Intelligent people live longer... really?

I came across this abstract from a 2008 Nature essay:

Why do intelligent people live longer? 

We must discover why cognitive differences are related to morbidity and mortality in order to help tackle health inequalities.

The statistics show that children with high IQs tend to live longer than those with less intelligence. What the statistics don't tell us is why. What thing or things do intelligent people do that can delay mortality? Ian Deary explains how cognitive epidemiologists are trying to answer the question, and potentially contribute to the redistribution of health.

Prof. Deary is the director of the Centre for Cognitive Epidemiology of the University of Edinburgh. I never met him, but I looked up his research and what he does, and it is certainly impressive. The essay looks at a number of retrospective studies where the IQ was measured earlier in life, and the longevity of the subjects was measured. The article goes on trying to give possible explanations in order to, like the abstract says, "help tackle health inequalities."

The title seemed provocative enough to spark some discussion, so I thought I'd start by giving my two cents. I won't get into the whole issue of "how do we measure intelligence," as that is not my field (though I'd love to hear from experts). Instead, I tried to read the paper from a purely statistical point of view. And this is the part that got me puzzled:

"First, what occurs to many people as an obvious pathway of explanation, is that intelligence is associated with more education, and thereafter with more professional occupations that might place the person in healthier environments. Statistical adjustment for education and adult social class can make the association between early-life intelligence and mortality lessen or disappear."

You see, to a statistician that statement settles the argument. If correcting for education and social class makes the association disappear, then the association is spurious. Instead, the Nature essay deems it an "over-adjustment."

Whenever you are trying to fit a statistical model you have to make sure that your independent variables are truly independent. Example: suppose I take a population of ten Asians and ten Caucasians, follow them for forty years, and learn that after forty years all Caucasians are dead and all Asians are still alive. I might naively conclude that Asians live longer than Caucasians. Now suppose I tell you that eight out of ten Caucasians were smokers. Well, smoke turns out to be what statisticians call a confounding factor, in other words, a variable that's correlated to both the dependent and the independent variables. Not including it in the analysis leads to spurious relationships. In my made-up example, if I stratify my analysis between smokers and non-smokers and repeat the statistical test, this time I will find no significant difference in the longevity of Caucasians versus that of Asians.

In the case of intelligence and longevity, "income/social class" is an obvious confounding factor. We all know that a healthy lifestyle is expensive. A diet high in vitamins and fibers, the time to exercise, medications, regular medical check-ups: sadly, in today's world, they are all privileges for the well-off. Furthermore, the earlier you make healthy lifestyle choices, the better your odds later in life. So, income at birth also weighs in: children born in poor environments may not have access to vaccinations, medications, healthy foods, and other more general healthy lifestyle choices, all things that will affect them as adults. I may be missing something crucial, but it really doesn't seem like an over-adjustment to me.

In a way, we're saying the same thing. There is an association. But: is the association causal or is it masking an underlying, stronger association? The essay seems to suggest that the association is indeed indicative of something else, but somehow it leaves the question open-ended, concluding: "The things that people with higher intelligence have and do that makes them live longer may be found and, we hope, shared, towards the goal of better and more equal health."

Still, I would really like to see a similar study with education and social class folded in. Because if it turns out that the true underlying drive for a better and longer life is income, well, in that case I do have a suggestion to answer the above question: let's make health care and healthy life styles more affordable to people.

Picture: Colors at the Pike Place Market, Seattle. Canon 40D, focal length 70mm, exposure time 1/40.