185

British Journal of Clinical Psychology (2012), 51, 185–196C© 2011 The British Psychological Society

TheBritishPsychologicalSociety

www.wileyonlinelibrary.com

IQ as a predictor of clinician-rated mental healthproblems in children and adolescents

Børge Mathiassen1,2∗, Per Hakan Brøndbo1, Knut Waterloo3,2,Monica Martinussen4, Mads Eriksen5, Ketil Hanssen-Bauer6,7

and Siv Kvernmo1,8

1Division of Child and Adolescent Health, Department of Child and AdolescentPsychiatry, University Hospital of North Norway, Tromsø, Norway

2Faculty of Health Sciences, Department of Psychology, University of Tromsø,Norway

3Division of Neurology, Department of Child and Adolescent Psychiatry, UniversityHospital of North Norway, Tromsø, Norway

4RKBU-North, Faculty of Health Sciences, University of Tromsø, Norway5Alta Child and Adolescent Mental Health Service, Finnmark Hospital Trust, Alta,Norway

6Centre for Child and Adolescent Mental Health, Eastern and Southern Norway,Oslo, Norway

7Department of Research and Development, Division of Mental Health Services,Akershus University Hospital, Lørenskog, Norway

8Faculty of Health Science, Department of Clinical Medicine, University of Tromsø,Tromsø, Norway

Objective. Previous studies indicate that low IQ is a substantial risk factor fordeveloping mental health problems. Based on these results, we hypothesized that IQmay predict some of the variance in clinician-rated severity of children’s mental healthproblems measured with the Children’s Global Assessment Scale (CGAS) and Healthof the Nation Outcome Scales for Children and Adolescents (HoNOSCA). The otheraims of this study were to examine if there was any difference in the predictive abilityof the different IQ scores of the Wechsler Intelligence Scale for Children, Third edition(WISC-III) and to examine if parent-rated measure of child mental health problems couldpredict the scores on CGAS and HoNOSCA after controlling for IQ, age, and gender.

Methods. In this study, 132 patients at three outpatient clinics in North Norway wereassessed with the parent version of the Strength and Difficulties Questionnaire (SDQ),HoNOSCA, CGAS, and with the WISC-III.

∗Correspondence should be addressed to Børge Mathiassen, Department of Child and Adolescent Psychiatry, Divisionof Child and Adolescent Health, The University Hospital of North-Norway, P.O. Box 19, N-9038 Tromsø, Norway(e-mail: borge.mathiassen@unn.no).

DOI:10.1111/j.2044-8260.2011.02023.x

186 Børge Mathiassen et al.

Results. Hierarchical regression analyses were conducted with HoNOSCA andCGAS as dependent variables. Demographics, WISC-III IQ scores, and SDQ wereentered as independent variables. The model with HoNOSCA as the dependent variablepredicted 25% of the total variance. The WISC-III full-scale IQ predicted an additional6% of the variance. The analyses with CGAS as the dependent variable gave no significantresults.

Conclusion. When a patient has a high HoNOSCA score, an intelligence test inaddition to an evaluation of symptoms on mental health should be considered. Futureresearch ought to examine whether HoNOSCA’s ability to detect change might beaffected by patients IQ.

In mental health services, intelligence tests and the concept of IQ have severalapplications. According to the diagnostic guidelines in ICD-10, an assessment ofintellectual ability is necessary when diagnosing mental retardation and most disordersof psychological development (WHO, 1993). For several of the psychiatric disorders, IQis a predictor of prognosis. For example, in schizophrenia, autism, and attention deficithyperactivity disorder (ADHD), low IQ is an indicator of poor prognosis (Hollis, 2000;Howlin, Goode, Hutton, & Rutter, 2004; Owens et al., 2003). Low IQ is also a significantrisk factor for developing mental health problems. Population studies have indicatedthat approximately one-third of children with intellectual disability have diagnosablepsychiatric disorders (Emerson, 2003; Emerson & Hatton, 2007; Rutter, Tizard, Yule,Graham, & Whitmore, 1976). It is estimated that 14% of all British children with apsychiatric disorder have an intellectual disability (Emerson & Hatton, 2007). In ourstudy, we examined the associations between IQ and clinician-rated mental health inchildren and adolescents.

The cognitive reserve (CR) model has been proposed as an explanation for the rela-tionship between IQ and neuropsychiatric disorders among adults (Barnett, Salomond,Jones, & Sahakian, 2006). The construct of ‘cognitive reserve’ has been used to explainthe disjunction between severity of neurological diseases or damage and clinical outcome(Stern, 2009). CR is a protective factor and a proxy measure of brain reserve capacityavailable to cope with brain damage. Education, occupation attainment, and IQ areexamples of proxy measures. The validity of the CR model has received limited researchattention among persons with mental health disorders. To our knowledge, most of thestudies that have used IQ as a measure of CR have been conducted among adults. TheDunedin birth cohort study showed that low IQ in childhood (IQ ≤ 85) increasedthe risk for developing psychiatric problems at 32 years of age (Koenen et al., 2009).In addition, low IQ predicted co-morbidity and the persistence of psychiatric disease. Inanother study from this cohort, it was found that low IQ at age 5 predicted post-traumaticstress disorder (PTSD) at age 26 among persons exposed to traumatic events (Koenen,Moffitt, Poulton, Martin, & Caspi, 2007). Several studies among military personnel havefound similar results (Gale et al., 2008; Macklin et al., 1998). In a cohort study of Swedishconscripts (N = 50,053), it was found that low IQ at enrolment (18–20 years) increasedthe risk of schizophrenia, severe depression, and other non-affective psychoses during a27-year follow-up period (Zammit et al., 2004).

The process of diagnosing and assessing mental health problems should be basedon information obtained from multiple sources (e.g., parents, patients, and teachers).The clinician’s evaluation of the severity of a patient’s symptoms is an important partof this process. The Children’s Global Assessment Scale (CGAS; Shaffer et al., 1983) andHealth of the Nation Outcome Scales for Children and Adolescents (HoNOSCA; Gowers

IQ as a predictor of mental health problems 187

et al., 1999b) are frequently used for this purpose. CGAS is a single factor measureof global functioning in children and adolescents. HoNOSCA is a broad measure ofbehavioural, symptomatic, social, and impairment domains in children and adolescents.A study of concurrent validity has shown that there is a large negative correlation(r = -.64) between CGAS and HoNOSCA (Yates, Garralda, & Higginson, 1999), indicatinga large overlap between the measures.

Only to a limited degree has the relationship between IQ and CGAS and HoNOSCAbeen studied. Among children admitted to a psychiatric inpatient unit, a moderateassociation has been found between IQ and CGAS (Green, Shirk, Hanze, & Wanstrath,1994). In a study comparing offspring of depressed and non-depressed parents, low IQpredicted CGAS scores in the clinical range (Weissman, Warner, & Fendrich, 1990).The correlation between IQ and HoNOSCA has been examined in a study of cognitiveperformance among youth with schizophrenia and bipolar disorder at a psychiatricinpatient unit (Pogge et al., 2008). The Wechsler Intelligence Scale for Children, Thirdedition (WISC-III; Wechsler, 1992) was used to assess cognitive performance in this study.The results showed no significant associations between the IQ scores at admission andthe HoNOSCA score at follow-up 6 years later.

Research on the association between parent-rated emotional and behaviour symp-toms measured with the Strength and Difficulties Questionnaire (SDQ; Goodman, 1997)and clinician-rated measures has been found to be small to moderate. Becker et al.(2006) found that among 1,459 children with ADHD, there were small correlationsbetween CGAS and the parent-rated SDQ scales measuring emotional problems, conductproblems, hyperactivity, peer problems, and prosocial behaviour. The associationbetween HoNOSCA and the parent version of the SDQ has been studied among patientstreated at outpatient and day-patient clinics (Yates et al., 1999). A moderate correlationbetween HoNOSCA and the SDQ scale measuring conduct problems was detected, andsmall correlations with the SDQ scales measuring emotional problems, hyperactivity,peer problems, and prosocial behaviour were observed.

The aim of the current study was to examine if IQ could predict the severity ofchildren’s mental health problems as measured with CGAS and HoNOSCA. Based onthe results from previous studies indicating that low IQ is a substantial risk factor fordeveloping mental health problems, we hypothesized that IQ may predict some of thevariance in the severity of children’s mental health problems. Based on the CR model,we would expect more severe problems among children with lower IQ scores (Barnettet al., 2006). We also wanted to examine if there were any differences in the predictiveability of the different WISC-III IQ scores. Furthermore, we wanted to examine if parent-reported emotional and behavioural symptoms measured with the SDQ had any impacton clinician-rated mental health problems after controlling for age, gender, and IQ. Toour knowledge this has not been studied previously.

MethodsParticipantsThe participants were children and adolescents referred to three Child and AdolescentMental Health Outpatient Clinics in Northern Norway. Demographic and clinicalcharacteristics of the participants are presented in Table 1 for boys, girls, and the totalsample. A total of 132 patients, 55% boys (n = 72) and 45% girls (n = 60) who had validCGAS, HoNOSCA, and IQ tests were included in the study. The mean age was 11.5 years

188 Børge Mathiassen et al.

Table 1. Descriptive data for boys, girls, and the total sample

Boys Girls Total(n = 72) (n = 60) (N = 132)

M SD M SD M SD t

HoNOSCA 11.50 (3.85) 10.77 (5.16) 11.17 (4.49) −0.91, p = .37CGAS 68.01 (8.79) 69.35 (11.53) 68.62 (10.11) 0.76, p = .45WISC-III IQ scoresFSIQ 85.44 (18.83) 83.77 (19.54) 84.68 (19.10) −0.50, p = .62VIQ 86.28 (17.40) 81.07 (17.18) 83.91 (17.43) −1.72, p = .09PIQ 87.99 (20.49) 90.75 (21.76) 89.24 (21.04) 0.75, p = .45

SDQ parent ratedEmotional problems 2.96 (2.49) 3.72 (2.20) 3.30 (2.38) 1.84, p = .07Conduct problems 2.39 (1.84) 2.18 (2.05) 2.30 (1.92) −0.61, p = .54Hyperactivity 6.06 (2.84) 4.65 (2.71) 5.42 (2.86) −2.89, p � .00Peer relationshipproblems

2.92 (2.34) 2.85 (1.99) 2.89 (2.18) −0.18, p = .86

Prosocial behaviour 7.38 (2.09) 7.83 (1.97) 7.58 (2.04) 1.29, p = .20

(SD = 2.9), and the girls (M = 12.1, SD = 3.0) were significantly older (t(130) = 2.16,p = .033) than the boys (M = 11.0, SD = 2.8).

The results from the SDQ parent-rated measure of child mental health are described inTable 1. Classified according to British norms (Goodman, 2001), the proportion of SDQscales in the abnormal/borderline range were as follows: emotional problems 47.9%,conduct problems 37.1%, hyperactivity 51.5%, peer relationship problems 54.5%, andprosocial behaviour 18.2%.

Referrals to a Norwegian child and adolescent mental health outpatient clinic mustbe made using a form, which includes a reason for referral. It is possible to list up tothree reasons. The aggregated categories of reasons for referral of the participants arepresented in Table 2.

Table 2. Reasons for referrals for the total sample (N = 132)

n %

Autism 2 1.5Psychosis 1 0.8Suicidal risk 5 3.8School absence 8 6.1Emotional problems 26 19.7Hyperactivity 60 45.5Conduct problems 52 39.4Learning or language problems 32 24.2Somatic symptoms 7 5.3Eating disorder 6 4.6Other problems not specified 17 12.9No problems 1 0.8Missing 5 3.8

IQ as a predictor of mental health problems 189

MeasuresThe CGAS (Shaffer et al., 1983) is a rating scale that measures general functioning inchildren aged 4–16 years, with a range from 100 (superior functioning) to 1 (needsconstant supervision). The most impaired level of functioning for the last 2 weekswas rated. CGAS has been evaluated in several studies and is widely used to assessseverity of mental health problems and outcome (Rey, Starling, Wever, Dossetor, & Plapp,1995; Schorre & Vandvik, 2004). In a study of inter-rater reliability among cliniciansworking in the Norwegian child and adolescent mental health service, an intra-classcorrelation coefficient (ICC) of .61 was found for the CGAS (Hanssen-Bauer, Aalen,Ruud, & Heyerdahl, 2007). In a comparable cross-national study, a similar ICC was found(Hanssen-Bauer, Gowers et al., 2007).

The HoNOSCA (Gowers et al., 1999a; Gowers et al., 1999b) consists of 15 scalesthat are rated from 0 (no problem) to 4 (severe to very severe problem). In the currentstudy, only the first 13 scales were used and its total score was used to indicate overallseverity of mental health problems (range 0–52). The age range for the HoNOSCA is3–18 years. The ratings were taken from the most impaired level of functioning in thesame time frame (2 weeks) as the CGAS. HoNOSCA has been evaluated in several studiesand has been found to be easy to use, reliable, valid, and sensitive to change (Bilenberg,2003; Brann, Coleman, & Luk, 2001; Garralda, Yates, & Higginson, 2000; Hanssen-Bauer,Aalen et al., 2007; Hanssen-Bauer, Gowers et al., 2007; Pirkis et al., 2005). In a study ofinter-rater reliability among clinicians working in the Norwegian child and adolescentmental health service an ICC of .81 was found for HoNOSCA (Hanssen-Bauer, Aalen et al.,2007). In a comparable cross-national study, the ICC was found to be .84 (Hanssen-Bauer,Gowers et al., 2007).

The WISC-III, Norwegian version (Sonnander, 1998), is an intelligence test forchildren aged 6–16 years. The test consists of 13 subtests that are combined into three IQscores: full-scale IQ (FSIQ), verbal IQ (VIQ), and performance IQ (PIQ). Both the split-halfand test–retest reliability of the WISC-III IQ scores are high (rxx > .93) (Wechsler, 1992).

The SDQ (Goodman, 1997) is a behavioural screening questionnaire designed forchildren and adolescents aged 3–16 years. It has been widely used in research inthe Nordic countries (Obel et al., 2004). There are separate SDQ forms for youths,parents, and teachers. In this study, the parent version was used. Each form consists of25 items divided into the following scales: emotional symptoms, conduct problems,hyperactivity/inattention, peer relationship problems, and prosocial behaviour. Thefactor structure of the SDQ has been replicated with a confirmatory factor analysesin a sample of Norwegian children (Ronning, Handegaard, Sourander, & Morch, 2004).

ProcedureAt the intake session (T1) referred patients were informed about the study by theclinician. Parents and/or children above the age of 12 had to give their written informedconsent to take part in the study. At T1, the participants were assessed with HoNOSCA,CGAS, and the parent version of the SDQ. After being placed on a waiting list, thepatients were reassessed (T2) when the treatment was initiated. The same measures asat T1 were used. In addition, the patients were assessed with an intelligence test. In thisstudy, data from the T2 assessment were used to minimize the time differences betweenthe administration of the intelligence test and the different measures of mental health.Missing data at T2 were replaced with data from T1. At T2, nine HoNOSCA and 24 SDQswere replaced with T1 data.

190 Børge Mathiassen et al.

The study was approved by the Regional Committee for Medical Research Ethics andthe Norwegian Data Inspectorate.

Statistical analysesAll statistical analyses were performed with SPSS version 16.0 (SPSS Inc. Chicago, IL).The difference between boys and girls were investigated using independent samplest-test. Six hierarchical regression analyses were conducted to examine how much of thevariance in clinician-rated mental health problems was predicted by the different WISCIQ scales. In three of the analyses HoNOSCA was the dependent variable, while CGASwas the dependent variable in the other three analyses. In all analyses, the independentvariables were entered in three steps. In step 1, age and gender were entered. In step2, the WISC-III scales FSIQ, PIQ, and VIQ were entered in separate analyses. In step 3,the SDQ scales emotional symptoms, conduct problems, hyperactivity/inattention, peerrelationship problems, and prosocial behaviour were entered.

Age and gender were entered in the first step of the regression models, as we wantedto examine the effect of IQ after controlling for the effect of these variables (Cohenet al., 1993). IQ was entered in the next step before entering the SDQ scales. The orderof the variables was based on results from longitudinal studies indicating that low IQis both a risk factor for developing mental health problems and, in time, precedes thedevelopment of mental health problems (Koenen et al., 2007; Koenen et al., 2009).In addition, IQ is a measure that is quite stable during development (Neisser et al.,1996).

The interpretations of effect sizes followed the guidelines suggested by Cohen (1988).Correlations r = .10 are interpreted as small, r = .30 medium, and r = .50 large.

ResultsThe means, standard deviations, and gender differences for all measures included inthis study are presented in Table 1. The boys had significantly more symptoms on theSDQ hyperactivity scale than the girls (t(130) = -2.89, p < .00). There were no othersignificant gender differences. The mean WISC-III FSIQ score was 84.46 (SD = 19.10).The distribution of the FSIQ scores was as follows: 22.7% had a FSIQ < 70, 28.8% had aFSIQ in the range of 70–85, 25.0% had a FSIQ in the range of 86–100, and 23.5% had aFSIQ > 100.

The correlations between HoNOSCA, CGAS, age, gender, WISC-III IQ scores, and theSDQ parent scales are shown in Table 3. The differences in the correlations betweenHoNOSCA and FSIQ, PIQ, and VIQ, respectively, were not statistically significant.

Hierarchical regression analysisAll regression models with HoNOSCA as the dependent variable and the WISC-III FSIQ (F(8,123) = 5.10, p < .00), VIQ (F (8,123) = 4.64, p < .00), PIQ (F (8,123) = 5.24, p < .00)as independent variables in step 2 were significant (see Table 4). The model with FSIQin step 2 in the regression analysis predicted 25% of the variance, while the models withVIQ and PIQ in step 2 predicted 23% and 25%, respectively. After controlling for age andgender, FSIQ, VIQ, and PIQ predicted an additional 6%, 4%, and 7%, respectively, of the

IQ as a predictor of mental health problems 191

Table 3. Correlations between clinician-rated measures, SDQ, and WISC-III IQ scores (N = 132)

1 2 3 4 5 6 7 8 9 10 11

1. Total HoNOSCA score2. CGAS −.54**3. Age .19* −.044. Gender (0 = girl,

1 = boy).08 −.07 −.19*

5. FSIQ −.28** .14 −.20* .046. VIQ −.22* .10 −.20* .15 .89**7. PIQ −.29** .16 −.17 −.07 .90** .61**8. Emotional problems .07 −.05 −.01 −.16 .12 .14 .089. Conduct problems .25** −.23** −.08 .05 −.04 −.04 −.03 .13

10. Hyperactivity .24** −.20* −.24** .25** −.16 −.15 −.13 .10 .44**11. Peer relationship

problems.26** −.14 −.08 .02 −.05 .04 −.12 .26** .18* .15

12. Prosocial behaviour −.24** .18* −.03 −.11 −.11 −.05 −.16 −.15 −.42** −.17 −.27**

Note. *p � .05; **p � .01, (two-tailed test).

Table 4. Hierarchical regression analysis results for predicting HoNOSCA (N = 132)

HoNOSCA

FSIQ in step 2 VIQ in step 2 PIQ in step 2

R2 � R2 � R2 � R2 � R2 � R2 �

Step 1. Demographics .05* .05* .05* .05* .05* .05*Age .22* .24** .22*Gender (0 = girl, 1 = boy) .08 .10 .04

Step 2. IQ .11** .06** .09** .04* .12** .07**FSIQ/VIQ/PIQ −.22* −.17 −.24**

Step 3. Parent rated symptoms .25** .14** .23** .14** .25** .14**Emotional problems .02 .02 .01Conduct problems .10 .11 .09Hyperactivity .14 .15 .16Peer relationship problems .19* .21* .17Prosocial behaviour −.12 −.09 −.15

Note. All �-coefficients were taken from the last step in the regression analysis. *p � .05; **p � .01.

variance in the HoNOSCA score. SDQ parent-rated mental health symptoms predictedan additional 14% of the variance after controlling for age, gender, and the differentIQ scores.

The results of the hierarchical regression analysis of the models with CGAS as thedependent variable and the WISC-III IQ scores as independent variable in step 2, arepresented in Table 5. Neither the models with FSIQ (F (8,123) = 1.70, p = .11), VIQ(F (8,123) = 1.57, p = .14), or PIQ (F (8,123) = 1.77, p = .09) were significant.SDQ parent-rated mental health symptoms did not predict any of the variance inCGAS.

192 Børge Mathiassen et al.

Table 5. Hierarchical regression analysis results for predicting CGAS (N = 132)

CGAS

FSIQ in step 2 VIQ in step 2 PIQ in step 2

R2 � R2 � R2 � R2 � R2 � R2 �

Step 1. Demographics .01 .01 .01 .01 .01 .01Age −.06 −.08 −.06Gender (0 = girl, 1 = boy) −.04 −.05 −.03

Step 2. IQ .03 .02 .02 .01 .03 .02FSIQ/VIQ/PIQ .12 .08 .14

Step 3. Parent-rated symptoms .10 .07 .09 .08 .10 .08Emotional problems −.02 −.01 −.01Conduct problems −.13 −.14 −.13Hyperactivity −.10 −.11 −.11Peer relationship problems −.07 −.08 −.05Prosocial behaviour .09 .08 .11

Note. All �-coefficients were taken from the last step in the regression analysis. *p � .05; **p � .01.

DiscussionThe main aim of this study was to examine whether IQ predicted clinician-rated severityof mental health problems in children. The results varied for HoNOSCA and CGAS asmeasures of clinician-rated severity. The model with HoNOSCA as the dependent variablepredicted 25% of the total variance, whereas the model with CGAS as the dependentvariable was not significant. After controlling for age and gender, FSIQ predicted anadditional 6% of the variance in the HoNOSCA score.

In the only previous study of the association between IQ and HoNOSCA, no significantcorrelations between these measures were found (Pogge et al., 2008). One possibleexplanation for why their results do not match the result in our study could be thatthe studies differ in terms of clinical sample surveyed and the time interval betweenthe assessment with HoNOSCA and WISC-III. In our study, all assessments were carriedout at the same time, while the assessment with HoNOSCA in Pogge’s study (2008) wascompleted 6 years after the cognitive evaluation with the WISC-III.

In our study, there was a large negative correlation (r = -.54) between the CGASand the HoNOSCA scores. This result corresponds with Yates et al. (1999) study,and indicates that there is a substantial overlap between these measures when usedamong outpatients. Therefore, it is surprising that IQ did not predict CGAS scores. Oneexplanation for the difference in the prediction of CGAS versus HONOSCA scores by IQcould be that these measures of clinician-rated mental health problems are constructedin different ways. CGAS consists of one rating scale while HoNOSCA includes a total of 13scales. Two of the scales in HoNOSCA cover ‘Problems with scholastic or language skills’and ‘Problems with self-care and independence’. It is well documented that scholasticand language skills are highly correlated with IQ (Neisser et al., 1996), while problemswith self-care are common among persons with an IQ < 70 (Sparrow, Cicchetti, &Balla, 2005). This may explain the stronger relationship between IQ and HoNOSCAcompared to CGAS. In studies where the association between CGAS and IQ has beenexamined (Green et al., 1994; Weissman et al., 1990), moderate correlations between

IQ as a predictor of mental health problems 193

these variables have been detected. Those findings do not correspond to the resultsin this study. Different samples with different severity of problems may be a possibleexplanation of the inconsistent results. In Green’s study (1994), the participants wereinpatients and their mean CGAS was 38.22 (SD = 8.85), while the present study wasconducted with an outpatient sample with a mean CGAS score of 68.62 (SD = 10.11).In the study by Weissman et al. (1990), only offspring of depressed and non-depressedparents were examined. Differences in psychometric properties may also be a possibleexplanation of the different results with CGAS and HoNOSCA as dependent variables.HoNOSCA has a higher inter-rater reliability than CGAS (Hanssen-Bauer, Aalen et al.,2007; Hanssen-Bauer, Gowers et al., 2007), and low reliability will attenuate the observedcorrelations between the measures and other variables.

Our results showed that the WISC-III FISQ and PIQ predicted slightly more of thevariance of the HoNOSCA score than did the VIQ. The analysis of whether there wereany significant discrepancies between the correlations between HoNOSCA and the threeIQ scores showed no significant differences. This also indicates no significant differencesin the predicting power of the FSIQ, PIQ, and VIQ.

The findings showed that parent-rated symptoms could also predict a considerablepart of the variance of the HoNOSCA score after controlling for age, gender, and IQ. Thiseffect was not found for CGAS. Even if mental health symptoms in this study did not haveany impact on the CGAS score in the regression analysis, there were small correlationsbetween CGAS and the parent-rated SDQ scales conduct problems, hyperactivity, andprosocial behaviour. Possible explanations of why these associations were not detectedin the regression analysis could be that the sample size in our study was too small and that23% of the sample in this study had an IQ below 70. The CGAS has been criticized for notcovering all relevant domains of functioning in children with developmental disabilities(Wagner et al., 2007). It could be that the part of the sample in this study with an IQunder 70 primarily has difficulties related to problems that are common among childrenwith developmental disabilities, and that impairment related to psychiatric symptomsare not the most prominent problem.

Finally, some limitations of this study should be noted. The sample size (N = 132)is too small to detect small effects in the regression analyses. According to Greensformula (Green, 1991), at least 407 subjects are needed to yield sufficient statisticalpower to detect predictions with small effect sizes in this study. A larger samplewould also have made it possible to examine more complex models for the relationshipbetween IQ and mental health problems, examining possible mediators and moderatorsto the relationship. The CR model has potential to explain this relationship, butmechanisms behind the association between low IQ and mental health related problemsare unknown (Koenen et al., 2009). Koenen et al. (2009) have proposed four potentialmechanisms. Low IQ may (1) be a proxy measure of neuroanatomical deficits increasingthe vulnerability to some mental health diseases, (2) reduce the ability to cope withstressful life events, (3) moderate health-seeking behaviour and the knowledge aboutmental health problems, and (4) there could be a spurious relation between low IQ andmental health disorders explained by, for example, a common genetic vulnerability. Itis recommended that future studies designs in a way that makes it possible to examinewhether these mechanism moderates or mediates the association between IQ and mentalhealth related problems.

194 Børge Mathiassen et al.

ConclusionThe finding that IQ predicted clinician-rated mental health problems with the HoNOSCAcould be important for clinical practice. A high HoNOSCA score indicates that anassessment of intelligence should be considered in addition to an evaluation of symptomson mental health.

ReferencesBarnett, J. H., Salomond, C. H., Jones, P. B., & Sahakian, B. J. (2006). Cognitive reserve in

neuropsychiatry. Psychological Medicine, 36 , 1053–1064. doi:10.1017/S0033291706007501Becker, A., Steinhausen, H.-C., Baldursson, G., Dalsgaard, S., Lorenzo, M., Ralston, S., . . .

Rothenberger, A. (2006). Psychopathological screening of children with ADHD: Strengths andDifficulties Questionnaire in a pan-European study. European Child & Adolescent Psychiatry,15(Suppl. 1), 56–62. doi:10.1007/s00787-006-1008-7

Bilenberg, N. (2003). Health of the Nation Outcome Scales for Children and Adolescents(HoNOSCA): Results of a Danish field trial. European Child & Adolescent Psychiatry, 12,298–302. doi:10.1007/s00787-003-0343-1

Brann, P., Coleman, G., & Luk, E. (2001). Routine outcome measurement in a child and adolescentmental health service: An evaluation of HoNOSCA. Australian and New Zealand Journal ofPsychiatry, 35, 370–376. doi:10.1046/j.1440-1614.2001.00890.x

Cohen, J. W. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale,NJ: Lawrence Erlbaum Associates.

Cohen, P., Cohen, J., Kasen, S., Velez, C. N., Hartmark, C., Johnson, J., . . . Streuning, E. L. (1993).An epidemiological study of disorders in late childhood and adolescence—I. Age- and gender-specific prevalence. Journal of Child Psychology and Psychiatry, 34, 851–867.

Emerson, E. (2003). Mothers of children and adolescents with intellectual disability: Socialand economic situation, mental health status, and the self-assessed social and psychologicalimpact of the child’s difficulties. Journal of Intellectual Disability Research, 47, 385–399.doi:10.1046/j.1365-2788.2003.00498.x

Emerson, E., & Hatton, C. (2007). Mental health of children and adolescents with intellectual disabil-ities in Britain. British Journal of Psychiatry, 191, 493–499. doi:10.1192/bjp.bp.107.038729

Gale, C. R., Deary, I. J., Boyle, S. H., Barefoot, J., Mortensen, L. H., & Batty, G. D. (2008). Cognitiveability in early adulthood and risk of 5 specific psychiatric disorders in middle age: The Vietnamexperience study. Archives of General Psychiatry, 65, 1410–1418.

Garralda, M. E., Yates, P., & Higginson, I. (2000). Child and adolescent mental health service use:HoNOSCA as an outcome measure. British Journal of Psychiatry, 177, 52–58.

Goodman, R. (1997). The Strengths and Difficulties Questionnaire: A research note. Journal ofChild Psychology and Psychiatry, 38, 581–586. doi:10.1111/j.1469-7610.1997.tb01545.x

Goodman, R. (2001). Psychometric properties of the Strengths and Difficulties Questionnaire.Journal of the American Academy of Child & Adolescent Psychiatry, 40, 1337–1345.doi:10.1097/00004583-200111000-00015

Gowers, S. G., Harrington, R. C., Whitton, A., Beevor, A., Lelliott, P., Jezzard, R., & Wing, J.K. (1999a). Health of the Nation Outcome Scales for Children and Adolescents (HoNOSCA):Glossary for HoNOSCA score sheet. British Journal of Psychiatry, 174, 428–431. doi:10.1192/bjp.174.5.428

Gowers, S. G., Harrington, R. C., Whitton, A., Lelliott, P., Beevor, A., Wing, J., & Jezzard, R. (1999b).Brief scale for measuring the outcomes of emotional and behavioural disorders in children.Health of the Nation Outcome Scales for children and Adolescents (HoNOSCA). British Journalof Psychiatry, 174, 413–416. doi:10.1192/bjp.174.5.428

Green, B., Shirk, S., Hanze, D., & Wanstrath, J. (1994). The Children’s Global Assessment Scalein clinical practice: An empirical evaluation. Journal of the American Academy of Child &Adolescent Psychiatry, 33, 1158–1164.

IQ as a predictor of mental health problems 195

Green, S. B. (1991). How many subjects does it take to do a regression analysis? MultivariateBehavioral Research, 26 , 499–510.

Hanssen-Bauer, K., Aalen, O., Ruud, T., & Heyerdahl, S. (2007). Inter-rater reliability of clinician-rated outcome measures in child and adolescent mental health services. Administra-tion and Policy in Mental Health and Mental Health Services Research, 34, 504–512.doi:10.1007/s10488-007-0134-y

Hanssen-Bauer, K., Gowers, S., Aalen, O. O., Bilenberg, N., Brann, P., Garralda, E., . . . Heyerdahl,S. (2007). Cross-national reliability of clinician-rated outcome measures in child and adolescentmental health services. Administration and Policy in Mental Health and Mental HealthServices Research, 34, 513–518. doi:10.1007/s10488-007-0135-x

Hollis, C. (2000). Adolescent schizophrenia. Advances in Psychiatric Treatment, 6 , 83–92.Howlin, P., Goode, S., Hutton, J., & Rutter, M. (2004). Adult outcome for children with autism. Jour-

nal of Child Psychology and Psychiatry, 45, 212–229. doi:10.1111/j.1469-7610.2004.00215.xKoenen, K. C., Moffitt, T. E., Poulton, R., Martin, J., & Caspi, A. (2007). Early childhood factors

associated with the development of post-traumatic stress disorder: Results from a longitudinalbirth cohort. Psychological Medicine, 37, 181–192. doi:10.1017/S0033291706009019

Koenen, K. C., Moffitt, T. E., Roberts, A. L., Martin, L. T., Kubzansky, L., Harrington, H., . . . Caspi,A. (2009). Childhood IQ and adult mental disorders: A test of the cognitive reserve hypothesis.American Journal of Psychiatry, 166 , 50–57. doi:10.1176/appi.ajp.2008.08030343

Macklin, M. L., Metzger, L. J., Litz, B. T., Mcnally, R. J., Lasko, N. B., Orr, S. P., & Pitmann, R. K.(1998). Lower precombat intelligence is a risk factor for posttraumatic stress disorder. Journalof Consulting and Clinical Psychology, 66 , 323–326.

Neisser, U., Boodoo, G., Bouchard, T. J., Jr., Boykin, A. W., Brody, N., Ceci, S. J., . . . Urbina, S.(1996). Intelligence: Knowns and unknowns. American Psychologist, 51, 77–101.

Obel, C., Heiervang, E., Rodriguez, A., Heyerdahl, S., Smedje, H., Sourander, A., . . . Olsen, J.(2004). The Strengths and Difficulties Questionnaire in the Nordic countries. European Child& Adolescent Psychiatry, 13(Suppl. 2), II32–II39. doi:10.1007/s00787-004-2006-2

Owens, E. B., Hinshaw, S. P., Kraemer, H. C., Arnold, L., Abikoff, H. B., Cantwell, D. P., . . . Wigal,T. (2003). Which treatment for whom for ADHD? Moderators of treatment response in theMTA. Journal of Consulting and Clinical Psychology, 71, 540–552.

Pirkis, J. E., Burgess, P. M., Kirk, P. K., Dodson, S., Coombs, T. J., & Williamson, M. K. (2005). Areview of the psychometric properties of the Health of the Nation Outcome Scales (HoNOS)family of measures. Health & Quality of Life Outcomes, 3, 76. doi:10.1186/1477-7525-3-76

Pogge, D. L., Insalaco, B., Bertisch, H., Bilginer, L., Stokes, J., Cornblatt, B. A., & Harvey,P. D. (2008). Six-year outcomes in first admission adolescent inpatients: Clinical andcognitive characteristics at admission as predictors. Psychiatry Research, 160, 47–54.doi:10.1016/j.psychres.2007.06.024

Rey, J. M., Starling, J., Wever, C., Dossetor, D. R., & Plapp, J. M. (1995). Inter-rater reliabilityof global assessment of functioning in a clinical setting. Journal of Child Psychology andPsychiatry, 36 , 787–792.

Ronning, J. A., Handegaard, B. H., Sourander, A., & Morch, W.-T. (2004). The Strengths and Dif-ficulties Self-Report Questionnaire as a screening instrument in Norwegian community samples.European Child & Adolescent Psychiatry, 13, 73–82. doi:10.1007/s00787-004-0356-4

Rutter, M., Tizard, J., Yule, W., Graham, P. & Whitmore, K. (1976). Isle of Wight studies, 1964–1974. Psychological Medicine, 6 , 313–332.

Schorre, B. E. H., & Vandvik, I. H. (2004). Global assessment of psychosocial functioning inchild and adolescent psychiatry: A review of three unidimensional scales (CGAS, GAP, GAPD).European Child & Adolescent Psychiatry, 13, 273–286. doi:10.1007/s00787-004-0390-2

Shaffer, D., Gould, M. S., Brasic, J., Ambrosini, P., Fisher, P., Bird, H., & Aluwahlia, S. (1983). Achildren’s global assessment scale (CGAS). Archives of General Psychiatry, 40, 1228–1231.

Sonnander, S., Ramund, B., & Smedler, A.-C. (1998). WISC-III: Swedish version. Stockholm:Psykologiforlaget.

196 Børge Mathiassen et al.

Sparrow, S. S., Cicchetti, D. V., & Balla, D. A. (2005). Vineland Adaptive Behavior Scales, SurveyForms Manual (2nd ed.). Livonian, MN: Pearson Assessments.

Stern, Y. (2009). Cognitive reserve. Neuropsychologia, 47, 2015–2028. doi:10.1016/j.neuropsychologia.2009.03.004

Wagner, A., Lecavalier, L., Arnold, L. E., Aman, M. G., Scahill, L., Stigler, K. A., . . . Vitiello, B.(2007). Developmental disabilities modification of the Children’s Global Assessment Scale.Biological Psychiatry, 61, 504–511. doi:10.1016/j.biopsych.2007.01.001

Wechsler, D. (1992). Wechsler Intelligence Scale for Children (3rd ed.). London: The PsychologicalCorporation.

Weissman, M. M., Warner, V., & Fendrich, M. (1990). Applying impairment criteria to children’spsychiatric diagnosis. Journal of the American Academy of Child & Adolescent Psychiatry,29, 789–795. doi:10.1097/00004583-199009000-00019

WHO (1993). The ICD-10 classification of mental and behavioural disorders: diagnostic criteriafor research. Geneva: WHO.

Yates, P., Garralda, M. E., & Higginson, I. (1999). Paddington Complexity Scale and Health ofthe Nation Outcome Scales for children and adolescents. British Journal of Psychiatry, 174,417–423. doi:10.1192/bjp.174.5.417

Zammit, S., Allebeck, P., David, A. S., Dalman, C., Hemmingsson, T., Lundberg, I., & Lewis, G.(2004). A longitudinal study of premorbid IQ score and risk of developing schizophrenia,bipolar disorder, severe depression, and other nonaffective psychoses. Archives of GeneralPsychiatry, 61, 354–360.

Received 31 July 2009; revised version received 20 March 2011