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Does Executive Function Explain the IQ-Mortality Association? Evidence from the Canadian Study on Health and Aging

From the Departments of Kinesiology and Psychology (P.A.H.), University of Waterloo, Waterloo, Ontario, Canada; Department of Statistics and Actuarial Science (J.D.), University of Waterloo, Waterloo, Ontario, Canada; Department of Psychology (M.C.), University of Saskatchewan, Regina, Saskatchewan, Canada; Department of Clinical Health Psychology (M.E.H.), University of Manitoba, Winnipeg, Manitoba, Canada; and the Department of Psychiatry (C.D.), University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

Address correspondence and reprint requests to Peter A. Hall, Department of Kinesiology, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. E-mail: pahall{at}healthy.uwaterloo.ca

Objective: To assess the robustness of the association between intelligence quotient (IQ) and mortality in older adults and to examine whether or not the association can be explained by more specific cognitive processes, including individual differences in executive functioning.

Methods: We examined the associations among Full Scale IQ, individual IQ subtest scores, and 10-year mortality among older community-dwelling, adult participants in the Canadian Study of Health and Aging, who were verified as disease and cognitive-impairment free at baseline via comprehensive medical and neurological evaluation (n = 516). Survival analysis including Cox proportional hazards regression models were used to examine mortality risk as a function of Full Scale IQ and its specific subcomponents.

Results: An inverse association was found between IQ and mortality, but this did not survive adjustment for demographics and education. The association between IQ and mortality seemed to be predominantly accounted for by performance on one specific IQ subtest that taps executive processes (i.e., Digit Symbol (DS)). Performance on this subtest uniquely and robustly predicted mortality in both unadjusted and adjusted models, such that a 1-standard deviation difference in performance was associated with a 28% change in risk of mortality over the 10-year follow-up interval in adjusted models.

Conclusions: The association between IQ and mortality in older adults may be predominantly attributable to individual differences in DS performance.

Key Words: IQ • mortality • executive function

Abbreviations: CSHA = Canadian Study on Health and Aging; DS = Digit Symbol; IQ = intelligence quotient; RT = reaction time.