Background: High premorbid IQ test scores are related to a reduced rate of later total mortality, although little is known about the shape of this association (ie, dose-response versus threshold), or the role of mediating and confounding factors in explaining it. Additionally, the link between IQ and cause-specific mortality has been little explored.
Methods: A cohort of over 1 million Swedish men who underwent IQ testing at military service conscription at about 18 years of age was followed for mortality experience until middle age.
Results: An average of 20 years of follow-up gave rise to 14,498 deaths in an analytical sample of 994,262. In basic analyses adjusting for age, year of birth, and conscription testing center, lower IQ scores were associated with an elevated risk of all-cause mortality (HRper 1-SD decrease in IQ; 1.32; 95% confidence interval = 1.30–1.34). This relation was incremental across the full IQ range, and was robust to adjustment for indicators of childhood social circumstances. The association did not appear to be mediated by factors measured concurrent with IQ (blood pressure, body mass index, or cigarette smoking), nor was it attributable to reverse causality. However, controlling for education (a close correlate of IQ) led to marked attenuation. IQ was also associated with mortality from accidents, coronary heart disease, and suicides, but not cancer.
Conclusions: In this large cohort we found a robust stepwise relation between early adult IQ and risk of total and accident mortality in men.
From the aMRC Social & Public Health Sciences Unit, University of Glasgow, Glasgow, UK; bCentre for Cognitive Ageing and Cognitive Epidemiology, University of Edinburgh, Edinburgh, UK; cDepartment of Public Health Sciences, Karolinska Institute, Stockholm, Sweden; and dDepartment of Social Medicine, University of Bristol, Bristol, UK.
Submitted 3 April 2007; accepted 22 April 2008.
The Medical Research Council (MRC) Social and Public Health Sciences Unit receives funding from the UK MRC and the Chief Scientist Office at the Scottish Government Health Directorates. The Centre for Cognitive Ageing and Cognitive Epidemiology is supported by the UK Biotechnology and Biological Sciences Research Council, the Engineering and Physical Sciences Research Council, the Economic and Social Research Council, the MRC, and the University of Edinburgh as part of the cross-council Lifelong Health and Wellbeing initiative. David Batty is a UK Wellcome Trust Fellow (WBS U.1300.00.006.00012.01).
Correspondence: F. Rasmussen, Child and Adolescent Public Health Epidemiology Group, Department of Public Health Sciences, Karolinska Institute, SE-17176 Stockholm, Sweden. E-mail: firstname.lastname@example.org.