With longer lifespans and an aging population, it wouldn’t be surprising to see rates of Alzheimer’s disease and dementia climbing higher too. But that’s not what’s happening. In fact, it appears that the risk of dementia is actually decreasing in the U.S.—and one of the biggest contributing factors may be an increasingly educated population.

From 2000 to 2010, the odds of dementia among a national sample of adults over age 65 dropped 14 to 23 percent.

According to Kenneth Langa, a professor of internal medicine and health management and policy at University of Michigan, Ann Arbor, education could explain about 15 percentage points of difference in dementia prevalence between 2000 and 2010. "So education—or what goes along with education—seems to be really important in driving these changes," Langa said at the annual meeting of the American Association for the Advancement of Science in Washington, D.C., where he recently presented his research.

Langa’s team began collecting data in 1992 from about 20,000 adults over age 50 through a longitudinal study known as the Health and Retirement Study, which collects information about income, work, assets, pension plans, health insurance, disability, physical health and functioning, cognitive functioning, and health care expenditures. (The study is sponsored by the National Institute on Aging.) The researchers compared populations in 2000 and 2010. Over that decade, the average age of their respondents remained 75 years old, but their level of education increased from 11.7 years to 12.6 years, and their net worth increased from $295,000 to $354,000. Rates of diabetes, heart disease, hypertension, and obesity—all risk factors for dementia—also increased during this period.

And yet the dementia rate went down, not up. Langa said that more aggressive control of cardiovascular risk factors like high blood pressure and diabetes may play a role.

And what about education? The researchers aren't sure how to explain the association they identified. “It's still not entirely clear what the link is between education and decreased dementia risk," Langa told mental_floss. "There are likely multiple different complicated pathways."

One leading hypothesis is that more education builds more “cognitive reserve,” Langa said. “Education, by challenging and ‘exercising’ the brain, leads to more connections between neurons. A person is better able to compensate for the pathologies that build up in the brain as we age and so can keep thinking well for a longer period of time.”

A caveat to the findings is that pinning down just how much real change in prevalence of dementia is occurring is tricky. The diagnostic threshold for dementia "is ‘fuzzy’ and can drift over time,” Langa said. Without a blood test or similar test for firm diagnosis, a classification of dementia is based on interviews, cognitive assessments, and clinical exams.

Another factor may be that “more awareness leads to more people going for assessment, and that can affect estimates across decades," he added. So too can response rates to surveys, diagnostic coding in Medicare data, and changes in nursing home populations, since these residents are typically less likely to be included in research. “The bottom line is, it’s hard to tell how much is real change,” Langa said.

Yet other research has discovered similar trends. Langa’s findings match results from a large study published this month in the New England Journal of Medicine that found dementia rates among more than 5205 adults 60 years and older dropping between 1975 and 2008—but only among those with a high school diploma or a higher education level. By measuring the incidence of dementia in five-year intervals, the study found that the rate of dementia decreased from 3.6 percent in the late 1970s and early 1980s to 2 percent by the late 2000s. "Among participants in the Framingham Heart Study, the incidence of dementia has declined over the course of three decades," the researchers write. "The factors contributing to this decline have not been completely identified."