Looking at your observation that "a hundred years ago we could easily have the same conversation about simply reading and writing - they (the poor, women, or "lower class" ) luke not be taught to read." I would not be so sure that the fact that nowadays 99% of everyone can learn proves that this historical observation was mistaken.
It's known that intelligence can be significantly negatively affected by environmental factors like malnutrition (both of child and mother during pregnancy), childhood disease, pollution and injury, and there is some evidence (also mentioned in the article we're discussing) that a century or two ago the average person was significantly dumber than now, presumably because of those factors - especially to the poor and women, which often experienced more severe childhood malnutrition than their male siblings.
So it would seem plausible that a hundred years ago a nontrivial percentage of people actually were too dumb to succeed in literacy and perhaps the fix for that wasn't just starting young enough and putting effort in, but rather that we started getting much fewer kids with severe environmental damage to their intelligence.
It's known that intelligence can be significantly negatively affected by environmental factors like malnutrition (both of child and mother during pregnancy), childhood disease, pollution and injury, and there is some evidence (also mentioned in the article we're discussing) that a century or two ago the average person was significantly dumber than now, presumably because of those factors - especially to the poor and women, which often experienced more severe childhood malnutrition than their male siblings.
So it would seem plausible that a hundred years ago a nontrivial percentage of people actually were too dumb to succeed in literacy and perhaps the fix for that wasn't just starting young enough and putting effort in, but rather that we started getting much fewer kids with severe environmental damage to their intelligence.