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intelligence and longevity

Why Do Smarter People Live Longer?

What’s related to what?

A study published last week in the British Medical Journal states simply that, “Childhood intelligence was inversely associated with all major causes of death.”

The study focused on some 65,000 men and women who took the Scottish Mental Survey in 1947 at age 11. Those students are now 79 years old and many of them have passed away. By and large, those who scored lower on the test in 1947 were more likely to have died – from all causes — than those who registered higher scores.

This is certainly not the first study to link intelligence with longevity. (Click here, here, and here, for instance). But it raises again a fundamental question: why would smarter people live longer? There seem to be at least two competing hypotheses:

Hypothesis A: More intelligent people make better decisions about their health care, diet, exercise, etc. and — as a result — live longer.

Hypothesis B: whatever it is that makes people more intelligent also makes them healthier. Researchers, led by Ian Deary, call this hypothesis system integrity. Essentially, the theory suggests that a healthy system generates numerous positive outcomes, including greater intelligence and longer life. The theory derives from a field of study known as cognitive epidemiology, which studies the relationships between intelligence and health.

Hypothesis A focuses on judgment and decision making as causal factors. There’s an intermediate step between intelligence and longevity. Hypothesis B is more direct – the same factor causes both intelligence and longevity. There is no need for an intermediate cause.

The debate is oddly similar to the association between attractiveness and success. Sociologists have long noted that more attractive people also tend to be more successful. Researchers generally assumed that the halo effect caused the association. People judged attractive people to be more capable in other domains and thus provided them more opportunities to succeed. This is similar to our Hypothesis A – the result depends on (other people’s) judgment and there is an intermediate step between cause and effect.

Yet a recent study of Tour de France riders tested the notion that attractiveness and success might have a common cause. Researchers rated the attractiveness of the riders and compared the rankings to race results. They found that more attractive riders finished in higher positions in the race. Clearly, success in the Tour de France does not depend on the halo effect so, perhaps, that which causes the riders to be attractive may also cause them to be better racers.

And what about the relationship between intelligence and longevity? Could the two variables have a single, common cause? Perhaps the best study so far was published in the International Journal Of Epidemiology last year. The researchers looked at various twin registries and compared IQ tests with mortality. The study found a small (but statistically significant) relationship between IQ and longevity. In other words, the smarter twin lived longer. Though the effects are small, the researchers conclude, “The association between intelligence and lifespan is mostly genetic.”

Are they right? I’m not (yet) convinced. Though significant, the statistical relationship is very small with r = 0.12. As noted elsewhere, variance is explained by the square of r. So in this study, IQ explains only 1.44% (0.12 x 0.12 x 100) of the variance in longevity. That seems like weak evidence to conclude that the relationship is “mostly genetic”.

Still, we have some interesting research paths to follow up on. If the theory of system integrity is correct, it could predict a whole host of relationships, not just IQ and longevity. Attractiveness could also be a useful variable to study. Perhaps there’s a social aspect to it as well. Perhaps people who are healthy and intelligent also have a larger social circle (see also Dunbar’s number). Perhaps they’re more altruistic. Perhaps they are more symmetric. Ultimately, we may find that a whole range of variables depend partially – or perhaps mainly – on genetics.

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