A number of recent studies have been finding that people who score lower on intelligence tests (notice how careful I am not to say “smarter people”) tend to die earlier than those who score higher. The effect doesn’t seem to arise from socioeconomic factors (well-off people score higher on IQ tests and also tend to be healthier), leaving scientists to reach for hypotheses. Maybe high-IQ people smoke less? eat healthier? follow doctors’ advice more?

Now a new study, reported in the September issue of the journal Psychological Science, finds an even more intriguing connection. The first to examine whether IQ and dependability in childhood together are associated with risk of death by age 67, it comes to a stark conclusion: “brighter, more dependable children live longer,” write Ian J.Deary of the University of Edinburgh and colleagues.

The scientists studied 1,181 Scots who were born in 1936. Almost everyone born in Scotland in that year took the “Scottish Mental Survey of 1947” when they were 11, were assessed by teachers for dependability at age 14, and then were tracked from 1968 through early 2003 (when there were 193 deaths). These are the data that Deary combed.

The results should strike dread in the hearts of the intellectually challenged and non-conscientious. People who were smarter than 67% of the population when tested as children had a 30% lower chance of dying from 1968 to 2003; people who are smarter than 95% of the population have a 40% lower chance of dying. People who were more dependable than 67% of the population when tested as children had a 22% lower risk of dying by age 67, and the truly conscientious—more so than 95% of their peers—had a 46% lower mortality risk. Children in the bottom half of the distributions for intelligence and dependability were more than twice as likely to die by age 67 than those in the top half for both.

For IQ, there was a “dose-response” relationship for the top 75%: the smarter they had been as kids, the lower their risk of dying starting at age 32 and through age 67. (The lowest 50% had basically the same mortality risk.) For dependability, there was also a dose-response effect, with lower mortality the more dependable the participants had been at age 14.

These numbers are in the same ballpark as established risk factors such as cholesterol, blood pressure, and smoking in middle age. How can your smarts at age 11 and your dependability at age 14 have such a powerful effect on something as fundamental as when you die?

Neither education levels, socioeconomic status or other factors known to influence mortality risk explained it, the scientists find. Other studies have found that dying before age 65 is linked to childhood intelligence and that early intelligence predicts later cardiovascular disease and death, but again it’s not clear why. And there is no obvious way that being smart and diligent should reduce the risk of dying of cancer, except maybe by keeping you from smoking and getting you to be screened for the disease. Ideas, anyone?