ReviewAssociations between childhood intelligence (IQ), adult morbidity and mortality
Introduction
Tests of intelligence are available in many forms, probably as many as there are ways to be intelligent. The most widely used tests comprise a variety of items, verbal and non-verbal, in keeping with the principle that the wider the range the better the measure of general intelligence. Typically, intelligence test scores are converted to a scale whose mean is 100 and the standard deviation is 15. Originally, the age standardised test scores were referred to as the “Intelligence Quotient” (IQ) calculated by dividing the age-standardised score by the chronological age. This is no longer used so the acronym IQ is used to denote the intelligence test score itself [1].
Most interest in IQ scores has been in their ability to predict later achievements whether these are in schooling, educational attainments, social adjustment, and job success or job satisfaction [2]. More recently, in step with recent advances in neuroscience, associations between IQ and information processing speed, averaged evoked responses, regional brain volumes, haemodynamic responses to cognitive demands and neurogenetic variations have become areas of interest where research progress might be most informative (see for example [3]). Underlying this surge in neuroscientific interest is the suggestion that IQ and its neurobiological infrastructure is in some unknown way linked to the overall efficiency of the brain in its capacity to maintain the integrity of central regulatory physiological systems. These ideas have supported many explorations of suspected associations between a variety of social, psychological and, latterly, health outcomes.
Not surprisingly, links between IQ and individual differences in disease risk or mortality have proven complex where results (for reasons discussed below) have been difficult to interpret with confidence. For example, there are well-established links between disease morbidity, mortality and low education or low socioeconomic status [4]. Early IQ is a strong predictor of time spent in education and later job status [5]. So what is driving these relationships? Are they simply attributable to low social status and all that entails in terms of disadvantage, poverty and hardship? Or is it a more direct influence of IQ on the pathophysiology of disease and the maintenance of effective disease defence and repair systems? Some of this uncertainty becomes more immediate and less academic when social inequalities in health are considered. These are well established within and between societies but when a long term look is taken at health trends in changing social inequalities, it is clear that health inequalities have remained stubbornly fixed in the face of rising living standards, improved universal health care, better nutrition and an abundance of healthy lifestyle advice [6]. Gottfredson [7] has argued that underlying IQ differences explain social inequalities in health and that these are not necessarily mediated through socioeconomic status. Likewise, Whalley [8] has extended these arguments to question the causal nature of the link between low education and disease risk.
Changing environments over time are also relevant to understanding temporal trends in disease incidences and the stability of associations between IQ and health. Flynn [9] showed that IQ test scores have increased in many developed countries—typically by an average of 3 IQ points every decade (the “Flynn effect”). The extraordinary magnitude of this increase has been sufficient to question its existence: key methodological issues remain unresolved [9], [10].
The most plausible explanations of the ‘Flynn effect’ have been environmental and many of these are very relevant to an examination of an association between IQ and morbidity/mortality because exposure to these environmental factors could explain individual differences in IQ and disease risk.
At one level, improved IQ test performance might reflect increased emphasis on the mechanics of IQ test success in teaching because this success is linked to later selection for jobs or places in higher education. Or, at a second level, more complex activities of daily living in an information-based society improve general competencies in problem solving measured in non-verbal subscales of IQ tests. Such sociocultural changes may be strong enough to accelerate brain maturational processes underpinning developmental improvements in reasoning abilities. A third possibility seems most likely and concerns the nature of the environment in which IQ develops [11]. Although these are reciprocal interactions between developing IQ, environmental choices and the shaping or optimisation of environments, there are many contextual opportunities for sharing influences between developing IQ and disease risk [10].
The exact environmental effects on intelligence are unknown but there are well-established negative or harmful influences on IQ [12]. These include chronic poor health, poor nutrition and diverse privations (discussed below in Section 5). However, there are also opportunities for other positive influences on IQ and these include parenting styles, enrichment of a stimulatory childhood environment and the benefits of a consistent network of social support, including better trained school teachers. Alone or in concert, these factors may be sufficient to explain observed IQ gains over the past century. These same positive influences on childhood IQ could be important factors in the maintenance of good health in childhood and are, therefore, possible confounders in any study of an association between childhood IQ and adult health.
Section snippets
Criteria
One group where there could be a clear link between IQ in childhood and later morbidity and/or mortality is people with intellectual disability (PWID). Criteria for this diagnosis are: score 70 or less on an IQ test with additional evidence of both a developmental disorder and deficits in adaptive behaviour. Therefore, PWID are not only quantitatively different from the general population in terms of IQ, but PWID are also qualitatively different. Exceptions to these general rules are those with
IQ, body systems and disease
The previous section on intelligence and health in PWID indicated that IQ may act as an index of systemic integrity, as suggested in Section 1. Whether or not this is the case in the general population is a question attracting ongoing research, but the association of higher childhood intelligence with prolonged survival is suggestive. However, it may be that IQ relates to morbidity in adulthood through health behaviour and socio-environmental pathways already well established. This would make
Current adult IQ and survival
When healthy volunteers are tested in late adulthood, their current performance on cognitive tests can be strongly linked to their survival over follow-up periods between 5 and 10 years [42], [43], [44], [45]. At first, these studies did not seem straightforward to interpret. For example, measures of cognitive ability could be acting as proxies for lower socioeconomic status (SES) or might be accounted for by the “terminal drop” hypothesis (see Section 4.2). By 2001, however, current cognitive
Methodological issues
IQ test results preserved from early twentieth century surveys now make practical the study of the relationship between childhood IQ and lifetime mortality risk. These studies can test the predictive value of IQ for overall (all-cause) mortality up to mean ages at death for samples born before 1930 and followed up until the present. When first reported, the putative association was met with some scepticism [60]. It was argued that childhood IQ was providing a proxy for cumulative psychosocial
Conclusion
Follow-up studies on childhood IQ and health outcomes including survival in adulthood added momentum to much recent research into the mechanisms through which psychosocial factors (including cognitive abilities) could influence health. At first it appeared that childhood IQ was acting independently of socioeconomic factors but as studies explored other relevant data collected in adolescence then support for the association between childhood IQ and adult morbidity/mortality diminished. The
Contributors
All authors contributed equally to this manuscript.
Competing interest
The authors have no competing interest.
Conflict of interest
The authors have no conflict of interest.
Provenance
Commissioned and externally peer reviewed.
Acknowledgements
AHMK and JMS are funded through the University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology supported by the University of Edinburgh, the Biotechnology and Biological Sciences Research Council (BBSRC), the Engineering and Physical Sciences Research Council (EPSRC), the Economic and Social Research Council (ESRC), and Medical Research Council (MRC).
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