Dementia is on the decline, with the risk of developing it dropping 20 percent per decade since the late 1970s, according to a striking new study published Wednesday in the New England Journal of Medicine.

It's a finding that may be hard to believe, with a tsunami of new dementia cases looming in the near future. By 2025, Alzheimer's disease alone will rob 7.1 million people older than age 65 of their memories, their ability to function, their very personalities — a 40 percent increase from today. The Alzheimer's Association has predicted that, by 2050, the disease will cost $1.1 trillion.

But both facts appear to be true. The aging of the baby boomers means that dementia cases will surge to an all-time high — the sheer number of older people who are living longer means that even a drop in the incidence of the disease won't solve that problem, which will exact a huge health and financial toll. But despite the popular perception that getting old invariably means people go gray and begin to lose their memory, the new data strongly suggest that, over the past few decades, the risk of developing dementia has receded for people with at least a high school education, raising hope that it may be possible to prevent one of the scariest risks of aging.

"Can we, a couple of decades down the road, bend the arc? ... Stroke used to be the second leading cause of death, and now it's the fifth. Maybe we can do this for dementia, too," said Sudha Seshadri, who is a professor of neurology at Boston University School of Medicine and who led the study.

What the study can't answer is the obvious next question: Why? Some of the improvement is linked to better control of blood pressure and cardiovascular health and a decrease in the risk of developing dementia after a stroke, but not all. The drop was led by a decrease in dementia caused by blood-flow problems; the decline in Alzheimer's disease was not statistically significant, meaning it could be due to chance. Seshadri plans to dig deeper into the data to try and discern what environmental factors or lifestyle changes could explain the changes.

"What I would say is we need to take that message to redouble our efforts, not to become complacent. We are doing something right," Seshadri said. "So if we understand what we are doing right, we can perhaps activate it."

Seshadri and her colleagues examined data from the long-running Framingham Heart Study, which has a storied reputation in medical history. Scientists began collecting data from more than 5,000 residents of Framingham, Mass., in 1948. In the decades since, that data has been a gold mine, and researchers have continued to follow the original subjects' offspring. It is best known for helping scientists make the connection between cardiovascular disease and high cholesterol, blood pressure and obesity. But in 1975, the panel of tests that the study subjects took was expanded to include cognitive assessments.

The new study showed that the risk of developing dementia was declining, providing the strongest evidence yet of an unexpected phenomenon that had been sporadically reported in various data sets over the years.

There are caveats. The Framingham study does not include a diverse population, so it remains to be seen whether the trend is true across racial groups. The effect was only seen in people who had at least a high school education. It also does not mean the same thing is happening in other communities in the world.

As prevention and treatment of heart disease and cancer have improved, "many people think we can live even longer lives — but lives compromised by dementia, vision loss, and hearing loss," David Jones of Harvard Medical School and Jeremy Greene of the Johns Hopkins University School of Medicine wrote in an accompanying editorial. "Whether that fate is inevitable or whether these, too, are malleable scourges remains to be seen. ... Even if death and taxes remain inevitable, cancer, CAD [coronary artery disease], and dementia may not."

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