Time, Sex, Gender, History, and Dementia.

A growing body of epidemiologic evidence indicates a decline in the incidence or prevalence of dementia in high income countries in the past 25 years. In this commentary, I first suggest that the decline in the incidence or prevalence of dementia is not explained completely by the factors considered so far, and that a broader historical perspective may be needed. Second, I suggest that the overall declining trend may conceal trends in opposite directions for the two major subtypes of dementia, the neurovascular and the neurodegenerative type. Third, I suggest some areas of future research to further elucidate the trends. The future of dementia remains somewhat unclear. Even if the incidence continues to decline, the prevalence may remain the same or increase if survival of persons affected by dementia increases. In addition, even if the prevalence declines, the total number of persons affected by dementia may remain the same or increase if the size of the elderly population expands. Finally, we cannot be sure that the decline in incidence will continue in the coming decades. With cautious optimism, we may conclude that the burden of dementia may be modified over time by human practices, including public health and medicine.