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Higher childhood IQ associated with lower adult mortality

Tobacco smoking rates probably one reason – but are there others, including genetics?

Louise Prime

Thursday, 29 June 2017

People with higher intelligence (IQ) test scores in childhood have lower lifetime mortality from many causes including cardiovascular disease, smoking-related cancers, respiratory disease and dementia than those with lower childhood IQ, according to a large UK-based study* published today in The BMJ. Commentators pointed out that variations in smoking rates and other known risk factors probably explain at least part of the association, but they also suggest that there could be other, possibly genetic, reasons for the link between IQ and longevity.

A research team led from the University of Edinburgh pointed out that previous studies showing that, on average, people with higher IQs tend to live slightly longer than those with lower IQs were largely based on data from male conscripts followed up only to middle adulthood. They designed a study to examine the association between intelligence test scores measured at the age of 11 and leading causes of death in men and women up to age 79.

They analysed UK data covering 33,536 men and 32,229 women born in Scotland in 1936, who took a validated childhood intelligence test at the age of 11 as part of the Scottish Mental Survey of 1947, and who could be linked to cause of death data up to December 2015. Recorded causes of death included coronary heart disease (CHD), stroke, specific cancers, respiratory disease, digestive disease, external causes (including suicide and death from injury), and dementia.

They reported that even after accounting for several potential confounding factors including age, sex and socioeconomic status, intelligence at the age of 11 was inversely associated with all major causes of death until the age of 79. Higher intelligence (equivalent to a difference of about 15 points on IQ tests) was associated with a 28% reduced risk of death from respiratory disease, a 25% reduced risk of death from coronary heart disease, and a 24% reduced risk of death from stroke.

They also reported other notable significant associations for deaths from injury (19% lower risk), smoking-related cancers (18%), digestive disease (18%), and dementia (16%). They found only weak, non-significant associations for suicide and for non-smoking-related cancers.

Although the study authors pointed out possible causes of bias in their study, they said it also had major strengths such as large sample size, 68-year follow-up and its ability to adjust for important confounders.

They said smoking and socioeconomic status were unlikely to fully account for the associations they found, but future studies would benefit from measures of the lifetime cumulative load of such risk factors.

The authors of an accompanying editorial** commented: “The study confirms that intelligence test scores in childhood are significantly associated with subsequent mortality. Importantly, it shows that childhood IQ is strongly dependent on already known risk factors. Tobacco smoking and its distribution along the socioeconomic spectrum could be of particular importance here.”

But they added: “It remains to be seen if this is the full story or if IQ signals something deeper, and possibly genetic, in its relation to longevity.”

* Calvin CM, Batty GD, Der G, et al. Childhood intelligence in relation to major causes of death in 68 year follow-up: prospective population study. BMJ 2017; 357: j2708. doi: 10.1136/bmj.j2708

** Falkstedt D, Lager ACJ. Higher IQ in childhood is linked to a longer life. BMJ 2017; 357: j2932. doi: 10.1136/bmj.j2932

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