Despite fears that dementia rates were going to explode as the population grows older and fatter, and has more diabetes and high blood pressure, a large nationally representative survey has found the reverse. Dementia is actually on the wane. And when people do get dementia, they get it at older and older ages.

Previous studies found the same trend but involved much smaller and less diverse populations like the mostly white population of Framingham, Mass., and residents of a few areas in England and Wales.