Cognitive Epidemiology: IQ And The Rate of Cancer
Cognitive Epidemiology: IQ And The Rate of Cancer
Abstract & Commentary
By William B. Ershler, MD, Editor, INOVA Fairfax Hospital Cancer Center, Fairfax, VA; Director, Institute for Advanced Studies in Aging, Washington, DC.
Synopsis: To address studies that indicate an inverse relationship between intelligence and mortality rates, Batty and colleagues capitalized on a large cohort of Swedish men who, upon entry into military service in Sweden were subjected to an assessment of intelligence quotient (IQ) by comprehensive examination of cognitive function.
Source: Batty GD, et al. IQ in early adulthood and later cancer risk: cohort study of one million Swedish men. Ann Oncology. 2007;18:21-28.
Nearly one million young Swedish military recruits were subjected to intelligence testing approximately 20 years ago. Within this cohort, there are now known to be approximately 10,000 incident cancers. In the current analysis, cancer development (all cancers, and each of 20 specific types) was examined in the context of the intelligence test scores. There were a few minor trends detected (some up, some down) with only one reaching a level of statistical confidence; a positive correlation of skin cancer and intelligence test result. However, the subjects in this cohort are currently only in their early forties, well below the peak incidence of cancer, and future studies might find more substantial associations where currently there are only trends.
Several studies have indicated an inverse relationship between intelligence and mortality rates, yet little is known about this association and specifically whether it relates to cancer deaths. To address this, Batty and colleagues capitalized on a large cohort of Swedish men who, upon entry into military service in Sweden were subjected to an assessment of intelligence quotient (IQ) by comprehensive examination of cognitive function. The data for 959,540 men born between the years 1952 and 1976 and tested between 1970 and 1994 (at age 18 or 19 years) were linked with cancer registry and/or mortality data to examine the association. Hazard ratios for the relation between IQ and cancer outcomes were computed using Cox regression.
During the average follow-up of 19.5 years there were 10,273 new cancer cases. Although IQ showed few associations with most of the 20 cancer types catalogued, there was a suggestion of a positive correlation with lung cancer and an inverse relationship with stomach, esophageal, and liver malignancies. The only robust finding was the relationship between IQ and skin cancer (hazard ration [HR] 1.18; 95% confidence interval [CI] 1.13-1.24, P < 0.01).
Commentary
Within the evolving literature in the field of cognitive epidemiology1 there have been repeated demonstrations of an inverse association of intelligence and all-cause mortality (e.g., 2). However, an explanation for this has not been conclusively forwarded. Furthermore, with regard to chronic diseases, such as cardiovascular disease or cancer, the impact of cognitive function has not been satisfactorily addressed. On the other hand, observations that low childhood IQ scores are associated with smoking3, obesity4 and heavy use of alcohol5, are factors that might predispose to increased cardiovascular disease or cancer. Furthermore, low scores have been associated with deprived socioeconomic status6 which might, over the long term, result in less adequate health maintenance and vulnerability to diseases that might otherwise be prevented.
The findings in this large analysis are intriguing, but modest. First the positive association, albeit weak, of IQ and lung cancer runs counter to at least one other report in which a negative correlation was found.7 Second, the robust positive association with skin cancer (both melanoma and non-melanoma cancers) would seem related to socioeconomic factors as has been previously reported. However, when the current data is adjusted for socioeconomic status, the relationship between IQ and skin cancer remains significant, albeit somewhat muted.
The fact that any relationships were discovered by this analysis is in itself remarkable, and likely most attributable to the very large data base available. The individuals under investigation (all men) were only in their late forties; two to three decades younger than the peak incidence years for most human cancers, particularly those that seem modifiable by host behavior. Thus, for the emerging field of cognitive epidemiology, a repeat analysis of this valuable cohort is warranted in perhaps twenty or thirty years. At least until then, it's probably advisable to continue doing those crossword puzzles!
References
1. Deary I, Batty GD. J Epidemiol Community Health. 2005;59:656-663.
2. Whalley LJ, et al. BMJ. 2001;322:819-823.
3. Taylor MD, et al. J Epidemiol Community Health. 2003;57:464-465.
4. Chandola T, et al. Int J Obesity. 2006;30:1422-1432.
5. Batty GD, et al. J Epidemiol Community Health, in press.
6. Neisser U, et al. Am Psychol. 1996;51:77-101.
7. Hart CL, et al. Psychosom Med. 2003;65:877-883.
To address studies that indicate an inverse relationship between intelligence and mortality rates, Batty and colleagues capitalized on a large cohort of Swedish men who, upon entry into military service in Sweden were subjected to an assessment of intelligence quotient (IQ) by comprehensive examination of cognitive function.Subscribe Now for Access
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