A study of 1.1 million people conducted by a team of health experts found out that a person with a high IQ has a lower chance of dying at any given time than those with a lower IQ score. Because the researchers were interested whether there was some sort of correlation between IQ and unintentional injury, they primarily tracked mortality in groups of former Swedish soldiers who had to take detailed IQ tests. While it might seem like a convenience sample at first glance, the soldiers were conscripts which meant that many of the socioeconomic factors one would need to consider for the study were accurately represented.
When combining IQ with various health indicators and causes of death, the experts found that there’s a tiny but persistent correlation between a higher IQ and a smaller chance of death in a car crash or from unintentional poisoning, suicide and disease. Even when demographics and income were accounted for, the correlation was still there. The explanation for this result rests on the idea that people with higher IQs are slightly more aware of dangers and take better care of themselves to prevent major health issues like hypertension and heart disease. If we consider that the difference in mortality is less than 6% at most, it’s not really that much better care.
There’s another issue that comes to mind when reading the study. Soldiers need to meet some sort of standard before they’re admitted into the military so the differences between those who scored highest on the IQ tests and those who scored lowest are very likely to be minimized and limit the randomness of the sample. So while the sample is better than just a convenience one, it’s still not as random as it should be. This limitation isn’t noted in the paper, but researchers do point out that the study was focused only on men and can’t be extended to women which is another important point. So far we only know this is a correlation for men aged 18 to early 40s.
Finally, by focusing on soldiers and their rate of mortality by the time they reach middle age, the study doesn’t consider environments in which an IQ score to put MENSA to shame wouldn’t be of any use. For example, soldiers in active war zones and people who live in dangerous places survive partly thanks to luck. Their intelligence won’t save them from a stray bullet or a bomb placed in just the right place at just the wrong time. Incorporating these possibilities into the study may negate the small advantage high IQ scorers seem to have. Chance plays a big part in whether a person survives a day and that’s something that needs to be considered before we pin survival solely to scores on an IQ test, no matter how elaborate.
But all that said, there is a good reason why someone would try to correlate IQ scores with the likelihood of death. Human intelligence was encouraged by natural selection exactly because a smarter creature is more aware of the risks around it and tries to mitigate them, surviving long enough to pass on its genes and with it, it’s wits. The intelligence we generally measure on paper, deals with a wide variety of talents and a complex body of academic knowledge. Simple smarts that help us keep an eye out for danger in our surroundings would need to be hereditary. The big question is how to reconcile academic IQ tests designed to measure abstract knowledge and talent with primal intelligence we see in almost every mammal.
See: Batty et al. (2008). IQ in Early Adulthood, Socioeconomic Position, and Unintentional Injury Mortality by Middle Age: A Cohort Study of More Than 1 Million Swedish Men American Journal of Epidemiology, 169 (5), 606-615 DOI: 10.1093/aje/kwn381