While sampling equally from different income groups might clear up these variables, from what I've been able to find some of highest rates of dementia are in re

While sampling equally from different income groups might clear up these variables, from what I've been able to find some of highest rates of dementia are in relatively small rich European countries. Those countries have longer life expectancies so it's hard to do apples-to-apples for rich countries to poor countries. I do think the "use it or lose it" model for neurodegeneration makes a lot of sense, in the same way you lose muscles if you don't use them, so bilingual is an interesting proxy for greater brain use. Although I also think the lack-of-sleep model makes sense too so I really don't know.