In my previous post, I noted that the oft-mentioned association between obesity and poor health and “early” death may be a function of the lower average IQ of obese people. I suggested that the true correlate of these things was in fact low IQ. And indeed, I’ve stumbled on additional studies that suggests that this is in fact that case.
There are in fact many studies that look at the relationship between IQ and mortality. A 2011 meta-analysis by Catherine Calvin et al. corralled these to thoroughly examine this relationship. There is no doubt: higher IQ individuals live longer, on average, than lower IQ individuals do. This isn’t just from accidental death, but heart disease and other “natural” causes as well (but not cancer, I’ll soon return to this).
The studies in this analysis span many Western countries (Denmark, UK, USA, and Sweden) and collectively include well over 1 million subjects (mostly thanks to one Swedish study). They followed their subjects from a minimum of 17 years in some studies to a maximum of 69 years in the longest. IQ measured in childhood and early adulthood predicts mortality. Socioeconomic status (SES) in childhood doesn’t attenuate this relationship. Adult SES and educational attainment had at best only modest effects (34% and 54%, respectively). IQ alone stood as a predictor of mortality.
There are a few limitations. For one, ideally, the studies would follow the participants until they all, or mostly all died – and monitored the cause of death. After all, concerns about “healthy” behaviors obscure the fact that mortality rates are always eventually 100%. But, then we’d see the relationship between IQ and longevity all the way down. Obviously, such a study would be very difficult (impossible?) to do. One of the few that comes close is the Terman Life Cycle Study (“Terman’s Termites”) – which is analyzed in the paper.
The other limitation is the somewhat inconsistent measurements across these studies, which is common to most any meta-analysis. However, on that point, the largest of the studies, the Swedish conscripts study (N = 994,262) found similar findings (with the drawback that the subjects in this study were only followed for 20 years).
The consistency of the findings, especially in the largest studies (and even within race, as most of these studies were) shows the strong relationship between IQ and health. These may reflect “’Body system integrity’” (function of the neurologic system, which may correlate with function of other physiologic systems),” as the Swedish study put it. That is, mutational load and pleiotropic effects of these mutated genes coming to bear.
About the issue of the degree that the superior care higher IQ individuals take of themselves plays a role in this relationship, another study from Sweden (courtesy Staffan), which looked at 1 million military conscripts found NO association between IQ and cancer incidence (except for modest correlations with lung and skin cancers). Since behaviors, especially “healthy” ones are related to IQ, this suggest that behaviors have no effect on the incidence of most cancers. As Staffan put it:
It’s interesting because IQ is linked to lots of health behaviors. People love to think that health behaviors will fix everything but smart people eat their vegetables and get cancer anyway.
Indeed. This finding is interesting in its own right, because it suggests that the non-heritable contribution to cancer is essentially purely random. This weakens the case for adopting “the right, healthy behaviors” to ward-off unhealthy outcomes. Yet, none the less, that is the conventional wisdom – something which has not fared well around here.
As Staffan noted, the association between IQ and health can be attributed to behavioral patterns that differ by IQ. Quite likely that plays some role, but isn’t the primary, or perhaps not even a significant factor.
Edit, 5/13/13: [Also note that in this post, Kevin Mitchell comes to the same conclusion:
Various researchers have suggested that g may be simply an index of a general fitness factor – an indirect measure of the mutational load of an organism. The idea is that, while we all carry hundreds of deleterious mutations, some of us carry more than others, or ones with more severe effects. These effects in combination can degrade the biological systems of development and physiology in a general way, rendering them less robust and less able to generate our Platonic, ideal phenotype. In this model, it is not the idea that specific mutations have specific effects on specific traits that matters so much – it is that the overall load cumulatively reduces fitness through effects at the systems level.
Direct evidence for this kind of effect of mutational load was found recently in a study by Ronald Yeo and colleagues, showing that the overall burden of rare copy number variants (deletions or duplications of segments of chromosomes) negatively predicts intelligence (r = -0.3).
If g really is an index of a general fitness factor, then it should be correlated with other indices of fitness. This indeed appears to be the case. G is weakly positively correlated with height, for example, and also strongly correlated with various measures of health and longevity.
This correlation can be interpreted in two ways: one, less intelligent people have less healthy and/or riskier lifestyles (i.e., direct causation), or, two, both intelligence and rates of mortality at least partially reflect an underlying factor – general fitness.
Another good marker of general fitness is developmental stability. This refers to the robustness of the system and the ability of the genotype to reliably generate a phenotype within the species-specific normal range, despite genetic and environmental perturbations and intrinsic noise or randomness. It is a property that varies between people.
One can get a good measure of developmental stability by looking at how symmetric someone is. The two sides of the body develop independently from the same set of genomic instructions – if a particular genotype is very robust then it should generate a very similar outcome on each side of the body. If, however, the system is less robust, then the person may be more asymmetric in any number of features (arm lengths, finger widths, earlobe lengths, eye widths, etc.). This kind of asymmetry is called fluctuating asymmetry as the direction is random – one arm may be longer than the other, but it is equally likely to be the left or right (unlike the asymmetry of internal organs, for example, which is directional and a species-specific trait).
Fluctuating asymmetry should thus be a good indicator of general fitness and is fairly easy to measure (though it is important to look at multiple features to get an aggregate score in each individual). It is also a heritable trait – monozygotic twins are more similar to each other in degree of asymmetry than are dizygotic twins. There is no reason, however, to think this reflects variation in a set of genes whose function it is to make the organism more symmetric, or to make developmental systems more robust. Rather, mutations in any genes affecting development are likely to not just contribute to some specific phenotype, but also to generally decrease robustness of the system and increase variability.
You can probably guess what’s coming next – fluctuating asymmetry correlates negatively with various IQ measures. At least, most of the studies that have looked at it have found such a correlation – ranging from –0.2 to –0.4, which is fairly substantial. Not all studies have found this but a meta-analysis confirms a correlation with a value between –0.12 and –0.2. This correlation is weaker, but still significant, and means that there is at least some relationship between intelligence and symmetry. (It may also be an underestimate, as one study found that psychometric tests with heavier loadings on g showed greater correlations with fluctuating asymmetry). The most plausible interpretation is that this correlation reflects the effects on both parameters of a “latent variable” – general fitness.
This may, incidentally, also explain the recently demonstrated correlation between intelligence and physical attractiveness, which itself has been correlated with facial symmetry.]
EDIT, 2/4/14: Indeed, a new paper (co authored by Ian Deary) found that the same relationship, to the same degree, between IQ and longevity turns up when simple reaction time is used as the measure:
This strengthens the notion that intrinsic health – body system integrity (a bodily g factor if you will) – is in good part responsible for the IQ-longevity link.
What about proposition that the adverse outcomes associated with obesity might in fact be the result of low IQ? Well, fortunately, one of the studies in the Calvin meta analysis did take a look at that. The Vietnam Experience Study by Batty et al followed the outcomes of 4316 Vietnam War veterans, using their Armed Forces test scores upon entry into the military to gauge their IQ. The sample was subjected to a medical exam in 1986, which measured the subjects’ BMI, among many other things. The result: IQ was by far the strongest predictor of death. Indeed, “marital status, alcohol consumption, systolic and diastolic blood pressure, pulse rate, blood glucose, body mass index, psychiatric and somatic illness at medical examination) was negligible (10% attenuation in risk)!”
In other words, weight and other “common markers of health” had little to no effect on death rates of the subjects for the 15+ years they were followed after their exam in the 1980s.
Sure, we could (correctly) argue that these men were veterans and presumably had to have a modicum of physical and mental fitness to have served in the first place – hence, their results were unrepresentative of the general population. But, it’s hard to ignore a result that is quite consistent with my previous prediction on health, obesity and cardiovascular markers.
Putting all that I found and posted on this topic together, it would seem that the conventional wisdom – that we need to eat “right”, exercise, keep thin, etc., to live a long, healthy life is largely, if not entirely, bullshit. People who live long lives often do those things because they were healthier to begin with. It’s quite likely that a few (or more than a few) extra pounds won’t hurt you if you’re an otherwise all-around healthy (and mentally sharp) person.
Yet, people, including the medical establishment, continue to give this advice, (sometimes in spite of evidence). The reason? My own suspicion is that health advice is one of the last remaining straws of the Utopian wish to perfect man. Forced to let go (but not without great resistance) of the idea that we can socially engineer an end to poverty, war, strife, jealousy in a Star Trek-esque manner if we just try hard enough, and still coming to grips with the idea of cognitive and behavioral inequalities, Utopians have latched to idea that we can make people healthier and live longer (and of course, thinner) if we just adopt the right behaviors. It may turn out that much of that might not be possible even in principle (not via behaviors, anyway). A serenity moment if there ever was one.
Edit, 5/13/13: [Indeed, if ever there was a shining example of this, it would be this poor family:
Early heart disease ran in Rick Del Sontro’s family, and every time he went for a run, he was scared his heart would betray him. So he did all he could to improve his odds. He kept himself lean, stayed away from red meat, spurned cigarettes and exercised intensely, even completing an Ironman Triathlon.
Rick Del Sontro keeps himself lean and watches his diet, but despite his efforts, he has heart disease like many in his family.
“I had bought the dream: if you just do the right things and eat the right things, you will be O.K.,” said Mr. Del Sontro, whose cholesterol and blood pressure are reassuringly low.
But after his sister, just 47 years old, found out she had advanced heart disease, Mr. Del Sontro, then 43, and the president of Zippy Shell, a self-storage company, went to a cardiologist.
An X-ray of his arteries revealed the truth. Like his grandfather, his mother, his four brothers and two sisters, he had heart disease. (One brother, Michael, has not received a diagnosis of the disease.)]
As well, as noted, the anti-obesity people may have to come to terms with the notion that their crusade against obesity is more about aesthetics than health.
It is worth noting two pieces of evidence that may serve as counterarguments. There is a study of post-Soviet collapse Cuba that claims to have monitored marked declines in obesity and cardiovascular disease there for a time, only to rebound (at least obesity) after conditions improved. My impression is that, in this case, just as with sub-Saharan Africa, it’s hard to have weight problems when food is a scare commodity (smoking also greatly declined). As well, the authors admit to having problems with missing and unreliable data in their study. Nonetheless, there is it is.
Just the same, another study claims similar levels of good health for Victorian England. This study attributes the health of Victorian people to low salt consumption, lack of smoking and drinking, and much higher levels of (continuous) physical activity. The authors claim that degenerative diseases were generally absent in the day. On this one, I am doubtful because of the lack of anything resembling solid measurements and my lack of trust in the reliability of the diagnoses of 19th century doctors.
The field of health, going forward, is – contrary to what we might have thought about our time – rather wide open. There is much we still have to learn, a lot of it because of our reliance on poor research design and the failure to appreciate the power of heredity.