There is good news and bad news in this story.
New studies show that the rate of Alzheimer's disease and other dementias is falling in the United States and some other rich countries.
But the total number of cases continues to rise because more people are living to an old age.
Scientists think the trend is due to better education and control health factors such as lowering blood pressure and cholesterol.
One study finds that an American over age 60 today has a 44 percent lower chance of developing dementia than a similar-aged person did roughly 30 years ago.