This is probably due to the fact that IQ is a signal of good overall health, and that different occupational and social worlds are inhabited by people depending on their cognitive ability.
Int J Epidemiol. 2006 Jan 30; [Epub ahead of print]
The association between cognitive ability measured at ages 18-20 and mortality during 30 years of follow-up--a prospective observational study among Swedish males born 1949-51.
Hemmingsson T, Melin B, Allebeck P, Lundberg I.
OBJECTIVES: An association between childhood cognitive ability measured with IQ tests and mortality has been reported recently. It is not clear from those studies if the relative risk is increased only among those in the lower end of the IQ score scale or if there is graded increase in mortality from the lowest to the highest. This study aims to investigate the association between cognitive ability measured at age 18-20 and mortality during a 30 year period of follow-up. METHODS: Data on cognitive ability was collected from 49 323 men, born in 1949-51, who were conscripted for compulsory military training in 1969/70. Data on mortality were obtained from the Causes of Death register 1971-2000. RESULTS: Cognitive ability was a strong predictor of all-cause mortality, cardiovascular disease (CVD)-mortality, mortality from violent causes, and alcohol-related mortality. A striking finding was a pronounced gradient in mortality risk across all IQ score groups. Adjustment for adult socioeconomic position attenuated the increased risk somewhat [for all-cause mortality: crude hazard ratio (HR) 1.16 (1.13-1.19), adjusted HR 1.12 (1.09-1.15)]. CONCLUSION: IQ test score measured in late adolescence (only males) was a significant predictor of all-cause, as well as cause-specific (CVD and injuries), mortality during 30 years of follow-up. The risk increased from high to low IQ test score results for all outcomes.