Objective: To examine the role of potential mediating factors in explaining the IQ–mortality relation.
Design, setting and participants: A total of 4316 male former Vietnam-era US army personnel with IQ test results at entry into the service in late adolescence/early adulthood in the 1960/1970s (mean age at entry 20.4 years) participated in a telephone survey and medical examination in middle age (mean age 38.3 years) in 1985–6. They were then followed up for mortality experience for 15 years.
Main results: In age-adjusted analyses, higher IQ scores were associated with reduced rates of total mortality (hazard ratio (HR)per SD increase in IQ 0.71; 95% CI 0.63 to 0.81). This relation did not appear to be heavily confounded by early socioeconomic position or ethnicity. The impact of adjusting for some potentially mediating risk indices measured in middle age on the IQ–mortality relation (marital status, alcohol consumption, systolic and diastolic blood pressure, pulse rate, blood glucose, body mass index, psychiatric and somatic illness at medical examination) was negligible (<10% attenuation in risk). Controlling for others (cigarette smoking, lung function) had a modest impact (10–17%). Education (0.79; 0.69 to 0.92), occupational prestige (0.77; 0.68 to 0.88) and income (0.86; 0.75 to 0.98) yielded the greatest attenuation in the IQ–mortality gradient (21–52%); after their collective adjustment, the IQ–mortality link was effectively eliminated (0.92; 0.79 to 1.07).
Conclusions: In this cohort, socioeconomic position in middle age might lie on the pathway linking earlier IQ with later mortality risk but might also partly act as a surrogate for cognitive ability.
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Funding: Mortality surveillance of the cohort in the post-service VES was funded by the National Center for Environmental Health in Atlanta, USA. GDB is a Wellcome Trust Fellow. MJS is supported by the British Heart Foundation, LHM and MG by the National Institute of Public Health, Denmark, and JB and SHB by the National Heart Lung and Blood Institute and the National Institutes on Aging of the US National Institutes of Health (grant no RO1-HL54780). IJD is the recipient of a Royal Society–Wolfson Research Merit Award.
Competing interests: None.
Contributors: GDB and LHM generated the idea for these analyses, which was developed by IJD and MJS. LHM built the data set which was analysed by MJS. SHB and JB provided guidance on the data collected during the telephone and medical surveys. GDB wrote the first draft of this manuscript to which all authors subsequently contributed.
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