If bright people live longer, is it because of the good things that social status provides?
This question may appear to be very hard to answer, because one thing leads to another, and bright people are clever enough to build themselves agreeable places in which to live. One approach is to “control” for this, on the outrageous assumption that bright people have done nothing to deserve their social status, but have it handed to them on a plate by the random vagaries of fate, or the accumulated greed of their pushy ancestors.
First of all, what influence does intelligence measured at age 11 have on longevity? The good news is that a standard-deviation increase in IQ score is associated with a 24% decrease in mortality risk. So, at IQ 115 lifespan is 24% longer than average*. This is good news, together with the 60% increase in wages above the average level from an OECD study.
On the wages front, the effect of intelligence had already been shown by Charles Murray in his well-known 1998 “Income inequality and IQ” in which he compared the earnings of one child in a family with that of another sibling, showing that the effect of intelligence was powerful at creating later life differences even between siblings brought up within the same family environment.
He extended his analysis in subsequent work, looking at the effects of divorce and poverty, finding them to have contributed relatively little to income measures once intelligence was taken into account.
IQ and Income Inequality in a Sample of Sibling Pairs from Advantaged Family Backgrounds. Charles Murray. AMERICAN ECONOMIC REVIEW VOL. 92, NO. 2, MAY 2002 (pp. 339-343)
Having matched individuals to their nearest siblings, Murray then examined whether within-sibling-pair intelligence differences could predict income differences within pairs. Examining the contribution of intelligence within families circumvented the need to obtain a range of SES measures, as it removed the variance accounted for by the shared family environment (e.g. parental income, occupation and education). Examining outcomes within families therefore allows researchers to account for early-life SES without the need to operationalize and measure it.
Murray found that brighter siblings started out earning less as they carried out further studies, and then surpassed their less bright siblings. Patience and intellect rewarded with delayed but then sustained greater earnings.
Iveson and colleagues have extended the within-family method by linking children in their 1947 national sample to younger siblings in a “Six day” sample, such that they had families tested at the same age with the same Moray House intelligence tests, and longevity measured up to November 2015.
Intelligence and all-cause mortality in the 6-Day Sample of the Scottish Mental Survey 1947 and their siblings: testing the contribution of family background.
Matthew H. Iveson Iva Čukić Geoff Der G. David Batty Ian J. Deary
International Journal of Epidemiology, dyx168,https://doi.org/10.1093/ije/dyx168
Published: 21 August 2017
Background: Higher 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.
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. Cox regression was used to estimate the relative risk of mortality associated with higher IQ scores after adjusting for shared family factors.
Results: A 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)].
Conclusions: Although 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.
Here are the results of the contrast between the living and the dead.
These are scary figures, and worth showing to friends who doubt that IQ has any practical meaning.
Here are the survival rates for 1 sd above average in intelligence, average intelligence, and 1 sd below average intelligence, with the chilling reminder that a good life is probably no more than 30,000 days. Carpe diem.
Here is the key paragraph in which they explain the significance of calculating the effects of SES by using the within-family method:
The somewhat attenuating effect of adjusting for family-related SES, although small, is broadly consistent with previous work using discrete proxy measures of early-life SES.19 However, the fact that a substantial IQ-mortality association persists beyond such adjustment is interesting, particularly given that the present study accounts for early-life SES in a different and more comprehensive way. Adjusting for within-family variance is notably more conservative than simply accounting for an explicit measure of SES, as it likely captures shared factors not directly related to SES such as genetic factors, environmental health (air pollution, etc.), childhood diet and exposure to passive smoking. Notably, adjusting for shared family factors led to a slightly larger attenuation of the intelligence–mortality association than adjusting for an explicit proxy measure of SES. However, the advantage of the family-based approach adopted in the present study is that it avoids any measurement error or residual confounding associated with using proxy measures such as parental occupation and income. Where previous studies have used proxy measures, residual confounding may erroneously underestimate the attenuating effect of early-life SES on the intelligence–mortality association.
Here are the additional summary points on this study
• After adjusting for all shared family factors, a standard-deviation increase in IQ score was associated with a 21% decrease in mortality risk.
• Early-life socio-economic circumstances are not sufficient to explain the intelligence–mortality association.
The key finding is that intelligence leads to a longer life, and childhood socioeconomic status does little to alter that effect. This is an important result, so please find a kind way of breaking this news to any sociologist of your acquaintance.
* In fact: So, at IQ 115 your chance of getting to a particular age, say 79, is 24% better than average.