A unique study looking at the difference in cardiovascular disease (heart disease and stroke) and life expectancy between people of high and low socio-economic status has found that a person's IQ may have a role to play.
Authors of the study published in Europe's leading cardiology journal, the European Heart Journal [1] today (Wednesday 15 July), analysed data from a group of 4,289 former soldiers in the USA. They found that IQ explained more than 20% of the difference in mortality between people from socio-economically disadvantaged backgrounds compared to those from more advantaged backgrounds. Importantly, this was in addition to the classical, known risk factors for heart disease, such as smoking and obesity.
Dr David Batty, who led the research, said: "We already know that socio-economically disadvantaged people have worse health and tend to die earlier from conditions such as heart disease, cancer and accidents. Environmental exposures and health-related behaviours, such as smoking, diet and physical activity, can explain some of this difference, but not all of it. This raises the possibility that, as yet, unmeasured psychological factors need to be considered. One of these is intelligence or cognitive function, commonly referred to as IQ. This measures a person's ability to reason and problem solve. IQ is strongly related to socio-economic status."
Dr Batty, an epidemiologist and Wellcome Trust Research Fellow at the Medical Research Council's Social and Public Health Sciences Unit, University of Glasgow (UK), and at the MRC Centre for Cognitive Ageing and Cognitive Epidemiology, University of Edinburgh (UK), and his colleagues were able to look at the relationships between IQ, socio-economic status and heart disease in the study of US former soldiers because, unusually, this group had this detailed information.
"We wanted to know what is it about low socio-economic status that gives you a higher risk of cardiovascular disease," explained Dr Batty. "In other words, how is socio-economic adversity getting 'under the skin' to cause people to have a reduced life expectancy or increased risk of cardiovascular disease? This is important to know so that we can actually try to do something about the problem."
Dr Batty found that, as expected, people on low incomes (in both early and mid-life), in jobs with low prestige and with less education, had an approximately two to seven-fold higher risk of dying from all causes and cardiovascular disease than people of a higher socio-economic status. "This confirmed what we already knew," he said. "However, when we took into account IQ, we found this explained approximately 50% of that difference."
A second analysis showed that taking into account nine known risk factors for heart disease (such as smoking and obesity) explained about 40% of the difference. The final analysis that adjusted for all three (age, known risk factors and IQ) explained between 63-65% of the difference in deaths from all causes and heart disease between people with high and low socio-economic status.
"The difference between the second and third analyses showed that IQ alone explained a further 23% of the differences in mortality between the higher and lower ends of the socio-economic spectrum, in addition to the other, known risk factors," said Dr Batty. "IQ wasn't a magic bullet in this study, but this psychological variable had additional explanatory power on top of the classic variables such as smoking, high blood pressure, high blood glucose and obesity. It has partially explained the differences in death from heart disease and all causes."
In an associated editorial [2], Professors Sir Michael Marmot (who leads the long-running Whitehall II study of London-based civil servants and who chaired the WHO's Commission on Social Determinants of Health) and Mika Kivimäki, both from the Department of Epidemiology and Public Health at University College London (UK), suggest that there could be three possible explanations for Dr Batty's findings: "(i) intelligence might lead to greater knowledge about how to pursue healthy behaviours; (ii) intelligence may "cause" socioeconomic position, i.e. more intelligence leads to more education, income, occupational prestige . . .; and (iii) intelligence may be a marker for something else, and it is that something else, early life exposures, for example, that leads to mortality." They speculate as to whether conditions in early life, ranging from foetal programming to parental interest in a child's education might influence both IQ and subsequent risk of disease, and they suggest that a combination of the second and third explanations "have much to recommend them and point to interventions at societal level".