In the linked study (doi:10.1136/bmj.d7622), Singh-Manoux and colleagues present new analyses that track changes in cognitive function in several cohorts of men and women aged 45-70 years within the Whitehall II study.1 The study has several strengths including repeated measures of cognitive function over a decade of follow-up across multiple cognitive domains. Using these data, the authors convincingly show that decline can be detected in almost all the cognitive domains tested during follow-up, even in the youngest participants—those aged 45-49 years.

To understand the importance of the findings it is necessary to appreciate the link between cognitive function and the clinical syndrome of dementia. Although cognitive function is not itself a defined clinical entity, and it can be difficult to understand the meaning of small changes in cognition over time, previous data show that modest differences in cognitive performance in earlier life predict larger differences in risk of dementia …