1 Department of Social Medicine, Institute of Public Health, University of Copenhagen, DK-2200 Copenhagen N, Denmark
2 Department of Mental Health, Medical School, University of Aberdeen, Aberdeen AB24 2ZD, Scotland
3 Geriatric Medicine Unit, Edinburgh University, Royal Victoria Hospital, Edinburgh EH4 2DN, Scotland
4 Department of Psychology, School of Arts and Humanities, University of Edinburgh, Edinburgh EH8 9JZ, Scotland
In a recent issue of the Journal, Karp et al. (1) reported an association between total years of formal education (typically completed by early adulthood) and dementia, a condition whose etiology is poorly understood. As the authors indicate, their finding of a lower risk in persons with higher educational credentials is well supported. That this relation held despite adjustment for adult occupational social class led Karp et al. to justifiably implicate early life education-related factors as risk indicators for dementia. However, they comment that "many studies, like ours, lack an independent measure of intelligence" (1, p. 181), indicating a scarcity of data on the early life intelligence-dementia relation. We are aware of two studies that were not discussed by Karp et al. that provide important information in this regard.
In a follow-up of American nuns into old age, scores of linguistic ability were derived from the density of information in selected passages from convent members handwritten autobiographies at around the age of 22 years when they were inducted into the order (2). Nuns with low linguistic scores were more likely not only to record low scores on tests of cognitive abilities in later life but also to develop Alzheimers disease, a subcategory of dementia. While the findings of the Nun Study are intriguing, the data are hampered by the use of an indirect measure of intelligence and a nonrepresentative sample. In the population-wide 1932 Scottish Mental Survey (3), almost everyone born in 1921 and attending school in Scotland in June 1932 was administered a valid test of psychometric intelligence (the Moray House Test) at age 11 years. The 1932 Scottish Mental Survey, therefore, covers the full range of intelligence scores. Recent linkage between these intelligence test scores and a national survey of early onset dementia showed no significant association. However, follow-up of a subset (4) of participants in the Aberdeen area showed that individuals with higher psychometric intelligence test scores in 1932 were at lower risk of late-onset dementia up until 1998 (4).
In addition to dementia, findings from follow-up studies of the 1932 Scottish Mental Survey suggest that lower psychometric intelligence in early life has a role in the development of a number of other adult health outcomes including total mortality (5), cardiovascular disease (6), some cancers (6), high blood pressure (7), and other psychiatric disorders (8). That these relations are apparent in adults who completed a brief psychometric intelligence test up to 65 years earlier (5) is particularly compelling. The association between most of these health outcomes and childhood cognitive ability tends to remain significant after adjustment for early life socioeconomic position, while further examination of the potential confounding/mediating effect of adult social circumstances is needed (9).
We agree with Karp et al. (1) that the most informative studies of dementia risk will contain measures of intelligence, as well as details of education and parental and own socioeconomic position. The causal directions among these intercorrelated variables are moot and require more research. However, there is more information concerning the relation between psychometric intelligence and dementia than they indicate.
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