IJE Advance Access originally published online on March 28, 2006
International Journal of Epidemiology 2006 35(3):670-672; doi:10.1093/ije/dyl053
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Commentary |
Commentary: Pre-morbid IQ and later health—the rapidly evolving field of cognitive epidemiology
1 Department of Psychology, University of Edinburgh, UK
2 Medical Research Council Social and Public Health Sciences Unit, University of Glasgow, UK
* Corresponding author. Department of Psychology, University of Edinburgh, 7 George Square, Edinburgh EH8 9JZ, UK. E-mail: I.Deary{at}ed.ac.uk
The new, specialist field of cognitive epidemiology is growing.1 Scores on mental ability tests (IQ-type tests) are replicable predictors of some health outcomes, especially death from all causes.2 The first empirical inkling of such an association was probably obtained in 1933, when a moderate-sized relationship was reported between the average IQ scores of children in New York city districts and the health of their residents, as indexed by death rates (r = –0.43; Figure 1).3 Several decades later the association was replicated at the level of the individual when male Australian Vietnam War veterans with higher mental test scores were shown to experience lower rates of total mortality and motor vehicle accidents.4,5 More recently, the follow-up studies of the Scottish Mental Survey of 1932 showed an association between IQ at age 11 years and survival to 
80 years.6,7
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The association between IQ, even childhood IQ, and all-cause mortality is reasonably well established, but there are still questions to be answered. To which disease-specific outcomes does the association with IQ apply? Is there a linear association between IQ level and death, or is there a threshold effect? Is the relation apparent in men and women? What is the mechanism(s) of the association between IQ and mortality? What is the role of genetic factors in these associations? The paper by Hemmingsson et al.8 both confirms the association and sheds light on some of these issues.
Given the limitation that the study includes only men—a characteristic of others in this field4,9—there are many strengths: the sample studied is large (providing high statistical power); coverage is high (allaying concerns of selection bias); the mental test was given in early adulthood (guarding against reverse causality owing to existing disease); the mental test battery appears quite comprehensive; the range of IQ scores examined is wide (affording an examination of the IQ-mortality relation across a wide IQ range); the follow-up period was 30 years; specific as well as all-cause mortality were examined as outcomes and obtained from national registers of deaths; and childhood as well as adult (in their 30s) socioeconomic position (both were obtained from the census and not from recall) were examined as possible confounder and mediator, respectively. There are only minor quibbles about this useful report. The test battery might have been described in more detail. It is surprising to see how little information is given on this key predictor variable when the possible confounders or mediators and outcomes are described more fully. The associations are given per unit increase in IQ, which in this study appears to refer to a ‘standard-nine’ scale. A true continuum would have preserved more information on IQ for individuals, but such data may not have been available to the investigators. To facilitate comparing the study's findings with other reports, the authors report the effect on mortality per standard deviation of IQ, but this was obtained from logistic rather than Cox regression.
Hemmingsson et al. report clear findings of associations between IQ scores and all-cause mortality, cardiovascular disease mortality [coronary heart disease (CHD) and non-CHD], mortality from injuries (including suicide and accidents), and alcohol-related mortality. There was no significant association with death from all cancers combined. With some evidence pointing to lung7,10 and possibly stomach10 cancers being associated with earlier IQ scores, further examination of the influence, if any, of IQ on organ-specific malignancies is warranted.
Childhood socioeconomic position (SEP) was examined as a possible confounding variable. In fact, adjustment for childhood SEP had almost no effect on the associations between IQ scores and all the outcomes. This finding is in accordance with those from previous studies.9,11 There are other reasons for thinking that childhood SEP is not a source of the covariance between IQ scores and mortality outcomes. Extensive research on the environmental and genetic contributions to intelligence differences shows that shared environment (the environment common to siblings, also called between-family effects), of which childhood SEP is a component, makes almost no contribution to intelligence differences by late adolescence and thereafter.12
Adult SEP (in the subjects' 30s) was examined as a possible, partial mediating variable. There is some statistical evidence in the report that it might be acting as such: however, the attenuation of the IQ-mortality effect was never >37% after adjustment for adult SEP. Statistical evidence alone will not reveal the mechanisms of these associations. It is clear from different studies that adult SEP in part accounts for the IQ-mortality association, just as, statistically, IQ accounts for some (sometimes quite a lot) of the association between SEP and mortality.7,11,13 It is not yet known to what extent IQ acts via adult SEP in affecting mortality, but this available evidence suggests that there is still a substantial residual effect of IQ on health outcomes (including mortality) that does not proceed via adult SEP.
A particularly valuable aspect of the report by Hemmingsson et al. is that it affords, because of large numbers and a wide spread of ability levels, an examination of the association between different levels of IQ and mortality risk. Their Figure 1 suggests a threshold effect: there is little difference in hazard ratio across the three highest categories of IQ score. There appears to be a steadily increasing risk of mortality thereafter, with an especially large difference in hazard ratios between the second-lowest and lowest IQ groups. The other literature is not clear on this. Even within the Scottish Mental Survey follow-ups, one sample indicated a dose–response effect of IQ,10 whereas another showed more evidence of a markedly raised risk in the lowest IQ quartile,7 an observation also made in the British Birth Cohort of 1946.11 Quite the most provocative contribution to this issue is the report of the Terman Life Cycle Study of Children with High Ability.14 These intellectually gifted children (the ‘Termites’), with IQs of 135–200 (mean = 151, SD = 11), have been studied every 5–10 years since 1922. They took the Stanford Binet IQ test as children and were followed-up for health outcomes from age 18 for a mean of 48 years. Following adjustment for childhood illness, sex, and father's occupation, even in people with IQs of between 135 and 163 a standard deviation increase in score was associated with a 32% reduced risk of mortality. These data suggest that the upper limit for IQs being beneficial to health could be as high as 163 points. It is hard to imagine what the mechanism might be that protects health incrementally as IQ rises from 120 to 160.
Overall, this new study8 provides further evidence of a link between pre-morbid IQ and some mortality outcomes, suggests an effect along much of the IQ range, and suggests that childhood and adult SEP do not have a major role in explaining these associations. The report's Discussion section is rather light on possible mechanisms (data on some risk indices such as smoking,15 blood pressure,16 and obesity17 were, we believe, available) and context, but its empirical contributions are themselves worthwhile.
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Acknowledgments |
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I.J.D. is the recipient of a Royal Society-Wolfson Research Merit Award; G.D.B. is a Wellcome Fellow.
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References |
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 Top References |
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1 Deary IJ. Intelligence, health and death: the new field of cognitive epidemiology. Psychologist 2005;18:610–13.[Web of Science]
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3 Maller JB. Vital indices and their relation to psychological and social factors. Hum Biol 1933;5:94–121.
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7 Hart CL, Taylor MD, Davey Smith G et al. Childhood IQ, social class, deprivation and their relationships with mortality and morbidity risk in later life: Prospective observational study linking the Scottish Mental Survey 1932 and the Midspan studies. Psychosom Med 2003;65:877–83.
8 Hemmingsson T, Melin B, Allebeck P, Lundberg I. The association between cognitive ability measured at ages 18–20 and mortality during 30 years of follow-up: a prospective observational study among Swedish males born 1949–1951. Int J Epidemiol 2006;35:665–70.
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10 Deary IJ, Whalley LJ, Starr JM. IQ at age 11 and longevity: Results from a follow up of the Scottish Mental Survey 1932. In: Finch CE, Robine J-M, Christen Y (eds). Brain and Longevity: Perspectives in Longevity. Berlin: Springer, 2003, pp. 153–64.
11 Kuh D, Richards M, Hardy R, Butterworth S, Wadsworth ME. Childhood cognitive ability and deaths up until middle age: a post-war birth cohort study. Int J Epidemiology 2004;33:408–413.
12 Deary IJ, Spinath FM, Bates TC. Genetics of intelligence. Eur J Hum Genet (in press).
13 Batty GD, Der G, Macintyre S, Deary IJ. Does IQ explain socioeconomic inequalities in health? Evidence from a population based cohort study in the west of Scotland. BMJ 2006;332:580–84.
14 Martin LT, Kubzansky LD. Childhood cognitive performance and risk of mortality: a prospective cohort study of gifted individuals. Am J Epidemiol 2005;162:1–4.
15 Taylor MD, Hart CL, Davey Smith G et al. Childhood mental ability and smoking cessation in adulthood: prospective observational study linking the Scottish Mental Survey 1932 and the Midspan studies. J Epidemiol Community Health 2003;57:464–65.
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