People are living longer than ever, says a report from the World Health Organization (WHO), but smart people live longer yet. People who are, shall we say, less smart die younger than more intelligent folks, and various studies around the world are attempting to find out why.

IQ affects how long you manage to stick around in this life, with a 15% increase in IQ giving a 21% better chance of not dying. These numbers come from a cohort study by researchers Lawrence Whalley and Ian Deary, using the Scottish Mental Surveys, a historic survey in which almost all 11-year-olds in Scotland got the same IQ test on the same day in 1932. The new study found out which of these subjects were still alive, and at which age others had died.

Scientific American cites one example, where “a person with an IQ of 115 was 21% more likely to be alive at age 76 than a person with an IQ of 100 (the average for the general population).”

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Even when controlled for other factors (economic status, employment, and so on), those with a higher IQ still come out on top. Research in other countries suggests a link between IQ and mortality, and studies using identical and non-identical twins show that the link is genetic.

There are two main theories in play. One says that IQ might be linked to general body durability–that is, those with a sturdy constitution might naturally enjoy a higher IQ. That would mean that being smart is more a symptom or a signifier of longevity rather than its cause.

Another, much more amusing theory says that smart people are better at avoiding things that kill them. This would mean that those with a higher IQ take fewer unnecessary risks, and that they don’t do stupid things like smoking, or eating a bad diet. Scientific American again: “Consistent with this hypothesis, in the Scottish data, there was no relationship between IQ and smoking behavior in the 1930s and 1940s, when the health risks of smoking were unknown, but after that, people with higher IQs were more likely to quit smoking.”

One problem with these theories is that IQ tests may not be a good indicator of intelligence. They may just indicate how good you are at IQ tests, with cultural and other factors playing a part (some people may be preconditioned to believe they’ll do badly, affecting their results) in the outcome. It’s difficult, too, to separate IQ from education and social class, both of which have potentially enormous effects on health and therefore longevity.