JAMA: Dementia among Americans of the same age has become about two thirds less common over 40 years
JAMA: Dementia among Americans of the same age has become about two thirds less common over 40 years
The JAMA authors combined data from three nationally representative US surveys and found a long-term decline in the age-specific risk of dementia. The total number of people with dementia may still rise because the population is growing rapidly at the oldest ages.
The phrase “dementia wave” usually suggests a single projection: more older people will mean more people with dementia. In a JAMA article published on May 13, 2025, P. J. Eric Stallard, Svetlana Ukraintseva, and Murali Doraiswamy propose separating this projection into two components.
The first is the number of people who survive to age 80, 85, and 95. The number of people aged 80+ in the United States is expected to double by 2050. The second is the proportion of people at each age who are already living with dementia. US surveys indicate that this age-specific prevalence has declined. The authors estimate a reduction of about 67% over 1984–2024, equivalent to 2.5–3% per year.
This estimate draws on several sources. Part of the time series was measured directly, while the authors extrapolated the estimate for 2024 by extending the previous rate of decline. In NHATS, a nationally representative survey of older Americans, the proportion of people aged 72 and older with probable dementia fell from 11.9% in 2011 to 9.2% in 2019 and 8.2% in 2021. The decline was especially marked among participants aged 80–89.
Prevalence measures how many people already have a condition. Incidence measures how often the condition is diagnosed for the first time. These measures can follow different patterns, for example because of mortality after diagnosis. In this case, both are moving in the same direction. An analysis of seven cohorts from the United States and Europe, which included 49 202 participants, found that dementia incidence declined by 13% per calendar decade during 1988–2015.
The reasons for this trend can currently be described only as a set of hypotheses. Using Framingham data, a systematic analysis of 27 studies linked the decline in incidence to reductions in the proportions of smokers and people with low educational attainment. The JAMA authors also list better blood pressure treatment, statin therapy, and other changes in life before age 65 as possible explanations. Their combined analysis does not establish how much each factor contributed.
The available evidence comes mainly from the United States and Europe. Some studies in Japan found an increase, and trends differed among groups within US samples. Obesity, diabetes, and hypertension also became more common over the same decades and may alter the trajectory for subsequent generations.
Projections of the number of people with dementia combine demographic change with age-specific risk. Both quantities therefore need to be tracked: how many people survive into old age, and what happens to their brains during the decades before they reach it.