"The significant interaction found between IQ and deprivation suggests that IQ in childhood is less important in terms of mortality for people who live in more affluent areas in adulthood than for people who live in deprived areas," says Carole L. Hart, Ph.D., of the University of Glasgow and colleagues from other universities in Scotland.
Their findings are published in the September issue of Psychosomatic Medicine.
The high-IQ individuals may have lived long lives because they learned better health behaviors early in life or because higher mental ability allowed them to better cope with adverse situations over the years, the researchers suggest.
The study examined results of the Scottish Mental Survey, an intelligence test held June 1, 1932, for 87,498 Scottish schoolchildren born in 1921.
The researchers then compared this data to records from a group of medical studies of adults in Scotland carried out in the 1960s and 1970s. These "Midspan" studies collected data about the subjects' home address, age and occupation, along with information about their cardiovascular and respiratory health.
Two of the Midspan studies enrolled more than 24,000 working-age participants -- including some born in 1921. Hart's team was able to match 938 of these subjects to their mental ability scores on the 1932 test. Based on home neighborhood and occupation, they then categorized the subjects according to deprivation and social class. Finally, they used health and hospital records to determine the causes and dates of death of this group over the next quarter-century.
Over those 25 years, 51 percent of the men and 38 percent of the women in the study died. In simple terms, there was a 17 percent greater chance of death for every 15 points of lower childhood IQ. After adjusting for deprivation and social class, this difference was reduced to 12 percent. These adjustments separated socioeconomic effects from IQ and explained some, but not all, of the differences associated with lower IQ.
"The analysis by IQ, divided into four groups, showed that it was the lowest group that had an increased relative rate of all-cause mortality," Hart says.
She adds that there is still much work to be done to explain the mechanisms by which childhood IQ is reflected in death rates decades later.
"It is possible that low childhood IQ leads to adult deprivation, which in turn leads to earlier death," she says. But it is also true that low birth weight or adverse circumstances in childhood lead to lower IQ at age 11 and also to poorer health outcomes. Hart's team will keep studying the combinations of data provided by others starting 70 years ago and continuing to the present.
Funding for the study was provided by the Chief Scientist's Office of the Scottish Executive.
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