Estimates of the prevalence of dementia vary considerably by the age group on which the estimates are based. Prevalence among those age 85 and above, for example, is likely to be considerably higher than estimates based on those age 65 and above. In addition, prevalence data are often categorized more broadly or more narrowly than "dementia." The Centers for Disease Control and Prevention, for example, cites prevalence data for specific causes of dementia, typically Alzheimer's disease, while the National Institutes of Health (NIH) subsumes dementia under the category of Serious Mental Illness. Data on the prevalence of Alzheimer's indicate increasing prevalence. Starting at age 65, the risk of developing the disease doubles every 5 years. By age 85 years and older, between 25% and 50% of people will exhibit signs of Alzheimer's disease. Up to 5.3 million Americans currently have Alzheimer's disease (Hebert, Scherr, Bienias, Bennett, & Evans, 2003). By 2050, the number is expected to more than double due to the aging of the population. Alzheimer's disease is the sixth leading cause of death in the United States and is the fifth leading cause among persons age 65 and older (Heron et al., 2009).

A recent meta-analysis (Prince et al., 2013) found global prevalence of dementia from all causes to be between 5% and 7% of adults age 60+. Two recent studies of dementia prevalence have shown some indication that prevalence may be declining. In one (Matthews et al., 2013), prevalence surveys of adults age 65+ were conducted almost 2 decades apart (1989 and 2008). After controlling for differences in the patient populations, the researchers found that the 2008 cohort had significantly lower prevalence of dementia. The second study (Christensen et al., 2013) took a slightly different approach. Those researchers assessed two cohorts of patients. One cohort was born in 1905 and was assessed in 1998 at age 93. The second cohort was born 10 years later, in 1915, and was assessed in 2010 at age 95. The 1915 cohort was found to have significantly lower prevalence of dementia. Both research teams concluded that the likely explanation was improved primary prevention of causes such as stroke.

Neither of the research teams found changes in the prevalence of Alzheimer's specifically, and it should also be noted that one study was conducted in the United Kingdom and the other in Denmark, so it cannot necessarily be concluded that primary prevention efforts in the United States have been similarly successful.

A limited number of studies have examined the prevalence of dementia among racial and ethnic groups. Differences in sampling methods and definitions of dementia, as well as difficulties controlling for variables such as level of education and bias in assessment batteries, limit the generalizability of results. However, results from individual studies suggest that the incidence and prevalence of dementia varies across racial and ethnic groups (Manly & Mayeux, 2004).

The Aging, Demographics, and Memory Study (ADAMS) used a nationally representative Health and Retirement Survey to estimate the prevalence of Alzheimer's disease (AD) and other dementias in the United States. Results were analyzed to determine the overall prevalence of dementia, as well as the relationship between dementia and variables such as education, gender, race (African American or Caucasian), and Apolipoprotein E (APOE) genotype. Controlling for education, gender, and APOE genotype, researchers found that African Americans were at a greater risk for dementia than Caucasians, although this difference was not statistically significant (Plassman et al., 2007).

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