Today's lower risk may be due, at least in part, to the fact that older Americans have spent more years in school.
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Finally, there is some heartening news about dementia: The percentage of American seniors who develop this debilitating condition is falling, according to new research.

The hopeful finding may be due—at least in part—to more people staying in school to get a high school or college diploma. In the study, which compared data from 2000 and 2012, senior citizens in the later group were more likely to have graduated from high school than those in the earlier one. Overall, people with the most years of schooling had the lowest chances of getting dementia.

The study is in line with previous research that suggests that education can help protect the brain from a mental decline in old age.

Other studies, in the United States and elsewhere, have also seen a downward trend in dementia rates overall. That’s especially good news considering that Baby Boomers, the largest generation in American history, are now reaching the age at which memory problems often begin.

The new research, published in JAMA Internal Medicine, found that 11.6% of seniors interviewed in 2000 met the criteria for dementia, while only 8.8% did in 2012. That’s about a 24% difference.

Each of the study samples included more than 10,500 adults, all 65 and older. Between the two time periods, the participants’ average length of education increased from 11.8 to 12.7 years (say, high school, plus some time in college).

While the study authors can’t say for sure, they suspect that schooling has a positive effect on brain function later in life—either through a direct effect on brain development, or because higher education may lead to healthier behaviors and better opportunities down the road.

Another theory: Cardiovascular health may have played a role dementia rates over time, the researchers say. While diabetes, high blood pressure, and high cholesterol—all conditions that can interfere with blood flow to the brain—are on the rise (although heart disease is declining), they are also being treated more aggressively. In other words, more people are successfully managing their cardiovascular risk factors, which may lower their risk for dementia.

These two theories are likely only pieces of the puzzle, however. The full set of social, behavioral, and medical factors that have led to this decline is still unknown, the authors wrote. And just because the percentage of seniors with dementia has gone down, that doesn’t mean the total number of people affected is less. "The number of older adults is growing so rapidly that the overall burden of dementia is still going up," said senior author David R. Weir, PhD, a research affiliate at the University of Michigan’s Population Studies Center, in a press release.

But the findings may help economic forecasters adjust their predictions for the total impact of Alzheimer's disease and other conditions. According to lead author Kenneth Langa, MD, professor of internal medicine at the University of Michigan, the results “add to a growing body of evidence that this decline in dementia risk is a real phenomenon, and that the expected future growth in the burden of dementia may not be as extensive as once thought."

Of course, the overall economic impact won’t make much of a difference to the millions of patients and caregivers who will still be faced with the reality of dementia. And today’s smaller families (compared with those of previous generations) will face new and unique challenges, the authors note, with fewer young and healthy relatives to provide long-term care and financial assistance.

“This is still going to be a top priority issue for families, and for health policy, now and in the coming decades,” Dr. Langa said.