Department of Social Medicine, Institute of Public Health, University of Copenhagen, Copenhagen, Denmark
Department of Health Psychology, Institute of Public Health, University of Copenhagen, Copenhagen, Denmark
Department of Social Medicine, Institute of Public Health, University of Copenhagen, Copenhagen, Denmark
Correspondence: Dr G. David Batty, MRC Social and Public Health Sciences Unit, University of Glasgow, 4 Lilybank Gardens, Glasgow G12 8RZ,UK. e-mail: david-b{at}msoc.mrc.gla.ac.uk
Declaration of interest None. Funding detailed in Acknowledgements.
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ABSTRACT |
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INTRODUCTION |
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METHOD |
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Study participants were linked to the Danish Psychiatric Central Register (Munk-Jorgensen & Mortensen, 1997) using their CPR 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 covariate data (birth weight and paternal social class) were available for 7022 singleton-born men who were known to be alive and living in Denmark or Greenland on 1 April 1969 when the Danish psychiatric 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 came first.
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RESULTS |
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The combined IQ score was inversely related to total psychiatric disorder (Table 1). The highest rates in adulthood were evident in children who had lower IQ test scores (HRlowest quintile v. highest=1.70, 95% CI 1.34-2.14). This effect was incremental (P value for linear trend 0.0001). Adjusting for paternal social class and birth weight resulted in little attenuation of this association. When we related the three IQ sub-test scores to psychiatric disorder, the patterns of association were essentially the same as that for the combined score (data not shown).
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DISCUSSION |
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Chance, confounding, selection bias and reverse causality are all alternative explanations for the IQ-psychiatric disorder gradient. Given the large sample size in this study, chance is unlikely to explain the results; moreover, the IQ-psychiatric disorder relationship was apparent in all our analyses. A deprived social background is a candidate confounder given its association with both some psychiatric disorders (Bruce et al, 1991) and childhood IQ (Neisser et al, 1996). Following adjustment for paternal social class and birth weight, the IQ-psychiatric disorder 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 target population).
The analytical sample had more favourable levels of some characteristics: they
had higher mean IQ test scores (68.28 v. 62.69; P
<0.001), and were more likely to come from the highest social group (6.71%
v. 5.08%, P=0.002) and be in the heaviest (43500 g) birth
weight group (46.55% v. 43.70%;. 43.70%; P0.001).
However, importantly, absolute differences between the groups were small. Our
study sample would only be biased if the IQ-psychiatric disorder gradient
differed between those included and those who were not. Although this is of
course possible, we believe it to be very unlikely. Finally, the IQ assessment
was performed early in life but was not accompanied by psychiatric screening.
This raises the issue of reverse causality whereby existing illness, either
clinical or subclinical, led to reduced IQ test performance and this generated
the observed inverse IQ-psychiatric disorder relationship.
Although these alternative explanations for the IQ-psychiatric disorder association cannot be disregarded, a direct mechanism underpinning the relationship may be that low IQ scores are an indicator of early subclinical cerebral disease processes that precede a psychiatric event. Childhood IQ may also represent a 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 representative nature of the childhood survey; the availability of covariate data, particularly socio-economic position; and the use of a population-based case register. The study has its limitations. First, by using data on hospital discharge from a psychiatric ward we have identified males with disorders serious enough to warrant contact with mental health services, so under-estimating the incidence of mental illness by failing to capture more minor episodes. Second, because only males participated in the schools-based survey, it is not clear whether the present findings 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 admissions register (1969) at age 15 years that did not require subsequent hospitalisation.
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ACKNOWLEDGMENTS |
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REFERENCES |
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Received for publication June 15, 2004. Revision received November 25, 2004. Accepted for publication November 30, 2004.
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