Childhood IQ in relation to later psychiatric disorder: Evidence from a Danish birth cohort study

Studies examining the relationship between early-life IQ and the risk of subsequent psychiatric disorder in adulthood arescarce. In the present investigation, the childhood IQ scoresof 7022 singleton-born Danish males were linked to psychiatrichospital discharge records in adulthood. IQ scores were inverselyrelated to the risk of total psychiatric illness, with thehighest levels apparent in the lowest scoring IQ group (HR_{lowest
quintile v. highest}=1.70, 95% CI 1.34-2.14). Adjusting for paternal occupational social class and birth weight had onlya small attenuating effect. Low childhood IQ may have an aetiologicalrole in the development of adult total psychiatric disorder.

INTRODUCTION

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INTRODUCTION

A series of studies has shown elevated rates of schizophrenia,depression and dementia in adults who had low childhood scoreson standard IQ tests (Whalley et al, 2000; Zammit et al, 2004). However, the possibility that pre-adult IQ may be linked withlater total psychiatric illness, in addition to these specificconditions, has rarely been examined. The only two studiesexamining this association reveal null (Deary et al, 2004)and inverse relationships (Walker et al, 2002). In these studies,case ascertainment was based on hospital admissions databasesestablished when study participants were already middle-aged, raising concerns that earlier cases had not been captured.Moreover, there was limited information on potential confoundingvariables: adjustment for socio-economic position was basedon area-rather than individual-level characteristics and therewas a lack of data on other candidate confounders, such asfoetal growth. Analyses of data from a Danish birth cohort (Osler et al, 2004)allow us to explicitly address the aforementionedissues in a more contemporary cohort.

METHOD

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METHOD

In 1965, all male school children born in the metropolitan areaof Copenhagen (Denmark) in 1953 (n=12 270) were invited toparticipate in a schools-based survey (Høgh & Wolf, 1981);a total of 9537 (77.7%) did so. Teachers group-administeredthe Härnqvist IQ test (Härnqvist, 1968), which consistedof three sub-categories (spatial, inductive and verbal), theresults of which were aggregated to produce a combined score.Paternal occupation was reported by the child and allocatedto one of five social class strata (Hansen, 1978). Birth weight was extracted from registers and certificates.

Study participants were linked to the Danish Psychiatric CentralRegister (Munk-Jorgensen & Mortensen, 1997) using theirCPR number, a unique person identifier. In the present study,a psychiatric disorder was defined as any psychiatric discharge diagnosis from a psychiatric ward. IQ scores and covariatedata (birth weight and paternal social class) were availablefor 7022 singleton-born men who were known to be alive andliving in Denmark or Greenland on 1 April 1969 when the Danishpsychiatric register was established. Follow-up ended upon first discharge date from a psychiatric ward for a psychiatric disorder,or death or emigration, or 22 January 2002, whichever camefirst.

RESULTS

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RESULTS

During 209 310 man-years of follow-up, 746 (10.6%) men werefound to have developed a psychiatric disorder. This outcomecomprised specific psychiatric disorders related to alcohol(21%) or drug misuse (14%); personality disorder (13%); transitionalsituational disturbance (12%); schizophrenia (9%); depression(8%); anxiety neurosis (4%); and ‘other’ conditions (19%).

The combined IQ score was inversely related to total psychiatricdisorder (Table 1). The highest rates in adulthood were evidentin children who had lower IQ test scores (HR_{lowest
quintilev. highest}=1.70, 95% CI 1.34-2.14). This effect was incremental(P value for linear trend 0.0001). Adjusting for paternal socialclass and birth weight resulted in little attenuation of this association. When we related the three IQ sub-test scores topsychiatric disorder, the patterns of association were essentiallythe same as that for the combined score (data not shown).

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Table 1 Hazards ratios (95% Cl) for the association between combined psychometric IQ score and total psychiatric disorders (n=7022)

DISCUSSION

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DISCUSSION

In the present study, Danish men with high IQ test scores at12 years of age had lower rates of later psychiatric disorder,an effect evident across the range of IQ scores. Similar resultshave been reported elsewhere (Walker et al, 2002).

Chance, confounding, selection bias and reverse causality areall alternative explanations for the IQ-psychiatric disordergradient. Given the large sample size in this study, chanceis unlikely to explain the results; moreover, the IQ-psychiatricdisorder relationship was apparent in all our analyses. A deprivedsocial background is a candidate confounder given its associationwith both some psychiatric disorders (Bruce et al, 1991) andchildhood IQ (Neisser et al, 1996). Following adjustment forpaternal social class and birth weight, the IQ-psychiatricdisorder relationship was only slightly weakened. We did not,however, have data on other potentially important covariates(e.g. alcohol and drug use).

The present analyses are based on 7022 men (57% of the targetpopulation). The analytical sample had more favourable levelsof some characteristics: they had higher mean IQ test scores(68.28 v. 62.69; P <0.001), and were more likely to comefrom the highest social group (6.71% v. 5.08%, P=0.002) andbe in the heaviest (43500 g) birth weight group (46.55% v.43.70%;. 43.70%; P $≤$ 0.001). However, importantly, absolute differencesbetween the groups were small. Our study sample would onlybe biased if the IQ-psychiatric disorder gradient differedbetween those included and those who were not. Although thisis of course possible, we believe it to be very unlikely. Finally,the IQ assessment was performed early in life but was not accompaniedby psychiatric screening. This raises the issue of reversecausality whereby existing illness, either clinical or subclinical,led to reduced IQ test performance and this generated the observedinverse IQ-psychiatric disorder relationship.

Although these alternative explanations for the IQ-psychiatricdisorder association cannot be disregarded, a direct mechanismunderpinning the relationship may be that low IQ scores arean indicator of early subclinical cerebral disease processesthat precede a psychiatric event. Childhood IQ may also representa ‘record’ of psychological and physiological insults(e.g. postnatal illness) occurring prior to test administration,which are predictive of some psychiatric conditions.

The strengths of this study are its size; the representativenature of the childhood survey; the availability of covariatedata, particularly socio-economic position; and the use ofa population-based case register. The study has its limitations.First, by using data on hospital discharge from a psychiatricward we have identified males with disorders serious enoughto warrant contact with mental health services, so under-estimatingthe incidence of mental illness by failing to capture moreminor episodes. Second, because only males participated inthe schools-based survey, it is not clear whether the presentfindings are generalisable to women. Third, we will have missed some cases that occurred between the childhood survey (1965)at age 12 years and the inception of the hospital admissionsregister (1969) at age 15 years that did not require subsequenthospitalisation.

ACKNOWLEDGMENTS

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ACKNOWLEDGMENTS

We thank all those who initiated or have continued involvementin the study: K. Svalastoga, E. Høgh, P. Wolf, T. Rishøj, G. Strande-Sørensen, E. Manniche, B. Holten, I. A.Weibull and A. Ortman. The study is supported by the DanishHeart Association, the Lund-beck Foundation and the HealthInsurance Fund. G.D.B. held a University of Copenhagen VisitingSenior Research Fellowship. He is now the recipient of a WellcomeAdvanced Training Fellowship.

REFERENCES

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