Brain size shrinkage seems to compete with some of those studies done between 1890 and 1930--- ..
I'd argue most of the above but ---- I could show a family that lived in poverty or near poverty, lived in fear and stress and lack of food for nearly 500 years Something that could be said of many European inhabitants over the last 5000 years. -That family has doctors, lawyers, PHD's , LLD's, engineers and mathematiticians. I believe that a certain amount of intellect comes from ancestors, some for environment, much from access to education and study habits. No doubt having access to food aids in being educated. Again--I won't argue your studies and their conclusion--they sound like the studies from earlier times I ask 2 questions: 1. How long does it take for shrinkage to be reversed 2 how long does the reversal process take?
However even that study wouldn't explain my pea headed cousin.
Since this is something I actively research, I would be interested in those studies done between 1890 and 1930, I don't know what you are referring to. As to your direct questions to me, exercise can restore brain tissue at virtually any age, though at old age that gets problematic. How long to reverse? I don't have an answer for that.
The virtue of exercise underlies a problem with your inferences from anecdotal data; everything is relative. The correlation of poverty with lack of exercise may be unique for this society, this time period. An impoverished family in Africa may walk 5 miles or more a day for water. An impoverished family in Flint draws water from the tap in their kitchen.
Your inference of greater stress in European culture may not be making a distinction between episodal and chronic stress; one is adaptive, the other a health hazard. I would guess that for most of those 5,000 years, most of the stress you infer was episodal, very acute at times, perhaps life-threatening, but not the type of chronic stress that occurs when all the images of mass media reveal to you just how much you are deprived on an hourly basis. Aboriginal cultures have next to nothing, but that does not cause stress because they do not know what they are missing.
Life expectancy in the "Dark Ages" was actually greater than the preceding Roman Empire or subsequent Industrial Revolution, because peasants did not have the stress, lack of exercise or even the poor diet of a modern day American "peasant." This is anecdotal, but I had a conversation with a Cooperative Extension worker at Kansas State University, whose goal it was to bridge local ranch with immigrant Hispanic cultures. The immigrants, who after all come to America for a better life, had a worse diet after they came to this country. Like I said, the impacts of poverty is relative to the society of the impoverished.