Childhood IQ and cardiovascular disease in adulthood: prospective observational study linking the Scottish Mental Survey 1932 and the Midspan studies
Introduction
Although rates of coronary heart disease (CHD) and stroke are declining in the west (Office for National Statistics, 1997; Warlow, 1998), they were the cause of 21% and 12% of deaths, respectively, in Scotland in 2000 (Registrar General for Scotland, 2001). Cardiovascular disease (CVD), which includes CHD and stroke, was the underlying cause in 41% of deaths (Registrar General for Scotland, 2001). In addition, CVD is a major contributor to the burden of illness, hospital admissions and disability.
There are well-established risk factors for CVD, such as tobacco smoking and raised blood pressure. People with poorer socioeconomic profiles, determined by education, social class and deprivation are known to be at higher risk of CVD (Davey Smith et al., 1998; Davey Smith, Bartley, & Blane, 1990; Carstairs & Morris, 1991). Personal traits, such as hostility, submissiveness and the type A personality more generally, have a link to some CVD outcomes (Whiteman, Deary, & Fowkes, 2000; Hemingway & Marmot, 1999). Childhood IQ, an intriguing new risk factor, has been found to be inversely related to all cause mortality, CVD and CHD (Whalley, & Deary, 2001; Hart et al., 2003b; Osler et al., 2003). In this paper, we expand our previous findings on mortality to investigate how childhood IQ contributes to CVD events (hospital admissions and deaths). We next examine the associations between childhood IQ and specific CVDs, viz., CHD and stroke. Given the suggestion that childhood IQ influences health behaviours throughout life (Deary, Whalley, & Starr, 2003), and the fact that childhood IQ is associated with birthweight (Shenkin et al., 2001), which is also linked to CVD in adulthood (Barker, 1994), it is possible that childhood IQ is associated with CVD outcomes relatively early in adult life. We therefore also explore whether the effects of childhood IQ are different in CVD, CHD and stroke occurring before or after the age of 65. Lastly, we hypothesise that the effect of childhood IQ on CVD may be mediated by traditional risk factors for CVD.
The Scottish Mental Survey 1932 (SMS1932), conducted under the auspices of the Scottish Council for Research in Education (SCRE), obtained data about the whole distribution of the intelligence of Scottish pupils (Scottish Council for Research in Education, 1933). On June 1, 1932, children born in the calendar year 1921 and attending schools in Scotland were given the same, well-validated mental ability test. The total number of children who sat the Moray House Test was 87,498 (44,210 boys and 43,288 girls). As far as we are aware, no other country collected information about the childhood mental ability differences of an entire year-of-birth cohort. The Moray House test is described in detail elsewhere (Deary, Whalley, Lemmon, Crawford, & Starr, 2000). Briefly, it has 71 items with a maximum possible score of 76. The time limit was 45 min for completion. It contains a preponderance of verbal reasoning items but also some numerical, spatial and abstract reasoning items. The score is expressed as a total score; there are no subscores.
The Midspan studies were large cardiorespiratory studies of adults carried out in Scotland in the 1960s and 1970s. Two studies are included in this analysis, the Collaborative study and the Renfrew/Paisley study. The Collaborative study was conducted between 1970 and 1973 in 27 workplaces in the west and central belt of Scotland (Davey Smith et al., 1998). The full sample consisted of 6022 men and 1006 women of working age. The Renfrew/Paisley general population study was carried out between 1972 and 1976 and involved 7052 men and 8354 women aged 45–64 years who were resident in Renfrew and Paisley (Hawthorne et al., 1995). The age ranges of the studies were such that some participants were born in 1921. Ethical permission was obtained from the Multi-Centre Research Ethics Committee for Scotland to link the SMS1932 data set with the 1921-born participants of the Midspan data sets.
There were 1251 Midspan participants who were born in 1921 and of these, 938 (75%) were matched to a mental ability score from the SMS1932. Full details of the matching procedures are reported elsewhere (Hart et al., 2003a). Since the children's ages varied between 10 and 11 years at the time of testing, the test scores were corrected for age in days and converted to usual IQ-type scores with mean 100 and standard deviation 15.
In both Midspan studies, participants completed a questionnaire and attended a physical examination (Davey Smith et al., 1998; Hawthorne et al., 1995). The questionnaire included questions about smoking habit, home address and occupation. Social class was coded according to the Registrar General's Classification (General Register Office, 1966) for occupation at the time of screening. The social class of women was allocated according to their own occupation, except for those women in the Renfrew/Paisley study who gave their occupation as housewives. In these cases, the social class was that of their husband. The home address at the time of screening was retrospectively postcoded, enabling deprivation category as defined by Carstairs and Morris to be ascertained (Carstairs & Morris, 1991). This measure is an area-based measure of deprivation, obtained from four census variables—male unemployment, overcrowding, car ownership and the proportion of heads of households in social classes IV and V. A deprivation score for each postcode sector is obtained which is converted to seven categories ranging from 1 (least deprived) to 7 (most deprived). The few missing values for social class (n=13) and deprivation category (n=3) were imputed as social class III manual and deprivation category 5 as these were the most commonly occurring. The physical examination included measurement of blood pressure, height, weight, respiratory function, plasma cholesterol and blood glucose (in the Renfrew/Paisley study only). Blood pressure was measured with the subject seated, using the London School of Hygiene sphygmomanometer, with a cuff of 12cm×22 cm. Diastolic pressure was recorded at the disappearance of the fifth Korotkoff sound. Two blood pressure readings were taken and the average diastolic and systolic pressure were recorded. The adjusted FEV1 was defined as the actual FEV1 as a percentage of the expected FEV1, derived from sex-specific linear regressions of age and height from healthy subsets of the study populations (Davey Smith et al., 1998; Hole et al., 1996). Body mass index in kg/m2 was calculated from the weight and height.
Midspan study participants were flagged at the National Health Service Central Register in Edinburgh. Causes and dates of death in a 25 year follow-up period were provided. Underlying causes of death were identified and grouped as all cardiovascular disease (CVD) (ICD9 codes 390-459), coronary heart disease (CHD) (ICD9 410-414) and stroke (ICD9 430-438). The few more recent deaths were coded in ICD10 and equivalent codes were used for each underlying cause. In addition, a computerised linkage with acute hospital discharges in Scotland provided records of acute hospital admissions in up to 25 years of follow-up, although hospital admissions were only provided to the end of 1995 for Renfrew/Paisley study participants (Hanlon et al., 1998). First occurrence of each cause of hospital admission from the discharge diagnosis was coded in the same way as the deaths.
Cox's models (Cox, 1972) were used to calculate proportional hazards regression coefficients for one standard deviation change in risk factor and for current and ex smokers compared to never smokers. The exponentiated proportional hazards regression coefficients are referred to as relative rates. Survival time in 25 years was taken from the date of screening until either the date of hospital admission, or the date of death if no hospital admission was found. One participant had embarked from the UK during the follow-up period and survival time was taken until the date of embarkation. Hospital admissions and deaths for the whole follow-up period were divided into those first occurring up to and including age 65 (the early period) and those first occurring after age 65 (the later period).
Section snippets
Results
There were significant negative correlations between childhood IQ and both adulthood diastolic blood pressure and systolic blood pressure, meaning that people with higher childhood IQ had lower blood pressure in adulthood (Table 1). We have reported this relationship elsewhere (Starr et al., 2004). There were significant positive correlations between childhood IQ and height and respiratory function, meaning that people with higher childhood IQ were taller and had better respiratory function in
Discussion
We have shown that childhood IQ was significantly related to CVD, CHD and stroke events occurring up to and including age 65, but not to these events occurring after age 65. Age 11 IQ has been shown to have substantial stability to IQ at age 77 (Deary et al., 2000; Crawford, Deary, Starr, & Whalley, 2001) so may be expected to predict events throughout life. It is possible that the lack of effect of lower childhood IQ at older ages is due to the higher risk people being removed from the
Acknowledgements
We thank the Scottish Council for Research in Education for making the data from the Scottish Mental Survey 1932 available to the authors. Victor Hawthorne was responsible for the original Midspan studies and Pauline MacKinnon updates the mortality information. Funding was provided by the Chief Scientist's Office of the Scottish Executive. I.J.D. is the recipient of a Royal Society-Wolfson Research Merit Award. L.J.W. holds a Wellcome Trust Career Development Award.
References (32)
- et al.
The stability of individual differences in mental ability from childhood to old ageFollow-up of the 1932 Scottish Mental Survey
Intelligence
(2000) - et al.
The Scottish Mental Survey 1932 linked to the Midspan studiesA prospective investigation of childhood intelligence and future health
Public Health
(2003) - et al.
Pulmonary function is a long-term predictor of mortality in the general population29-year follow-up of the Buffalo Health Study
Chest
(2000) - et al.
The effect of age on risk factors for ischaemic heart diseaseThe Manitoba Follow-up study 1948–1993
Annals of Epidemiology
(1998) Mothers, babies, and disease in later life
(1994)- et al.
Deprivation and health in Scotland
(1991) Regression models and life tables
Journal of the Royal Statistical Society (B)
(1972)- et al.
The NART as an index of prior intellectual functioningA retrospective validity study covering a 66-year interval
Psychological Medicine
(2001) - et al.
The Black report on socioeconomic inequalities in health 10 years on
British Medical Journal
(1990) - et al.
Education and occupational social classWhich is the more important indicator of mortality risk?
Journal of Epidemiology and Community Health
(1998)
IQ at age 11 and longevityResults from a follow-up of the Scottish Mental Survey 1932
Mortality in relation to smoking40 years’ observations on male British doctors
British Medical Journal
Hospital use by an ageing cohortAn investigation into the association between biological, behavioural and social risk markers and subsequent hospital utilization
Journal of Public Health Medicine
Childhood IQ, social class, deprivation and their relationships with mortality and morbidity risk in later lifeProspective observational study linking the Scottish Mental Survey 1932 and the Midspan studies
Psychosomatic Medicine
Cardiorespiratory disease in men and women in urban ScotlandBaseline characteristics of the Renfrew/Paisley (Midspan) Study population
Scottish Medical Journal
Cited by (121)
Cognitive ability in early life and risk of depression in adulthood: A systematic review and meta-analysis
2024, Journal of Affective DisordersIntelligence and life expectancy in late adulthood: A meta-analysis
2023, IntelligenceAPOE effects on cognition from childhood to adolescence
2019, Neurobiology of AgingCognition level and change in cognition during adolescence are associated with cognition in midlife
2019, Annals of EpidemiologyYoung adult cognitive ability and subsequent major depression in a cohort of 666,804 Danish men
2018, Journal of Affective Disorders
- 1
Tel.: +44-131-650-3452; fax: +44-131-651-1771.