Singh-Manoux, ArchanaKivimaki, MikaGlymour, MariaElbaz, AlexisBerr, ClaudineEbmeier, Klaus PFerrie, Jane EDugravot, Aline2012-04-192012Singh-Manoux, Archana, Mika Kivimaki, M Maria Glymour, Alexis Elbaz, Claudine Berr, Klaus P Ebmeier, Jane E Ferrie, and Aline Dugravot. 2012. Timing of onset of cognitive decline: results from Whitehall II prospective cohort study. British Medical Journal 344: d7622.0959-8138http://nrs.harvard.edu/urn-3:HUL.InstRepos:8603140Objectives: To estimate 10 year decline in cognitive function from longitudinal data in a middle aged cohort and to examine whether age cohorts can be compared with cross sectional data to infer the effect of age on cognitive decline. Design: Prospective cohort study. At study inception in 1985-8, there were 10 308 participants, representing a recruitment rate of 73%. Setting: Civil service departments in London, United Kingdom. Participants: 5198 men and 2192 women, aged 45-70 at the beginning of cognitive testing in 1997-9. Main outcome measure: Tests of memory, reasoning, vocabulary, and phonemic and semantic fluency, assessed three times over 10 years. Results: All cognitive scores, except vocabulary, declined in all five age categories (age 45-49, 50-54, 55-59, 60-64, and 65-70 at baseline), with evidence of faster decline in older people. In men, the 10 year decline, shown as change/range of test×100, in reasoning was −3.6% (95% confidence interval −4.1% to −3.0%) in those aged 45-49 at baseline and −9.6% (−10.6% to −8.6%) in those aged 65-70. In women, the corresponding decline was −3.6% (−4.6% to −2.7%) and −7.4% (−9.1% to −5.7%). Comparisons of longitudinal and cross sectional effects of age suggest that the latter overestimate decline in women because of cohort differences in education. For example, in women aged 45-49 the longitudinal analysis showed reasoning to have declined by −3.6% (−4.5% to −2.8%) but the cross sectional effects suggested a decline of −11.4% (−14.0% to −8.9%). Conclusions: Cognitive decline is already evident in middle age (age 45-49).en-USJournal Article2012-04-1910.1136/bmj.d7622