It’s interesting to look at the map of the Americas. You get out of the high populations of Canada and the USA and go down to Mexico which is in the mid range. You travel through Central America into the northwest part of South America and it’s low, except for Costa Rica which is high. The rest of South American looks like North America.
The question is, why?
likely its related to diet.
In many countries people die from a lot of causes at early ages that prevent them from ever contracting Alzheimer’s.