Perhaps it is because people have gotten fatter? Heavier people have stronger bones.

Heavier bodies, though, can account for at most half of the effect, said Dr. Steven R. Cummings of the California Pacific Medical Center Research Institute and the University of California at San Francisco. When asked what else was at play, he laughed and said, “I don’t know.”

Dementia rates, too, have been plunging. It took a few reports and more than a decade before many people believed it, but data from the United States and Europe are becoming hard to wave off. The latest report finds a 20 percent decline in dementia incidence per decade, starting in 1977.

A recent American study, for example, reports that the incidence among people over age 60 was 3.6 per 100 in the years 1986-1991, but in the years 2004-2008 it had fallen to 2.0 per 100 over age 60. With more older people in the population every year, there may be more cases in total, but an individual’s chance of getting dementia has gotten lower and lower.

There are reasons that make sense. Ministrokes result from vascular disease and can cause dementia, and cardiovascular risk factors are also risk factors for Alzheimer’s disease. So the improved control of blood pressure and cholesterol levels should have an effect. Better education has also been linked to a lower risk of Alzheimer’s disease, although it is not known why. But the full explanation for the declining rates is anyone’s guess. And the future of this trend remains a contested unknown.

The exemplar for declining rates is heart disease. Its death rate has been falling for so long — more than half a century — that it’s no longer news. The news now is that the rate of decline seems to have slowed recently, although it is still falling. While heart disease is still the leading cause of death in the United States, killing more than 600,000 people a year, deaths have fallen 70 percent from their peak. The usual suspects: Better treatment, better prevention with drugs like statins and drugs for blood pressure, and less smoking, are, of course, helping drive the trend. But they are not enough, heart researchers say, to account fully for the decades-long decline.

The heart disease effect has been examined by scientist after scientist. Was it a result of better prevention, treatment, lifestyle changes?

All three played a role, researchers said.

Dr. Jones said the current explanation reminds him of the Caucus-race in “Alice’s Adventures in Wonderland,” in which the Dodo, officiating, declared that “EVERYBODY has won, and all must have prizes.”