Abstract
BackgroundHigher early-life intelligence is associated with a reduced risk of mortality in adulthood, though this association is apparently hardly attenuated when accounting for early-life socio-economic status (SES). However, the use of proxy measures of SES means that residual confounding may underestimate this attenuation. In the present study, the potential confounding effect of early-life SES was instead accounted for by examining the intelligence–mortality association within families.MethodsThe association between early-life intelligence and mortality in adulthood was assessed in 727 members of the 6-Day Sample of the Scottish Mental Survey 1947 and, for the first time, 1580 of their younger siblings. These individuals were born between 1936 and 1958, and were followed up into later life, with deaths recorded up to 2015. Cox regression was used to estimate the relative risk of mortality associated with higher IQ scores after adjusting for shared family factors.ResultsA standard-deviation advantage in IQ score was associated with a significantly reduced mortality risk [hazard ratio = 0.76, p < 0.001, 95% confidence interval (CI) (0.68–0.84)]. This reduction in hazard was only slightly attenuated by adjusting for sex and shared family factors [hazard ratio = 0.79, p = 0.002, 95% CI (0.68–0.92)].ConclusionsAlthough somewhat conservative, adjusting for all variance shared by a family avoids any potential residual confounding of the intelligence–mortality association arising from the use of proxy measures of early-life SES. The present study demonstrates that the longevity associated with higher early-life intelligence cannot be explained by early-life SES or within-family factors.
Highlights
Identifying predictors of mortality and chronic disease has been the goal of many epidemiological studies from across various countries and populations
6-Day Sample members exhibited significantly longer survival times, both for those alive at the censor date and for those who had died, than members of the Sibling Sample (Table 1). This reflects the fact that all individuals included in the Sibling Sample were younger than those in the 6-Day Sample, and did not have the opportunity to accrue the same length of exposure period
In the most conservative of the analyses, we observed a 21% decrease in mortality risk with each standard-deviation advantage in IQ score. This observation is consistent with previous reports of socio-economic status (SES)-adjusted hazard ratios in the Scottish Mental Survey 1947 (20% decrease in mortality risk)[26] and in the 6-Day Sample.[24]
Summary
Identifying predictors of mortality and chronic disease has been the goal of many epidemiological studies from across various countries and populations. Methods: The association between early-life intelligence and mortality in adulthood was assessed in 727 members of the 6-Day Sample of the Scottish Mental Survey 1947 and, for the first time, 1580 of their younger siblings These individuals were born between 1936 and 1958, and were followed up into later life, with deaths recorded up to 2015. Results: A standard-deviation advantage in IQ score was associated with a significantly reduced mortality risk [hazard ratio 1⁄4 0.76, p < 0.001, 95% confidence interval (CI) (0.68– 0.84)] This reduction in hazard was only slightly attenuated by adjusting for sex and shared family factors [hazard ratio 1⁄4 0.79, p 1⁄4 0.002, 95% CI (0.68–0.92)]. Conclusions: somewhat conservative, adjusting for all variance shared by a family avoids any potential residual confounding of the intelligence–mortality association arising from the use of proxy measures of early-life SES. The present study demonstrates that the longevity associated with higher early-life intelligence cannot be explained by early-life SES or within-family factors
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