Children with lower-than-average intelligence are more likely to die young — but why? European researchers have some clues.

Dr. Markus Jokela at the University of Helsinki, Finland and colleagues wanted to know why a low IQ in childhood is linked with an increased risk of dying in early adulthood. Could the reduced life expectancy be explained by developmental or social disadvantages?

In the late 1950s, researchers began to track more than 10,000 children, recording their weight and height, problem behaviors, father's occupational class, parental interest in their education, family size, and family difficulties. As the children grew to adulthood, the researchers also recorded information on their education, occupation, marital status, tobacco and alcohol use, and health issues.

In the latest issue of the journal Pediatrics, Jokela and his co-authors report that 192 subjects in the study died between the ages of 24 and 46 years.

The risk of dying by midlife was about twice as high in individuals with low IQ at age 11 as in those with high IQ (3.4 percent vs. 1.7 percent).

The increased risk was "only partly explained by the 'usual suspects' of family background, sociodemographic factors or health behaviors," Jokela told Reuters Health.

"For instance," he said, "parental socioeconomic status (had) only a minor role in explaining the IQ-mortality association. However, we did find that children with parents who were very interested in their child's education (as reported by the children's teachers) had a lower mortality risk...than children whose parents had only little interest in how their child was faring in school."

Also, childhood problem behavior, which is known to be associated later on with risky and antisocial behavior, accounted for part of the increased risk that came with a lower IQ.

"Previous studies have shown that children whose parents' have a strong interest in their child's activities and whereabouts are less likely to participate in risky behavior, such as delinquency," Jokela added. "Our finding shows that the effect of parental monitoring may even extend to child's mortality risk."

"We currently have an incomplete understanding of health inequalities originating from individual psychological characteristics, such as IQ," Jokela concluded. "Identifying these mechanisms could inform us how to plan more effective public health interventions accessible to wider audiences."