The results of a life-long study suggest that children who score higher on IQ tests may live longer.
The researchers, from the University of Edinburgh in Scotland, U.K., report their findings in a paper published in the BMJ.
The report describes how higher scores on childhood IQ tests were linked to a lower lifetime risk of dying from known major causes, including heart disease, stroke, smoking-related cancer, digestive disease, external causes of death, respiratory diseases, and dementia.
For these diseases, the reductions in risk were largely similar for men and women. But the researchers also found that, in men only, a higher childhood IQ was linked to a lower risk of suicide.
The researchers say that the findings suggest that lifestyle - and smoking in particular - is an important factor in the link between childhood IQ and risk of death.
The study participants were 33,536 men and 32,229 women born in Scotland in 1936, all of whom completed a validated childhood intelligence test at 11 years old as part of the Scottish Mental Survey of 1947, and who could be linked to cause of death records up to the end of 2015.
Largest study to follow life course
The intelligence test, which was administered at school by the children's teachers, included 71 items that measured verbal and nonverbal reasoning. Studies carried out since 1947 have validated the test and found it to be on a par with other standardized measures of intelligence.
The Scottish study is thought to be the largest so far to have followed a group of men and women over the life course and related causes of death to childhood intelligence.
Previous studies have already suggested that people who score higher on intelligence tests tend to live longer, on average, than people with lower scores. However, most of these do not span the life course, or they only focus on particular groups.
For example, the largest study of this kind to date included a million participants, was confined to Swedish male conscripts, and only followed them until middle age.
Size of risk varied by cause of death
As well as finding a link between higher childhood IQ and lower risk of dying before the age of 80, the researchers found that the amount of reduced risk varies by cause of death.
For instance, their analysis reveals that a higher score on childhood IQ tests is linked to a 24 percent lower risk of dying from stroke, a 25 percent lower risk of dying from coronary heart disease, and a 28 percent lower risk of dying from respiratory disease.
The team also found links between higher childhood IQ and lower risk of death from injury, dementia, digestive diseases, and smoking-related cancers such as lung and stomach cancer.
However, they found no evidence of a link between childhood IQ and death from cancers that are not related to smoking.
When they adjusted the results to take into account smoking and socioeconomic status, the researchers found that some of the links remained strong, suggesting that these factors do not fully explain the differences.
They suggest that future studies should now investigate the "cumulative load of such risk factors over the life course."
In a linked editorial, Swedish researchers specializing in public health and population studies note that the Scottish study is "uniquely comprehensive" because it examines major causes of death and follows the participants to an age by which nearly half of them have died.
They draw attention to a section of the study report that looks at how increments in childhood intelligence relate to specific causes of death. They liken it to a drug trial that looks at the effect of different doses.
"The most obvious dose-response relations are those for cardiovascular disease, coronary heart disease, stroke, smoking-related cancer, and respiratory disease," they note.
Although "injuries could also be added to this list," they remark that, nevertheless, the study "tells us that lifestyle, and especially tobacco smoking, must be an important component in the effect of intelligence on differences in mortality." They conclude:
"It remains to be seen if this is the full story or if IQ signals something deeper, and possibly genetic, in its relation to longevity."