You may remember my dictum: If you are fatter than you want to be, eat less.
That post led to an outpouring of deeply lived personal experience, of almost French complexity, extolling the virtues of eating particular food types in particular combinations at particular times, and not paying too much attention to calories. Fine. If you wish to be befuddled, that is your perfect right.
So, with some trepidation, here is a summary of the current state of knowledge regarding intelligence and health. Indeed, it is my summary of a summary paper. A pointless redundancy, you may say, but I know you are busy, and I would not like to interrupt your lunch break.
Intelligent people lead healthier lives, and that is not just because they intelligently make healthy decisions, but also, it would appear, because they are inherently healthier. Spooky.
What genome-wide association studies reveal about the association between intelligence and physical health, illness, and mortality
Ian JDeary 1 Sarah EHarris 12 W DavidHill 1
1 Centre for Cognitive Ageing and Cognitive Epidemiology, Department of Psychology, University of Edinburgh, 7 George Square, Edinburgh EH8 9JZ, United Kingdom
2Medical Genetics Section, Centre for Genomic & Experimental Medicine, MRC Institute of Genetics & Molecular Medicine, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU, United Kingdom
The associations between higher intelligence test scores from early life and later good health, fewer illnesses, and longer life are recent discoveries. Researchers are mapping the extent of these associations and trying to understanding them. Part of the intelligence-health association has genetic origins. Recent advances in molecular genetic technology and statistical analyses have revealed that: intelligence and many health outcomes are highly polygenic; and that modest but widespread genetic correlations exist between intelligence and health, illness and mortality. Causal accounts of intelligence-health associations are still poorly understood. The contribution of education and socio-economic status — both of which are partly genetic in origin — to the intelligence-health associations are being explored.
Until recently, an article on DNA-variant commonalities between intelligence and health would have been science fiction. Thirty years ago, we did not know that intelligence test scores were a predictor of mortality. Fifteen years ago, there were no genome-wide association studies. It was less than five years ago that the first molecular genetic correlations were performed between intelligence and health outcomes. These former blanks have been filled in; however, the fast progress and accumulation of findings in the field of genetic cognitive epidemiology have raised more questions. Individual differences in intelligence, as tested by psychometric tests, are quite stable from later childhood through adulthood to older age. The diverse cognitive test scores that are used to test mental capabilities form a multi-level hierarchy; about 40% or more of the overall variance is captured by a general cognitive factor with which all tests are correlated, and smaller amounts of variance are found in more specific cognitive domains (reasoning, memory, speed, verbal, and so forth). Twin, family and adoption studies indicated that there was moderate to high heritability of general cognitive ability in adulthood (from about 50–70%), with a lower heritability in childhood. It has long been known that intelligence is a predictor of educational attainments and occupational position and success
In addition to mortality, intelligence test scores are associated with lower risk of many morbidities, such as cardiovascular disease, cerebrovascular disease, hypertension, cancers such as lung cancer, stroke, and many others, as obtained by self-report and objective assessment. Higher intelligence in youth is associated at age 24 with fewer hospital admissions, lower general medical practitioner costs, lower hospital costs, and less use of medical services, and intelligence appeared to account for the associations between education and such health outcomes. Higher intelligence is related to a higher likelihood of engaging in healthier behaviours, such as not smoking, quitting smoking, not binge drinking, having a more normal body mass index and avoiding obesity, taking more exercise, and eating a healthier diet.
All this work launched a new field: cognitive epidemiology. When studying health, factor in intelligence. If you read any research about a health problem, like for example obesity, always ask yourself the question: how much of this problem is associated with intelligence? Do they have early childhood data on ability and health? Without that, there is probable confounding.
The associations which are found between health and intelligence could be due to a direct genetic pathway shared by intelligence and health, and/or by better, more educated and wealthy intelligence choices.
Genome-wide association studies transformed the field. Box 1 summarises all the different statistical methods. This is a very good guide to the field. The main one is GWAS, which finds regions of the genome which are correlated with the trait in question and statistically significant at a P-value of <5 × 10−8 to control for the multiple comparison being made.
Here are all the correlations between the genetic code and health.
Table 1 here
Another part of understanding the genetic contribution to intelligence health correlations concerns other predictors of health inequalities, and intelligence’s correlations with them. Intelligence is related to education and socio-economic status (SES), and those were known to be related to health inequalities before intelligence was known to have health associations. Although education and SES are principally thought of as social-environmental variables, both have been found to be partly heritable, by both twin-based and molecular genetic studies, both have high genetic correlations with intelligence, Mendelian Randomisation results show bidirectional genetic effects between intelligence and education, and both have genetic correlations with health outcomes
What does all this mean? It may mean that the underlying causes of health, happiness, morbidity and mortality are unequally distributed, and favour some people more than others. Evolution does not have to conform to our imaginings or our notions of fairness. If genetics is a significant contributor within a genetic group, it is plausible that it contributes to between group variance. Perhaps the Japanese live longer because they are Japanese. This remains to be proved, but is worth testing. If we ever achieve the noble ambition of creating healthy environments all over the inhabited world we may yet have a residuum of health differences due to purely genetic causes.
Meanwhile, you may be wondering what is the intelligent thing to do about your health. Don’t smoke, don’t get fat, and don’t read too many health warnings.