Results from the Framingham Heart Study showed a progressive decline in the incidence of dementia over three decades. This temporal trend and a parallel improvement in cardiovascular health over time were both observed only in the cohort of persons who had at least a high school diploma. Rising educational levels might have contributed to the 5-year delay we observed in the mean age at onset of clinical dementia. However, the proportion of participants who did not have a high school diploma was low during the last two epochs, thus limiting a deeper investigation of trends in the incidence of dementia in this subgroup.

Few studies can accurately track the incidence of dementia over time, and our study provides robust evidence that indicates a declining trend. Data from Rochester, Minnesota, showed a 30% decline in the age-adjusted incidence of dementia during the second decade they studied (1985–1994), but no significant trend was observed when the entire study period (1975–1994) was considered.7 Moreover, in that study, estimates of the incidence of dementia could have been affected by changes in clinical practice and diagnostic criteria for dementia over time, since ascertainment of events was obtained by linkage with medical records rather than with the use of standardized protocols. Although we also use data from medical records in the Framingham Heart Study, we supplement these data with direct assessments of the participants and their families. The Rotterdam Study suggested a 25% reduction in the incidence of dementia over a 10-year period through a comparison of the incidence rates in 1990 and 2000,6 but the results did not reach significance. Studies from the United States,7 England,8 and Stockholm9 have indirectly suggested declines in the incidence of dementia on the basis of repeated prevalence estimates drawn from survey data. In these studies, a true decline in the incidence of dementia cannot be distinguished from a faster increase in life expectancy among persons who do not have dementia than among those who have dementia.

In parallel with the trend toward a lower incidence of dementia, participants in the Framingham Heart Study also had improvements in most indicators of cardiovascular health, with the exception of a trend toward increasing prevalences of diabetes and obesity; this trend is consistent with national and global statistics.27 Although the age-adjusted prevalence of some vascular risk factors has decreased, the effect of specific vascular risk factors (e.g., elevated blood pressure) on the risk of dementia appears to have remained constant across epochs. However, we observed a decreasing effect of cardiovascular events and an increasing benefit of the use of antihypertensive medications on the subsequent risk of dementia during successive epochs. These findings suggest that earlier diagnosis and more effective treatment of stroke and heart disease might have contributed to a lower incidence of dementia, particularly vascular dementia, during more recent epochs. This benefit seems to be more pronounced among persons who have a high school diploma, a finding that is consistent with our observation that improvement in cardiovascular health was seen only among persons with at least a high school education.

Since vascular risk factors increase the risk of stroke and, in turn, a history of stroke increases the risk of cognitive decline and dementia, we examined the effect of adjustment for incident stroke on the risk of dementia; however, such an adjustment did not appreciably diminish the observed trends. Similarly, adjustment for the Framingham Stroke Risk Profile score and its components (including systolic blood pressure, use of antihypertensive medications, diabetes status, smoking status, atrial fibrillation, and clinical cardiovascular events) did not explain the decline, regardless of whether the data were obtained at baseline or at midlife. Our observations do not rule out a role for vascular risk factors in explaining the observed trends, although they emphasize the need to simultaneously search for additional explanations.

In addition to changes in vascular risk factors, temporal trends in the prevalence of neurodegenerative processes have also been documented. A recent study from Switzerland that evaluated 1599 specimens of brain tissue obtained from autopsies performed over the course of three decades (1972–2006) from persons who were 65 years of age or older at death suggests a decline in the age-adjusted burden of amyloid deposition.28 This intriguing trend might be caused by changes in education and vascular risk factors that are similar to those we observed in the Framingham Heart Study or could be due to factors we could not consider, such as changes in diet, physical activity, exposures to environmental toxins, or other unknown factors. The Framingham Heart Study does not have brain autopsy data from before 1995, and thus we are unable to explore the contribution of trends in amyloid burden to the observed risk of dementia across epochs.

A strength of the Framingham Heart Study is the long period of surveillance; dementia events have been tracked since 1975 and continue to be tracked to date. This permitted the assessment of temporal trends over three decades in a single cohort with carefully ascertained longitudinal data on various vascular risk factors. The Framingham Heart Study–based risk estimates, which are intermediate between the highest29 and lowest30 national estimates, are considered to be reliable and have been used by the Alzheimer’s Association to educate the public on lifetime risks.14,31 Although our diagnostic criteria for dementia have evolved over time, we were able to rereview all available records to retrospectively apply consistent diagnostic criteria to the entire 30-year period, thus reducing the risk of bias due to differences in diagnostic thresholds. Our tracking system for dementia is as consistent and accurate as possible in the setting of a longitudinal study, but we acknowledge that the awareness of dementia as a diagnostic entity has grown over the past 15 years. However, any resulting bias is likely to increase sensitivity for incident dementia in more recent epochs and should create a bias against finding a declining trend. Although the increase in educational level could have reduced the sensitivity of our multistep protocol for dementia screening, we performed complete neuropsychological assessments in a large number of participants who met the MMSE screening criteria and found no evidence that our MMSE cutoffs, which were adjusted for educational level, were insensitive among participants with high levels of education (see Tables S13 and S14 in the Supplementary Appendix).

One of the limitations of the Framingham Heart Study is that the participants are overwhelmingly of European ancestry; therefore, our findings would need to be replicated in groups that include a larger number of participants of other races and ethnic backgrounds. Furthermore, data were not available to examine the effects of some putative risk factors for dementia, such as diet and physical activity, as possible explanations for the observed temporal trends. Also, we were unable to consider the burden of subclinical vascular brain injury as a possible explanation for the observed trends, since the participants have undergone magnetic resonance imaging of the head only since 1999.

Despite our observation of a declining trend in the age-specific incidence of dementia and the possible stabilization of dementia occurrence in Western Europe,32 the worldwide burden of dementia will continue to increase rapidly as the average life expectancy increases. This is especially true for the most economically vulnerable persons, the most elderly persons in high-income countries,33 and persons in low-to-middle-income countries,34,35 where the average life expectancy and the burden of vascular risk factors are increasing most rapidly.

In conclusion, although projections suggest an exploding burden of dementia over the next four decades owing to an increasing number of older persons at risk,4,36 primary and secondary prevention might be key to diminishing the magnitude of this expected increase.37 Our study offers cautious hope that some cases of dementia might be preventable or at least delayed. However, it also emphasizes our incomplete understanding of the observed temporal trend and the need for further exploration of factors that contribute to this decline in order to better understand and possibly accelerate this beneficial trend.