A new study has found that 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 that researchers say is probably occurring across developed countries and that could have major social and economic implications for families and societies.
Another recent study, conducted 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 had 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 subjects 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, also in the Lancet, soften alarms sounded by advocacy groups and some public health officials who have forecast a rapid rise in the number of people with dementia, as well as in the costs of caring for them. The projections assumed that the odds of getting dementia would be unchanged.
Yet experts on aging said the studies also confirmed something they had suspected but had had difficulty proving: that dementia rates would fall and mental acuity improve as the population grew healthier and better educated. The incidence of dementia is lower among those better educated, and among those who control their blood pressure and cholesterol, possibly because some dementia is caused by ministrokes and other vascular damage.
So as populations controlled cardiovascular risk factors better and had more years of schooling, it made sense that the risk of dementia might fall. A half-dozen previous studies had hinted that the rate was falling, but they had flaws that led some to doubt the conclusions.
Researchers said the two new studies were the strongest, most credible evidence yet that their hunch had been right.
Dallas Anderson, an expert on the epidemiology of dementia at the National Institute on Aging, the principal financer of dementia research in the United States, said the new studies were "rigorous and are strong evidence." He added that he expected that the same trends were occurring in the United States but that studies were necessary to confirm them.
"It's terrific news," said Dr. P. Murali Doraiswamy, an Alzheimer's researcher at Duke University, who was not involved in the new studies. It means, he said, that the common assumption that every successive generation will have the same risk for dementia does not hold true.
The new studies offer hope amid a cascade of bad news about Alzheimer's disease and dementia. Major clinical trials of drugs to treat Alzheimer's have failed. And a recent analysis by the Rand Corp. based on an assumption that dementia rates would remain steady concluded that the number of people with dementia would double in the next 30 years as the baby boom generation aged, as would the costs of caring for them.
But its lead author, Michael D. Hurd, a principal senior researcher at Rand, said that his projections of future cases and costs could be off if the falling dementia rates found in Britain hold true in the United States.
Dr. Marcel Olde Rikkert of Radboud University Nijmegen Medical Center in the Netherlands, who wrote an editorial to accompany the Danish study, said estimates of the risk of dementia in older people "urgently need a reset."
Maria Carrillo, vice president of medical and scientific relations at the Alzheimer's Association, an advocacy group, was not convinced that the trends were real or that they held for the United States.
The studies assessed dementia, which includes Alzheimer's disease and other conditions that can make mental functioning deteriorate. Richard Suzman, director of the division of behavioral and social research at the National Institute on Aging, said it was not possible to know from the studies whether Alzheimer's was becoming more or less prevalent.
The British researchers, led by Dr. Carol Brayne of the Cambridge Institute of Public Health, took advantage of a large study that tested 7,635 randomly selected people, ages 65 and older, for dementia between 1984 and 1994. The subjects lived in Cambridgeshire, Newcastle and Nottingham. Then, between 2008 and 2011, the researchers assessed a similar randomly selected group living in the same areas.
"We had the same population, the same geographic area, the same methods," Brayne said. "That was one of the appeals."
In the Danish study, Dr. Kaare Christensen of the University of Southern Denmark in Odense and his colleagues compared the physical health and mental functioning of two groups of elderly Danish people. The first consisted of 2,262 people born in 1905 who were assessed at age 93. The second was composed of 1,584 people born in 1915 and assessed at age 95.
In addition to examining the subjects for physical strength and robustness, the investigators gave them a standard dementia screening test, the mini-mental exam and a series of cognitive tests.
"With these two studies, we are beginning to see that more and more of us will have a chance to reach old age cognitively intact, postponing dementia or avoiding it altogether," Anderson said. "That is a happy prospect."