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Systematic Review with Meta-Analysis Catherine M Calvin, Ian J Deary, Candida Fenton, Beverly Roberts, Geoff Der, Nicola Leckenby, David Batty

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Systematic Review with Meta-Analysis Catherine M Calvin, Ian J Deary, Candida Fenton, Beverly Roberts, Geoff Der, Nicola Leckenby, David Batty Intelligence in youth and all-cause-mortality: systematic review with meta-analysis Catherine M Calvin, Ian J Deary, Candida Fenton, Beverly Roberts, Geoff Der, Nicola Leckenby, David Batty To cite this version: Catherine M Calvin, Ian J Deary, Candida Fenton, Beverly Roberts, Geoff Der, et al.. Intelligence in youth and all-cause-mortality: systematic review with meta-analysis. International Journal of Epidemiology, Oxford University Press (OUP), 2010, 10.1093/ije/DYQ190. hal-00637030 HAL Id: hal-00637030 https://hal.archives-ouvertes.fr/hal-00637030 Submitted on 29 Oct 2011 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Intelligence in youth and all-cause-mortality: systematic review with meta-analysis For Review Only Journal: International Journal of Epidemiology Manuscript ID: IJE-2010-05-0505.R1 Manuscript Type: Original Article Date Submitted by the 26-Aug-2010 Author: Complete List of Authors: Calvin, Catherine; University of Edinburgh, Dept of Psychology Deary, Ian; University of Edinburgh, Dept of Psychology Fenton, Candida; University of Glasgow, Medical Research Council Social & Public Health Sciences Unit Roberts, Beverly; University of Edinburgh, Dept of Psychology Der, Geoff; University of Glasgow Leckenby, Nicola; University of Edinburgh, Dept of Psychology Batty, David; University of Edinburgh, Dept of Psychology; University of Glasgow, Medical Research Council Social & Public Health Sciences Unit intelligence, mortality, meta-analysis, socioeconomic factors, Key Words: systematic review Page 1 of 49 1 2 3 Intelligence in youth and all-cause-mortality: systematic review with meta- 4 5 6 analysis 7 8 9 10 a 11 Catherine M. Calvin 12 a 13 5 Ian J. Deary 14 15 Candida Fenton b 16 17 a 18 Beverly Roberts 19 20 Geoff Der b For Review Only 21 22 Nicola Leckenby a 23 24 a, b, c 25 10 G. David Batty 26 27 28 29 30 31 32 33 34 aCentre for Cognitive Ageing and Cognitive Epidemiology (CCACE), Department of 35 36 37 15 Psychology, University of Edinburgh, UK 38 b 39 Medical Research Council Social & Public Health Sciences Unit, Glasgow, UK 40 41 cDepartment of Epidemiology & Public Health, University College London, 1-19 42 43 44 Torrington Place, London WC1E 6BT 45 46 47 48 20 -------- 49 50 51 Correspondence to Ian Deary, Centre for Cognitive Ageing and Cognitive 52 53 Epidemiology, Department of Psychology, University of Edinburgh, Edinburgh EH8 54 55 9JZ, UK . T. + 44 131 650 3452. E. [email protected] 56 57 58 59 60 25 Word count: 6022 1 Page 2 of 49 1 2 3 4 Abstract 5 6 Background. A number of prospective cohort studies have examined the association 7 8 9 between intelligence in childhood or youth and life expectancy in adulthood; 10 11 however, the effect size of this association have yet to be quantified and previous 12 13 30 reviews require updating. 14 15 16 Methods. The systematic review included an electronic search of EMBASE, 17 18 MEDLINE and PSYCHINFO databases. This yielded 16 unrelated studies that met 19 20 inclusion criteria, Forcomprising Review 22,453 deaths among Only 1,107,022 participants. 21 22 23 Heterogeneity was assessed, and fixed effects models were applied to the aggregate 24 25 35 data. Publication bias was evaluated, and sensitivity analyses were conducted. 26 27 Results. A one SD advantage in cognitive test scores was associated with a 24% (95% 28 29 30 CI 23-25%) lower risk of death, during a 17 to 69 year follow-up. There was little 31 32 evidence of publication bias (Egger’s intercept = 0.10, p = 0.81), and the intelligence- 33 34 35 mortality association was similar for men and women. Adjustment for childhood 36 37 40 socioeconomic status (SES) in the nine studies containing these data had almost no 38 39 impact on this relationship, suggesting that this is not a confounder of the intelligence- 40 41 42 mortality association. Controlling for adult SES in five studies and for education in 43 44 six studies attenuated the intelligence-mortality hazard ratios by 34% and 54%, 45 46 respectively. 47 48 49 45 Conclusions. Future investigations should address the extent to which attenuation of 50 51 the intelligence-mortality link by adult SES indicators is due to mediation, over- 52 53 adjustment, and/or confounding. The explanation(s) for association between higher 54 55 56 early life intelligence and lower risk of adult mortality require further elucidation. 57 58 Key words: Intelligence, Mortality , Meta-analysis, Socioceconomic Factors, 59 60 50 Systematic review . 2 Page 3 of 49 1 2 3 4 Introduction 5 6 Individual differences in intelligence (cognitive ability, mental ability) test scores, as 7 8 9 measured by standardised IQ-type tests in childhood, show an inverse association 10 11 with risk of death from all causes throughout adulthood. That is, higher intelligence 12 13 55 appears to confer protection. This finding is replicated in prospective cohorts from 14 15 1 2 16 several Westernized countries , across different ranges of intelligence , and in follow- 17 18 up periods from early through to late adulthood. 2-4 19 20 IntelligenceFor and somatic Review health may be inextricably Only linked throughout the life 21 22 23 course. However, longitudinal studies help to establish causal pathway models of the 24 25 60 effects of one upon the other. For example, morbidities such as diabetes, cancer, 26 27 stroke and peripheral atherosclerosis, and/or their treatments, are reported to cause a 28 29 5-10 30 decline in cognitive function after longitudinal follow-up. This illness-to-cognitive 31 32 ability direction of association is a commonplace finding. The reverse direction of 33 34 35 association is studied less often, and has only recently come to be recognized under 36 11, 12 37 65 the term ‘cognitive epidemiology’. That is, mental ability scores from early life 38 39 associated with later adulthood morbidities, and before any somatic symptoms or risk 40 41 42 factors of disease are manifest, provide evidence that cognitive abilities may be 43 44 predictive of later health outcomes. 45 46 The association between pre-morbid intelligence and adult all-cause mortality 47 48 1 49 70 was the subject of a systematic review , in which all nine studies that met the 50 51 inclusion criteria demonstrated an inverse relationship between intelligence and risk 52 53 of dying by follow-up. The review did not quantify the association. Furthermore, there 54 55 56 were insufficient studies to address comprehensively a number of pertinent questions 57 58 from this research domain. One issue is whether or not the association between 59 60 75 intelligence and mortality is the same in women as in men. For example, it is possible 3 Page 4 of 49 1 2 3 that sex differences in the incidence, age at onset of health behaviours, and the extent 4 5 13, 14 6 to which these act as risk factors for disease , could produce sex-specific 7 8 intelligence-mortality gradients. Data from many more men than women have been 9 10 11 included in intelligence-mortality cohort studies to date, mainly due to some studies 12 13 80 using military conscript databases. Moreover, when mixed-sex cohorts report 14 15 mortality risk as predicted by intelligence for men and women separately, they rarely 16 17 18 test for statistical difference but, rather, report the observed trend. With more studies 19 20 now reporting hazardFor ratios (HRs)Review for mortality by Onlysex, there is an opportunity to 21 22 quantify the predictive effects of intelligence on mortality separately for men and 23 24 25 85 women. 26 27 A second issue yet to be evaluated systematically is the extent to which 28 29 intelligence as a predictor of mortality is confounded by early life environmental 30 31 32 influences including socioeconomic factors. Socioeconomic status (SES) is 33 34 established as an important determinant of public health inequalities,15-18 including 35 36 37 90 risk of mortality, and it can carry influence in childhood, via factors such as family 38 39 income and parental education, to predict individual differences in childhood 40 41 intelligence. 19, 20 In this context, therefore, intelligence may be considered a mediating 42 43 44 variable on the pathway between early life influences and adult health outcomes. If 45 46 early social factors substantially confound the link between intelligence and longevity, 47 48 95 then adjusting for childhood SES would sizeably attenuate the effect size of the 49 50 1 51 association between intelligence and mortality. In their systematic review Batty et al 52 53 identified three out of nine studies that adjusted for childhood SES: one of these 54 55 showed no change from an unadjusted model, and two had modest attenuating effects, 56 57 58 suggesting that intelligence has independent effects on risk of mortality from those of 59 60 4 Page 5 of 49 1 2 3 100 early socioeconomic influences. Due to this small number of studies, the role of 4 5 6 childhood SES in the intelligence-mortality link requires further investigation. 7 8 One explanation why intelligence may exert an influence on life expectancy is 9 10 21 22 11 its ability to predict educational outcomes and occupational class, which can both 12 13 affect health outcomes via a number of mechanisms, for example the knowledge and 14 15 105 living conditions that contribute to better personal health risk assessment, behaviours 16 17 23 18 and management.
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