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A new study has found dementia rates among people 65 and older in England and Wales have plummeted by 25 percent over the past two decades, to 6.2 percent from 8.3 percent, a trend researchers say is likely occurring across developed countries and that could have major social and economic implications for families and societies.

Another recent study, in Denmark, found that people in their 90s who were given a standard test of mental ability in 2010 scored substantially better than people who reached their 90s a decade earlier.

Nearly one-quarter of those assessed in 2010 scored at the highest level, a rate twice that of those tested in 1998. The percentage of severely impaired fell to 17 percent from 22 percent.

The British study, published Tuesday in The Lancet, and the Danish one, which was released last week in The Lancet, soften alarms sounded by advocacy groups and some public-health officials who have forecast a steadily rising population of baby boomers with the same odds of getting dementia as older people now, as well as exploding costs to care for them.

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And experts on aging say the studies confirmed something they long suspected but lacked good evidence to prove: Dementia rates would fall and mental acuity improve as the population grew healthier and better educated. The findings may also give new impetus to efforts to get people to quit smoking and take other steps to lower their risks of heart disease and stroke.

Epidemiologists have long found that the incidence of dementia is lower among the better educated, as well as among those who control their blood pressure and cholesterol. Since some dementia is caused by ministrokes and other vascular damage, it made sense that as populations better control these risk factors, dementia rates might drop.

A half dozen previous studies hinted the hypothesis was correct but had methodological problems that cast doubt on such findings.

A recent RAND Corp. analysis — based on an assumption that dementia rates would remain steady — had concluded the number of people with dementia would double in the next 30 years as baby boomers aged, and so would the costs of caring for them.

But its lead author, Michael D. Hurd, a principal senior researcher at RAND, said in an interview that while his estimates of current costs were correct, the future projections could be off if the falling dementia rates found in Britain held true in the United States.

“If these trends continue, it would affect our estimates,” he said.

Maria Carrillo, vice president of medical and scientific relations at the Alzheimer’s Association, an advocacy group, said she was not convinced that the trends were real or that they held for the United States. She said that the British paper had a methodological flaw and that the Danish work might reflect the fact that people there were generally healthier than those in the United States.

The studies assessed dementia, which includes Alzheimer’s disease but also other conditions, like ministrokes, that can make mental functioning deteriorate.

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