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In describing the Kaplan-Meier survival graphs in “Vita Brevis, Dignitatis Inutilis” I correctly described the findings in the Iveson et al. paper, but then went a step too far in equating a decrease in mortality risk with an identical increase in lifespan. I said:
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.
The second sentence should have read “So, at IQ 115 your chance of getting to a particular age, say 79, is 24% better than average”.
Calculating the actual lifespan increase is somewhat more complicated. That is mostly because the living are still living, and their deaths are not yet precisely foretold. Nonetheless, the curves give the best estimates of survival, and show that chances of survival of brighter persons are better than average.
Regarding the findings in their paper, the authors tell me:
We usually find that people are satisfied with knowing the Hazard Ratio per Standard Deviation of intelligence. However, you could look at the horizonal (x axis; time) distance for a given probability of being alive for -1 Standard Deviation, mean, and +1 Standard Deviation of intelligence.
To make your own estimates, look at Fig 1 and decide on a probability level, or an age to be achieved. Then read the result off the other axis.
For example, if you accept that a .8 probability is a fair bet for life planning purposes, say as regards pensions, retirement expenditure generally, or starting new ventures then, very roughly and judged by eye:
IQ 115 live 26,500 days which is 72.6 years
IQ 100 live 24,500 days which is 67.1 years
IQ 85 live 23,000 days which is 63 years
So, the “cost” of being of average intelligence rather than 1 standard deviation above average is 5.5 years.
As a rough rule of thumb, those of IQ 115 live 10 years longer than those of IQ 85. Since the latter group will have much lower wages than those of IQ 115 it is natural to assume that social class of origin determines the difference in lifespan, and that wealth is what causes “inequalities” in health outcomes. In fact social class only slightly reduces the effect of intelligence on lifespan.
In summary, it appears that those born with “system integrity” have better brains and better bodies, an advantage which is theirs to cherish or to lose.