Elsevier

Intelligence

Volume 37, Issue 6, November–December 2009, Pages 520-528
Intelligence

The association of childhood intelligence with mortality risk from adolescence to middle age: Findings from the Aberdeen Children of the 1950s cohort study

https://doi.org/10.1016/j.intell.2008.11.004Get rights and content

Abstract

There is growing evidence that childhood IQ is inversely associated with mortality in later life. However, the specificity of this association in terms of causes of death, whether it is continuous over the whole range of IQ scores and whether it is the same according to age and sex is not clear. In a large cohort (N = 11,603) of a complete population of children born in one city in the UK in the early 1950s, IQ measured at age 7 years (using a routinely administered picture test) was found to be inversely associated with mortality between the ages of 15 and 57 years. For every 1 SD increase in IQ at 7, the all cause mortality hazard ratio was 0.79 (95% CI 0.73, 0.85). On adjustment for a range of perinatal factors, father's social class at birth, number of sibs in the household and childhood height and weight, this was attenuated slightly to 0.81 (0.74, 0.88). Almost identical associations of IQ with mortality were seen for men and women as well as at younger (15–39) and older (40+) ages. These associations were across the entire IQ range, although some of the high mortality in the lowest category of IQ (< 70) was accounted for by causes associated with congenital disorders. Overall, external causes of death showed the strongest association, with weaker associations being seen for cancer. Further work is required to understand the mechanisms whereby childhood IQ has such a robust association with mortality in later life.

Introduction

It has been known for several decades that individuals who are severely cognitively impaired have increased rates of mortality in childhood, adolescence (Simila, von Wendt, & Rantakallio, 1986) and adulthood (Patja et al., 2001, Patja et al., 2000). More recently, attention has been focussed on the inverse association of childhood cognition with mortality in adult life that appears to extend over the entire range of IQ (Batty, Deary, & Gottfredson, 2007). This association has been replicated in a number of independent studies (Hart et al., 2003, Osler et al., 2003, Kuh et al., 2004, Pearce et al., 2006, Whalley and Deary, 2001), and is even seen to hold even among individuals whose IQ in childhood was in the range 135 to 160 or more (based on the Stanford–Binet test) (Martin & Kubzansky, 2005). While most studies of childhood cognition as a predictor of adult mortality have only been able to look at deaths into middle age, an inverse association has also been found with mortality up to age 76 years in subjects included in the 1932 Scottish Mental Survey (Hart et al., 2003, Whalley and Deary, 2001). Childhood or early adulthood intelligence has been reported to be inversely associated with risk for cardiovascular disease (Hart et al., 2004, Batty et al., 2005, Lawlor et al., 2008, Batty et al., 2008b), risk factors for adult mortality (Lawlor et al., 2006, Chandola et al., 2006, Batty et al., 2007), and hospital admission from unintentional injury (Lawlor, Clark, & Leon, 2007). However, a recent study found no association of early adult IQ with cancer risk in adulthood (Batty et al., 2007).

A number of hypotheses, which are not mutually exclusive, have been put forward to explain the inverse associations of intelligence measured in childhood with adult mortality (Batty and Deary, 2004, Whalley and Deary, 2001). One type of explanation is that childhood IQ and later mortality risk may share common antecedents rather than being directly causally related. This possibility has also been formulated in terms of childhood IQ being a marker of individual constitution or “system integrity”. Genetic factors would be included, as would perinatal problems giving rise to irreversible damage to organ systems including the brain. However, these early life factors could include nutritional or other insults as indicated by impaired fetal or growth in infancy and/or childhood. A second class of alternative explanation is that there may be confounding factors such as socioeconomic position and parental intelligence or education, all of which are associated with intelligence in early life and later adverse health risks in the offspring. To the extent that the association of childhood intelligence with mortality is causal it might be mediated by one or more adult characteristics such as educational attainment, occupation and socioeconomic position. Intelligence might also directly influence an individual's ability to interpret and effectively utilise information about health-related risk factors and health service use.

Despite progress in trying to understand the mechanism and implications of the inverse association between childhood intelligence and adult mortality, several issues remain to be resolved (Deary & Batty, 2006). These include the extent to which the associations of childhood intelligence with different causes of death really do vary, how far these associations are truly linear, and whether associations differ depending upon the age in early life that intelligence is tested. Moreover, few large studies have been able to examine the association of childhood IQ with later mortality in a complete population comprised of individuals ranging from those with major cognitive deficits (due to congenital anomalies or severe brain damage in early life) to the most intellectually gifted children.

The aim of this paper is to add to existing literature on the association of childhood intelligence with mortality in adulthood. It substantially extends an earlier report of from this study that was based on far fewer deaths (Batty, Clark, Morton, Macintyre, & Leon, 2002). In particular we will examine associations of childhood IQ at age 7 years with all-cause mortality, by sex and age at death, as well as investigating associations with mortality from major causes of death, taking account of a number of early-life potential confounders.

Section snippets

Methods

Data from the Aberdeen Children of the 1950s (ACONF) cohort study were used. This study was based on the Aberdeen Childhood Development Survey (ACDS) conducted in the early 1960s into the determinants of “mental subnormality” in a complete population (Birch, Richardson, Baird, Horobin, & Illsley, 1970). The ACONF cohort consists of the 12,150 members of the ACDS who were born in Aberdeen between 1950–1956 and for whom comprehensive information was abstracted from the Aberdeen Maternity and

Results

Table 1 shows the baseline characteristics of participants and provides details about the extent of missing data by variable. Most variables were complete or almost complete. The variables with the highest percentage of missing values were IQ score at age 7 years (4%), height and weight at school entry (4%), father's occupational social class at birth of study member (6%) and gestational age at birth (10%). The subset of individuals with complete data on all variables shown in Table 1 (used in

Discussion

We have found that childhood cognition/IQ is inversely associated with mortality from all causes from age 15 years into middle age. This association is seen across the entire range of IQ scores from < 70 to 130+ and is equally strong among men and women. Perinatal factors, childhood socio-economic circumstances and growth appear to explain very little of this association. However, we have found evidence that the strength of this association varies to some degree by cause of death. After

Funding

The Aberdeen Children of the 1950s Study was funded as a component project of a Medical Research Council Co-operative Group. The Scottish Chief Scientist Office provided funds for a study of relationships of childhood intelligence. D A Lawlor is funded by a Department of Health (UK) Career Scientist Award and works in an MRC centre. David Batty is a Wellcome Trust Fellow. The Medical Research Council (MRC) Social and Public Health Sciences Unit receives funding from the UK MRC and the Chief

Acknowledgements

We are very grateful to Raymond Illsley for providing us with the data from the Aberdeen Child Development Survey and for his advice about the study. Graeme Ford played a crucial role in identifying individual cohort members and in helping us initiate the process of revitalisation. Doris Campbell, George Davey Smith, Bianca De Stavola, Marion Hall, David Godden, Diana Kuh, Glyn Lewis, Susan MB Morton and Viveca Östberg collaborated with the authors to revitalise the cohort. Margaret Beveridge

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