Ian Deary, University of Edinburgh, and Geoff Der, MRC Social and Public Health Sciences Unit, Glasgow, report on a study from the MRC Unit that measured both the IQs and the reaction times of middle-aged subjects. Both tests of mental ability were associated with life span, but reaction time was the stronger indicator.
These findings, presented in the study "Reaction Time Explains IQ's Association with Death," will appear in the January 2005 issue of Psychological Science, a journal of the American Psychological Society.
The new research builds on earlier studies showing that people with lower IQs tend to die at younger ages than those with higher IQs. Deary and Der, however, wanted to use a more fundamental measure of mental ability - which they define as efficiency in processing information. They thought IQ tests might relate to physical health because people with higher IQs typically are more likely to be in occupations with safer environments. Reaction time is moderately related to IQ, but is a simpler assessment of the brain's information-processing ability - one that doesn't bear so much on other, possibly confounding factors like knowledge, education, or background.
To test their theory they examined data from the MRC Unit that, back in 1988, had 412 male and 486 female 54- to 58-year-olds living in west Scotland. The participants took both an IQ test measuring their verbal and numeric cognitive abilities and a reaction-time test that measured how quickly they pressed a button after seeing a number on a screen. The researchers also recorded the participants' gender, employment, education, and smoking status. Over the next 14 years, 185 participants died, and Deary and Der compared their test results to see if the IQ or reaction-time responses predicted their mortality. The researchers learned that those with higher IQ scores lived longer, a result consistent with other studies. The study also showed that characteristics significantly related to death included male gender and smoking. But Deary and Der also found something new - faster reaction times seemed an even better predictor of long life than IQ.
There are different ways the results could be interpreted. Slow reaction times could reflect a degeneration of the brain, which in turn could reflect degenerating physical health (an obvious possible cause of earlier mortality). But in another study the IQs of 11-year-old subjects also were found to predict life span length, just as accurately as it did for the middle-aged participants in Deary and Der's 14-year study.
Future studies of reaction times in younger-aged people may shed more light on the IQ-mortality connection.
Professor Deary said, "It is only in the last few years that we have come to realize that IQ-type scores are related to mortality, even when the mental tests were taken decades before death. Now, several research teams have replicated this finding. What we need to do now is understand it. We and others are following up several possible explanations for this intriguing new association between intelligence and survival."
Psychological Science is ranked among the top 10 general psychology journals for impact by the Institute for Scientific Information. The American Psychological Society represents psychologists advocating science-based research in the public's interest.