. This study aimsto investigate the effect of ADHD symptoms on speed and performance on a non-verbal intellectual test.Forty three Icelandic male prisoners were screened for childhood ADHD using the Wender Utah RatingScale (WURS), and for adult ADHD using the DSM-IV checklist of symptoms. IQ was measured usingthe Ravens Standard Progressive Matrices (RSPM) total score, and test completion time was alsorecorded. Correlations were conducted on the measures and the effect sizes ranged between small (child-hood ADHD with RSPM total score and completion time) to large (adult ADHD with childhood symptoms,RSPM total score and completion time).

Multiple regression analysis indicated that intellectual performance was significantly negativelyaffected by fast test completion time (medium effect size), but even after controlling for this, performancewas further impaired by adult ADHD symptoms (large effect size). The results indicate that ADHD symp-toms in adulthood adversely affect intellectual test performance above the speed of performance alone.

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1. Introduction

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

ADHD has further been documented as a common comorbidityamong individuals with intellectual disability (Buckley et al.,2006). Intellectual measures which have been shown to discrimi-nate ADHD in adults from normal controls include subtests ofthe Wechsler Scales: Digit-Symbol Coding, Arithmetic, BlockDesign, Digit Span (Hervey, Epstein, & Curry, 2004; Quinlan,

ll rights reserved.

rensic Mental Health Sciencerk, London SE5 8AF, UK. Tel.:

ng@aol.com (S. Young).

2001). These are all subtests that rely on speed of processingand/or working memory and which may be affected by the impul-sivity and attention deficits characteristic of ADHD. Additionally,increased variability in reaction time has been a consistently re-ported deficit in children with ADHD (Castellanos & 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 measureof both visuoperception and abstract reasoning, it is easy to admin-ister, is reasonably culturally fair, and is a reliable and valid test ofnon-verbal intelligence (Lezak, 1995). It is a good measure of fluid(analytical) reasoning that allows people to solve novel problemsand it taps into several working memory systems (Prabhakaran,Smith, Desmond, Glover, & Gabrieli, 1997). This test was thereforeconsidered 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 AdultIntelligence Scale (WAIS-R, WAIS-III and WAIS-IV).

Carpenter, Just, and Shell (1990) used computer modelling toanalyse the processing involved by college students completingRSPM, and found that the ‘‘ability to induce abstract relationsand the ability to dynamically manage a large set of problem-solv-ing goals in working memory’’ (p. 404) are key processes that dis-tinguish between individuals completing the matrices. These arelikely to be some of the areas in which individuals with ADHD have

1274 E. Goodwin et al. / Personality and Individual Differences 50 (2011) 1273–1277

deficits (Barkley, 1997), thus individuals with ADHD may be disad-vantaged on tests that require these skills and scores obtained maynot always reflect true deficits. This was evident, for example, inthe case of Billy Joe Friend (Gudjonsson & Young, 2006). Prior totrial Billy Joe was reported to have an IQ score of 63, consistentwith learning disability, however later testing at appeal indicateda 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’sintellectual deficits were secondary to his ADHD and, at time ofthe original IQ assessment, ADHD symptoms had prevented himfrom completing the test at his true intellectual capacity.

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

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

2. Method

2.1. Design

To investigate the relationship between ADHD symptoms andperformance on the RSPM, a within-participants design was used.

2.2. Participants

An initial sample of 73 Icelandic prison inmates participated inthe study, of whom 19 did not complete a Wender Utah RatingScreen for childhood ADHD and so were not included in the currentanalysis as a childhood diagnosis of ADHD is a prerequisite of apossible diagnosis in adulthood. A further eight did not completethe RSPM and so were also not included, and three were excludedbecause they were female. For the remaining 43 participants agesranged from 19 to 44 years (mean = 28.9, standard deviation = 6.5),and all were of Caucasian ethnic background. The index offencesthat led to the current prison sentences being served were splitinto six categories and the number of participants in each were

as follows; property offence n = 18 (42%), drug offence n = 9(21%), violent offence n = 6 (14%), sexual offence n = 4 (9%), serioustraffic violation n = 3 (7%), and other offence n = 3 (7%). Details ofthe participants’ ADHD symptoms and personality traits usingthe Eysenck Personality Questionnaire are in Gudjonsson, Sigurds-son, Young, Newton, and Peersen (2009).

2.3. Measures

2.3.1. Wender Utah Rating Screen (WURS; Ward, Wender, & Reimherr,1993)

This measure provides an aid for retrospectively diagnosingchildhood ADHD. It was originally a 61-item scale including itemsrelating to childhood behaviours and medical health, although the25-item self-report version of the scale is commonly administeredand was the measure used in this study. The items are those orig-inally found to give the greatest mean differences between ADHDpatients (N = 81) and the non-ADHD control group (N = 100, Wardet al., 1993). Items are rated on a 5-point Likert-type scale; 0 = ‘notat all or very slightly’, 1 = ‘mildly’, 2 = ‘moderately’, 3 = ‘quite a bit’,and 4 = ‘very much’. It was found that a cut-off score of 46 or high-er correctly identified 86% of adults with ADHD in childhood, and99% of controls (Ward et al., 1993), and this cut-off was appliedin the current study.

2.3.2. Diagnostic statistical manual IV checklist of ADHD symptoms(APA, 1994)

This 18-item self-report checklist measures current symptomsof ADHD in adulthood. It comprises nine items relating to problemswith inattention and nine items relating to hyperactivity-impulsiv-ity. The checklist items are in line with the DSM-IV criteria forADHD (APA, 1994). Each item is scored on a 3-point Likert-typescale; 0 = ‘never’, 1 = ‘sometimes’, and 2 = ‘often’. Only those whohad met the cut-off criterion for childhood ADHD as measuredby the WURS could obtain a diagnosis of adult ADHD on theDSM-IV checklist. Full adult diagnosis (i.e. being fully symptomaticin adulthood) required that six or more symptoms from the inat-tention section or six or more symptoms from the hyperactivity/impulsivity section were rated as being present ‘often’ (i.e. scoredas 2) in the previous 6 months. Participants were classified as beingin partial remission of symptoms if they met criteria for ADHD inchildhood and scored 17 or more on the DSM-IV checklist butdid not meet criteria for being fully symptomatic in adulthood,as this score equates to one standard deviation above the meanfor a non-control group (Young, 1999).

2.3.3. Raven Standard Progressive Matrices (RSPM; Raven et al., 1998)This is an objective, non-verbal test of analytic intelligence con-

sisting of sixty matrices presented in five sets, A–E. Each matrixtest depicts a pattern from which one piece is missing. The partic-ipant is required to select the piece which correctly completes thematrix from several given options. Possible scores range from 0 to60.

2.4. Procedure

Prisoners were provided with an information sheet detailingwhat the research procedure would entail for them, in order thatthey could give informed consent to participate. They were in-formed that their participation would be confidential and anony-mous, and that their non-participation would not affect theirmanagement or length of stay within the prison. They were in-formed of their right to withdraw without consequence from thestudy at any time. For those who consented to take part, twoscreening measures were used to identify ADHD (the WURS and

Table 3Results of the multiple regression analyses of the effect of speed (test completiontime) and ADHD symptoms (DSM-IV checklist) on test performance (RSPM totalscore).

B Standard b t Adjusted R2

E. Goodwin et al. / Personality and Individual Differences 50 (2011) 1273–1277 1275

the DSM-IV checklist). The RSPM were administered, and bothscore and time taken to complete were recorded.

The study was approved by the Icelandic Data ProtectionAuthority, the Prison and Probation Administration, and the EthicsCommittee at the Institute of Psychiatry, King’s College, London.

error B

Block1

Constant 25.59 3.58 7.15***

Timetaken ontest

.70 .28 .47 3.38** 0.20F(1,40) = 11.45**

Block2

Constant 35.42 5.07 6.99***

Timetaken ontest

.48 .21 .32 2.25*

DSM-IVADHDscore

�.44 .17 �.37 �2.59* 0.30F(1,39) = 9.88***

* p < .05.** p < .01.

*** p < .001 level.

3. Results

Of the 43 participants, 21 met criteria for childhood ADHD. Ofthese, eight were in full remission and nine in partial remissionof their symptoms, and four met criteria for ADHD in adulthood(Inattentive type = 1; Hyperactive type = 2; and Combinedtype = 1). Table 1 provides descriptive data for the ADHD and RSPMmeasures (i.e. mean scores, standard deviations, and range ofscores). Assumptions of normality for all measures and linearitywere met, as investigated using the Kolmogorov–Smirnov test ofnormality (Wender Utah total score p = .20, DSM-IV total scorep = .19, RSPM total score p = .20, and RSPM time taken p = .08)and P–P plots.

Table 2 shows the Pearson product moment correlations be-tween ADHD symptoms (childhood and current), the total RSPMscore, and completion time on the RSPM, and the significance level.The effect sizes, according to Cohen’s (1992) recommendations,ranged between small (childhood ADHD with RSPM total scoreand completion time) to large (adult ADHD with childhood symp-toms, RSPM total score and completion time).

In order to further investigate if current ADHD symptoms con-tributed to the variance in intelligence on the RSPM beyond thespeed of performance, a multiple regression, using a hierarchical(Blockwise) entry method, was carried out. To account for the pos-sible confounding or moderating effect of speed on test perfor-mance (Young & Gudjonsson, 2005), possible effects of testcompletion time were controlled for by entering this variable intothe first block. ADHD symptoms, as measured by DSM-IV scorewere then entered in the second block. As shown in Table 3, bothmodels were significant. In Block 1, test completion time contrib-uted significantly explaining 20% of the variance in IQ (medium ef-fect size, p < .01), however in Block 2, when controlling for test

Table 1Descriptive data on the childhood and adult ADHD scales, RSPM total score and RSPMtime taken.

Measure Mean (SD) (Range)

Wender Utah Childhood ADHD 46.79 (26.32) (2–98)DSM checklist adult ADHD 14.19 (8.18) (0–29)RSPM total score obtained 36.70 (9.75) (11–56)RSPM time taken (minutes) 15.86 (6.63) (6.0–37.4)

Table 2Pearson product moment coefficients between childhood ADHD symptoms (mea-sured by the WURS), current ADHD symptoms (measured by the DSM-IV checklist),RSPM total score, and RSPM test completion time.

Childhood ADHD(total score obtainedon WURS)

Total scoreobtained onRSPM

Testcompletiontime onRSPM

Total score obtained onRSPM

�.23

Test completion timeon RSPM

�.20 .48*

Adult ADHD (totalscore obtained onDSM-IV checklist)

.52* �.50* �.41*

* p < .01 level (one-tailed).

completion time, a further 10% was explained by the inclusion ofADHD symptoms (large effect size, p < .001). This indicates thatwhile speed may be sacrificed for accuracy on the test, when con-trolling for this variable ADHD symptoms continue to affect testperformance.

4. Discussion

This study aimed to investigate the relationship between ADHDsymptoms and achievement on the RSPM as a performance-basedmeasure of intelligence. All three hypotheses were supported. Thetime taken to complete the RSPM was negatively correlated withadult ADHD symptoms, supporting H1. The total RSPM score wasnegatively correlated with adult ADHD, supporting H2. Longertest-completion times were positively correlated with higherRSPM scores (supporting H3). Multiple regression analysis indi-cated that, when controlling for test completion time, ADHD symp-toms were indeed significantly related to the total RSPM scoreobtained, and correlational analysis further suggested that thisrelationship is related to current, rather than childhood, symp-toms. These are important findings. They suggest that the prob-lems with speed and performance observed in the current studyamong prisoners who are symptomatic for ADHD are largelycaused by persistence of ADHD symptoms rather than a childhoodhistory of ADHD. The implication is that as ADHD symptoms remit,speed of performance becomes slower and more efficient. Thesechanges in speed and performance, their relationship with ADHDsymptoms and their precise causes would be most effectivelyinvestigated in longitudinal studies.

The association between ADHD symptoms and faster test com-pletion may be explained by the common deficits among ADHDindividuals in relation to their impulsivity, poor attention, andbehavioural disinhibition. It may be that these individuals rushthrough items without duly considering each one, and this hasbeen reported among ADHD adults in relation to performancebased tests such as the Matching Familiar Figures Task (Young &Gudjonsson, 2005), the Tower of London task (Young, Morris,Toone, & Tyson, 2007), and the Continuous Performance Test(Epstein, Conners, Sitarenios, & Erhardt, 1998). Increased errorsof omission and commission on the Continuous Performance Testindicate the role that ADHD symptoms play in relation to test per-formance, and as concluded by Epstein et al. (1998), are consistentwith the clinical presentation of those with ADHD.

The results of the multiple regression analysis showed that cur-rent ADHD symptoms added to the variance in performance on theRSPM beyond that of speed of performance. With this in mind, if

1276 E. Goodwin et al. / Personality and Individual Differences 50 (2011) 1273–1277

those with high and low levels of ADHD symptoms were given aset amount of time to spend on the test, those with a greater fre-quency/severity of symptoms would still be expected to performless well, possibly due to symptoms of inattention, impulsivity,and poor efficiency in effectively managing time. Due to the smallsample size we cannot establish from the present findings whichfacets of ADHD specifically influence the relationship; the resultsprovide some support for the suggestion that ADHD symptomsundermine intellectual performance and mask the true abilitiesof the individual. This has been documented in applied circum-stances such as the case of Billy Joe Friend whose ADHD symptomsprevented him from completing an intellectual test at his truecapacity (Gudjonsson & Young, 2006).

The study contributes to our knowledge about specific neuro-cognitive functions in ADHD that are associated with recovery.Young and Gudjonsson (2008) found that many residual functionaldeficits continue after remission of ADHD childhood symptoms,whereas the present findings suggest that non-verbal intelligenceand speed of performance recover once ADHD symptoms havefully remitted. The relationship, however, may be complex andvary according to measures used. For example, deficits in workingmemory (using a digit span subtest) have been found to persistinto adulthood in those with ADHD in childhood but not adulthood(i.e. remitters) (Barkley, Murphy, & Fischer, 2008) whereas theywere found to desist in remitters using a larger battery (theWAIS-III Working Memory Index) (Halperin, Trampush, Miller,Marks, & Newcorn, 2008). Omission and commission errors on aContinuous Performance Test may desist in remitters (Fischer,Barkley, Smallish, & Fletcher, 2005; Halperin et al., 2008).Undoubtedly higher order executive function deficits (e.g. atten-tion, impulsivity, disorganisation, poor planning ability) willgreatly influence test performance and even limited improvementin these types of problems may lead to a disproportionately greaterimprovement in speed of performance and tests of general intellec-tual abilities.

It should be noted that the findings reported here may not begeneralisable to the wider population. Although a fair representa-tion of Icelandic prison inmates, the sample is small and some lim-itations should be considered. Firstly, only screening measureswere used to identify ADHD, and it is documented that a compre-hensive, multi-informant approach will provide the broadest pic-ture of behavioural difficulties especially for offenders (Young &Gudjonsson, 2010). Symptoms of ADHD should also be consideredalongside other possible explanations: motivational factors mayaffect performance (e.g. Smith & Smith, 2002), and attitudinaland conduct factors may impact on the test performance andmay help to further explain the remaining variance (conduct disor-der, for example, is frequently found to occur alongside ADHD inchildren, Waschbusch, 2002; but this was not measured in thepresent study). Furthermore, data were not collected in relationto possible influencing factors such as neurological impairmentor family history, which may also provide alternative explanationsof the apparent relationship between ADHD symptoms and testperformance. The definition of remission used may also have somebearing on results (Biederman, Mick, & Faraone, 2004).

Within the context of these limitations however, and given theincreased rates of ADHD symptoms found in prison populations,the study indicates a need for the identification and proper evalu-ation of offenders with ADHD. It is important to determine thesefunctional deficits and consider how these will affect their rehabil-itation. If these are not accurately assessed the institution may setunrealistic targets which they fail to complete. In turn this maymean they are unable to meet early release criteria, which is costlyboth for the individual and the prison service.

Future work should be conducted within clinical populations toinclude those with a full clinical diagnosis of ADHD, and a medica-

tion follow-up study would further help to clarify the role thatADHD symptoms play regarding test performance, as it might beexpected that those whose symptoms are treated by medicationwould perform better if re-tested. The role of motivation factors,including level of effort, also needs to be investigated. It would alsobe of interest to examine subtype differences on test performanceas neuropsychological deficits may be attributed more specificallyto certain classes of symptoms (e.g. impulsivity on the MatchingFamiliar Figures Task, Young & Gudjonsson, 2005). The small num-ber of participants for which this information was available in thisstudy precluded such analysis.

Finally, the work of Prabhakaran et al. (1997) has demonstrateda relationship between performance on the RSPM and several areasin the brain related to working memory. Their results suggest thatfluid reasoning is mediated by a composite of working memory.Thus, the finding in the present study of impaired performanceon the RSPM being evident beyond speed among the ADHD symp-tomatic participants, suggests that the relationship between ADHDsymptoms and fluid intelligence is possibly caused by interferencewith working memory systems. This may partly explain thepresent findings and merits further investigation, including usingfunctional magnetic resonance imaging techniques during brainactivation.

Acknowledgement

The authors are grateful to Anna Kristin Newton and MariusPeersen, who conducted most of the interviews and administeredthe tests.

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