Social inequalities in mortality: a problem of cognitive function?

Michael Marmot* and Mika Kivimäki
Department of Epidemiology and Public Health, University College London, UK
European Heart Journal Advance Access published online on July 14, 2009
European Heart Journal, doi:10.1093/eurheartj/ehp264

Available online at : http://eurheartj.oxfordjournals.org/cgi/reprint/ehp264v1

This editorial refers to ‘Does IQ explain socio-economic differentials in total and cardiovascular disease mortality? Comparison with the explanatory power of traditional cardiovascular disease risk factors in the Vietnam Experience Study’, by G.D. Batty et al., on page 1903

Does IQ explain socio-economic differentials in total and cardiovascular disease mortality?
Comparison with the explanatory power of traditional cardiovascular disease risk factors in the Vietnam Experience Study

G. David Batty1,2,3,*, Martin J. Shipley4, Ruth Dundas1, Sally Macintyre1, Geoff Der1, Laust H. Mortensen5,6 and Ian J. Deary2
1 Medical Research Council Social and Public Health Sciences Unit, University of Glasgow, UK
2 Department of Psychology, MRC Centre for Cognitive Ageing and Cognitive Epidemiology, University of Edinburgh, Edinburgh, UK
3 The George Institute for International Health, University of Sydney, Sydney, Australia
4 Department of Epidemiology and Public Health, University College London, London, UK
5 National Institute of Public Health, University of Southern Denmark, Copenhagen, DK
6 Department of Psychiatry and Behavioral Sciences, Duke University Medical Center, Durham, NC, USA

Available online at:
http://eurheartj.oxfordjournals.org/cgi/content/full/ehp254v1?ijkey=67c476f65760629157c80fea8825464bf797c952

Aims: The aim of this study was to examine the explanatory power of intelligence (IQ) compared with traditional cardiovascular disease (CVD) risk factors in the relationship of socio-economic disadvantage with total and CVD mortality, that is the extent to which IQ may account for the variance in this well-documented association.

Methods and results: Cohort study of 4289 US male former military personnel with data on four widely used markers of socio-economic position (early adulthood and current income, occupational prestige, and education), IQ test scores (early adulthood and middle-age), a range of nine established CVD risk factors (systolic and diastolic blood pressure, total blood cholesterol, HDL cholesterol, body mass index, smoking, blood glucose, resting heart rate, and forced expiratory volume in 1 s), and later mortality. We used the relative index of inequality (RII) to quantify the relation between each index of socio-economic position and mortality. Fifteen years of mortality surveillance gave rise to 237 deaths (62 from CVD and 175 from ‘other’ causes). In age-adjusted analyses, as expected, each of the four indices of socio-economic position was inversely associated with total, CVD, and ‘other’ causes of mortality, such that elevated rates were evident in the most socio-economically disadvantaged men. When IQ in middle-age was introduced to the age-adjusted model, there was marked attenuation in the RII across the socio-economic predictors for total mortality (average 50% attenuation in RII), CVD (55%), and ‘other’ causes of death (49%). When the nine traditional risk factors were added to the age-adjusted model, the comparable reduction in RII was less marked than that seen after IQ adjustment: all-causes (40%), CVD (40%), and ‘other’ mortality (43%). Adding IQ to the latter model resulted in marked, additional explanatory power for all outcomes in comparison to the age-adjusted analyses: all-causes (63%), CVD (63%), and ‘other’ mortality (65%). When we utilized IQ in early adulthood rather than middle-age as an explanatory variable, the attenuating effect on the socio-economic gradient was less pronounced although the same pattern was still present.

Conclusion: In the present analyses of socio-economic gradients in total and CVD mortality, IQ appeared to offer greater explanatory power than that apparent for traditional CVD risk factors.