Association and causation: the link between childhood IQ and mortality
Around Christmas, we are often more aware of the scale of poverty than at other times of the year. Everybody is talking about social inequality and its impact on health, from the World Health Organisation (WHO) to our own UK policymakers , from researchers and journalists to health professionals . The WHO defines social determinants as “...the conditions in which people are born, grow, live, work and age, including the health system. These circumstances are shaped by the distribution of money, power and resources at global, national and local levels, which are themselves influenced by policy choices. The social determinants of health are mostly responsible for health inequities - the unfair and avoidable differences in health status seen within and between countries.”
The difficulty, of course, is how to tease out what is most important in causing ill health- poverty, education, childhood social environment or family situation. If we don’t understand what are the causes and effects of these inequalities, then it becomes very hard to design coherent policies to address them. The link between childhood intelligence and health is a great example. Previous studies have suggested that intelligence in childhood and early adulthood is associated with illness and death. Note the wording: an “association” or “link” does not necessarily imply “causation”; i.e. we cannot say whether differences in childhood intelligence actually cause changes in health in later life.
There are two competing hypotheses to explain this association or link: firstly, that early IQ is the fundamental explanation for socioeconomic differences in health; and secondly, that socioeconomic conditions in childhood and adulthood are responsible for health differences linked to early IQ. In the first hypothesis, we are assuming a true association. In the second hypothesis, early IQ may be a confounder of the relationship between socioeconomic conditions and health. Whereas bias involves error in the measurement of a variable (in this case childhood IQ or mortality), confounding involves error in the interpretation of what may be an accurate measurement. When we look at an association, we have to consider whether the results could be due to bias, confounding, or chance (considered in lesson 2 of “Understanding EBM in 4 days”), before we conclude that it is a true association.
In a Swedish study of 1500 children who were followed up over their lifetimes after taking an IQ test, this relationship between childhood IQ and mortality was examined. As with previous studies, they found that increased educational attainment was associated with reduced mortality, with a 9% reduction in men and a 12% reduction in women for each additional year in school. These findings were unchanged after adjustment for the childhood IQ, and so the researchers concluded: “mortality differences among participants by own educational attainment were not explained by childhood IQ, neither for men nor women. Hence, our results do not suggest that differences in early IQ can explain why people with longer education live longer.”
They also found that childhood IQ was independently linked to male adult mortality, and so “higher cognitive ability also seems to have an additional direct protective effect”. In women, the researchers found that the mortality risk was highest in those with the highest childhood IQ, and this risk was greatest in women over the age of 60. They explained this finding as follows: “intelligent women may have a greater underlying risk of dying that is masked by their having on average a longer education and, supposedly, a lower mortality risk than women who spent less time in education”. These findings make the first hypothesis for a direct effect of childhood IQ on health implausible, and it seems much more likely that “the link between IQ and mortality involves the social and physical environment rather than simply being a marker of a healthy body to begin with”.