Dementia on the Retreat in the U.S. and Europe

The reason for the confusion appears to be that there are more older women than older men in the population. At any age when dementia is likely, there will be more women with dementia in the population than men.

One puzzling aspect of the decline is that it seems to be confined to Europe and the United States — it was not seen in Asia, South America or, from limited data, in Africa. There have been reports of increasing dementia rates in Japan, China and Nigeria, the paper’s authors note.

Those increases are puzzling, Dr. Hofman said. The trend may be related to higher rates of smoking, which makes dementia more likely, in those countries.

One leading hypothesis for the decline in the United States and Europe is improved control of cardiovascular risk factors, especially blood pressure and cholesterol. Nearly all dementia patients have other brain abnormalities, including blood vessel damage likely to be the result of high blood pressure.

High blood pressure seems to be most damaging in middle age, Dr. Hofman said. Those with lower blood pressure earlier in life but higher blood pressure later tend to have reduced chances of dementia.

Large swings in blood pressure are a risk at any age, he added.

Another possible reason for declining dementia rates might be better education, which is thought to have a protective effect by giving the brain more capacity — for example, a memory cache of more synonyms for words that were forgotten.

Like control of blood pressure and cholesterol, education levels have gradually improved over the past few decades. “There is a theory, but still not much evidence, that education shifts dementia to a later age,” Dr. Hofman said.