The lower a young man's IQ, the higher his risk of dying by middle-age, a large study of Swedish men suggests.
Researchers found that among nearly 1 million men followed from the age of 18 onward, the risk of dying from any cause by middle-age gradually increased as the men's IQs dipped.
Those in the group with lowest average IQ at age 18 were more than twice as likely to have died by middle-age than their counterparts with the highest IQ scores — even with a range of other factors considered. Those included the men's family income during childhood, and weight and general health at age 18.
The study, published in the medical journal Epidemiology, does not reveal the reasons for the IQ-mortality link. But the findings add to evidence from past studies tying childhood IQ to longevity.
One potential reason for the link is that a higher IQ typically leads to more education and a better job, which gives people more resources for protecting their health, according to the researchers on the current study, led by Dr. Finn Rasmussen of the Karolinska Institute in Stockholm.
Supporting that idea, the researchers found that when they factored education levels into their analysis, the link between IQ and earlier death diminished somewhat.
In addition, the researchers say, people with higher IQs may respond to health messages differently and be more likely to exercise or avoid smoking, for example. They may also tend to have better mental health, which in turn protects their physical health.
The findings are based on data collected from 994,262 Swedish men at the time of their military conscription service, when they were 18 years old, on average. They were then followed for up to 32 years, during which time nearly 14,500 died.
Overall, the risk of early death grew as the men's young adulthood IQ declined. More specifically, the researchers found, the risks of death from accidents, heart disease and suicide all correlated with IQ.
The implications of the findings are not completely clear, according to Rasmussen and his colleagues. However, they point out, there is some evidence that IQ can be boosted during childhood, suggesting one potential way to improve people's long-term health.