Background: An association between childhood cognitive ability measured by IQ tests and mortality has been reported recently. It is not clear from those studies to what extent the increased relative risk associated with lower IQ scores may be attenuated by adjustment for other risk factors. This study aims to investigate the association between cognitive ability measured at age 18-20 years and mortality among middle-aged men adjusting for risk factors for mortality over the life course.
Methods: Data on cognitive ability, and other risk factors for premature mortality (indicators of mental health and social adjustment and behavioural factors), were collected among 49 321 men, born in 1949-51, at conscription for compulsory military training in 1969-70. Information on socioeconomic factors in childhood and adulthood, as well as information on mortality, was collected through national registers.
Results: Cognitive ability showed an inverse and graded association with mortality between 40 and 53 years of age (1297 cases, crude hazard ratio (HR) 1.15, 95% CI 1.12 to 1.18, for one-point decrease on the nine-point IQ scale). Adjustment for indicators of social misbehaviour, mental health problems and behavioural risk factors, measured in late adolescence, and adult social circumstances strongly attenuated the increased risks of mortality, and it was no longer significantly increased (adjusted HR 1.02, 95% CI 0.99 to 1.06, for one-point decrease on the nine-point IQ scale).
Conclusion: The association between IQ and mortality among men below 54 years of age was almost completely attenuated by adjustment for risk factors captured by our measures of achieved social positions.