Greater intelligence at age 11 years is linked to a lower risk for most causes of death at age 79 years, a prospective cohort study published June 28 in the BMJ has found.
“In general, the effect sizes were similar for women and men (albeit marginally greater for women), with the exception of death by suicide, which had an inverse association with childhood ability in men but not women,” Catherine M. Calvin, postdoctoral research assistant, University of Edinburgh, United Kingdom, and colleagues write. “In a representative subsample with additional background data, there was evidence that childhood socioeconomic status and physical status indicators had no more than a modest confounding impact on the observed associations.”
The researchers tracked a cohort of 33,536 men and 32,229 women, all born in 1936 in Scotland and all participants of the Scottish Mental Survey of 1947, over the course of 68 years, until December 2015.
The authors analyzed the link between intelligence on the basis of the Scottish Mental Survey of 1947 and specific causes of death, including coronary heart disease, stroke, specific cancers, respiratory disease, digestive disease, external causes, and dementia. They also conducted sensitivity analyses to address the potential for confounding variables.
The researchers used school attended as a proxy measure to adjust for potential confounding by background socioeconomic status. Some overadjustment may have occurred, as some schools require previous demonstration of cognitive ability, they write.
“In confounder adjusted models that included three indicators of childhood socioeconomic status, the relation between intelligence and all cause mortality or mortality from cardiovascular disease, any cancer, smoking related cancer, respiratory disease, or digestive disease was attenuated by 7-26%,” they report. “Addition of physical status to the model had modest impact (attenuation range 10-26%).”
All Causes of Death Inversely Related to IQ
All causes of death were inversely associated with childhood IQ. After adjustment for age and sex at time of testing, each additional standard deviation (SD) of higher intelligence score (approximately 15 points) was associated with a 20% reduced risk for death overall (hazard ratio, 0.80).
A 1 standard deviation greater score was linked to a 24% reduced risk for cardiovascular death and stroke death. Other causes of death associated with IQ included 25% lower risk for coronary heart disease, 28% lower risk for respiratory disease, 18% lower risk for digestive disease, 19% lower risk for injury, and 16% lower risk for dementia.
Further, each SD greater score was linked to an 18% lower risk for smoking-related cancer and a 14% lower risk for all cancer, but a 4% reduced risk for cancer unrelated to smoking did not reach significance. No significant association between suicide and intelligence existed for the full sample.
These findings corroborate those of multiple other studies, with the added strength that this study involved a full population, rather than only males, Daniel Falkstedt, PhD, from the Karolinska Institutet, and Anton Lager, PhD, from the Centre for Epidemiology and Community Medicine, both in Stockholm, Sweden, write in an accompanying editorial. They say this study confirms the significant association between mortality and childhood intelligence.
“New in this study is the uniquely comprehensive follow-up of major causes of death through to an age at which almost half of the participants had died,” they write. “By going beyond studies of all cause mortality, these authors are able to illuminate an otherwise hard to interpret association between early intelligence and subsequent risk of death.”
Is IQ an Indicator of “Bodily System Integrity,” Resilience?
Teasing out how much of the association is attributable to heredity compared with lifestyle or access to greater socioeconomic advantages, however, remains challenging, the editorialists suggest.
The primary attribution of this link to genetics differs “from the common perception among social epidemiologists that higher intelligence is associated with success in the educational system and with favourable environmental conditions and lifestyles (a culture of non-smoking, for example) linked to better education, work, and income, and, in addition, that higher intelligence can help people to recognise and deal with daily risks such as traffic,” they explain.
Dr Falkstedt and Dr Lager recommend contemplating the results of the study within the context of this genetic idea, in which “an individual’s IQ might signal individual differences in bodily ‘system integrity’ and resilience and, ultimately, individual differences in longevity.”
The clearest examples of dose–response relationships occurred for deaths caused by cardiovascular disease, coronary heart disease, stroke, smoking-related cancer, and respiratory disease, they write. The associations still reveal the influence of lifestyle factors, such as tobacco smoking, on the link between intelligence and mortality risk, and smoking’s distribution along the socioeconomic continuum “could be of particular importance here,” they explain.
“Importantly, [the study] shows that childhood IQ is strongly associated with causes of death that are, to a great extent, dependent on already known risk factors,” the editorialists conclude. “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.”
The study was funded by the UK cross council Lifelong Health and Wellbeing Initiative, Dementias Platform UK, the Biotechnology and Biological Sciences Research Council, and the Medical Research Council.
The authors and editorialists have disclosed no relevant financial relationships.
BMJ. 2017;357:j2708, j2932. Article full text, Editorial full text
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