At the end of Assistant Village Idiot's post Obama's IQ, he writes the following:
"Reaching much farther back, the ability to be competent in a range of subjects suggests that Thomas Jefferson and Theodore Roosevelt were likely of high intelligence. A few others showed such facility in writing and composition, even on highly abstract subjects, that they must surely have had sufficient wattage. Madison and John Adams come to mind there.
If one has an agenda about a president it is usually easy to 'prove' him stupid or brilliant by carefully selecting the data. Andrew Jackson may have been close to illiterate, so you can paint him a dull gray if you like. Yet he also had such a prodigiouis memory that when legislation was being discussed, he could recall every previous draft verbatim from what he had heard read out, sometimes over weeks. So bright colors there. It depends what you choose."
A commenter wrote:
"Herbert Hoover and his wife Lou translated Agricola's classic work on mining from Latin into English, just for fun."
What I find most noticeable about this analysis of US presidents is that there is a change from those presidents who were born before the age of mass media to those who were born after. As Assistant Village Idiot mentions, it is not straightforward to compare presidents from before the age of mass media with those after in terms of pure intellectual ability. However, we can compare them in terms of the character of their intellectual accomplishments.
There is a definite qualitative shift. Consider for instance Ignatius L. Donnelly, a US legislator both at the state and national level and most known for a book about Atlantis which theorized that the Biblical Deluge and the Atlantean catastrophe were the same thing. I have not read the book, so I cannot comment on that aspect, but the fact is that Donnelly wrote and researched the book himself, no ghostwriters, no use of books merely as a promotional tool.
The character of the thought of many individuals from before and after the age of mass media, especially with the rise of social media has undergone a definite change. It is not explicable in terms of intelligence both because of the speed of the change and also because with this change in the nature of thinking, high intelligence has been repurposed as efficient processing of media inputs.
Bruce Charlton has two posts from 2011 which describe this phenomenon well. One, titled "complexity of thought" says that too much time spent communicating (including taking in communications) diminishes time spent thinking and if the balance is too much in favor of communication, then complexity of thinking substantially diminishes
"This explains why the internet has not led to any advances in genuine understanding, since the millions-fold expansion in the amount of data (plus increased social interaction via electronic media, plus increased volume and usage of mass media) have led to an equally vast simplification of cognition.
The average modern human mind is now more like a relay station than a brain - performing just a few quick and simple processes on a truly massive flow-through of data.
This applies equally, or especially, to intellectuals who are plugged-into oceans of data in a way never before possible. When all (almost all) intellectual output is simply a summary of unassimilated input, as we see all around; then we can perceive that intellectual processing has become grossly simplified."
And it goes beyond simplicity and complexity. The question is whether one's mind is thinking and understanding on its own or just processing data from outside.
The other post "The hierarchy of authorities" compares the current situation to the great thinkers of the past:
"nobody really believes that a high school kid with Wikipedia at his fingertips, or a hotshot globetrotting research professor, actually 'know more' than Aristotle or Aquinas. Rather, students and academics now actually know almost-nothing, and are - presumably - stuffed and overflowing and mere-conduits-for billions of words, sounds and images of fashions, illusions, delusions, distortions and open-ended misunderstandings."
Consider Aristotle and Aquinas. They had both built up vast edifices of thought within their minds; combining both understanding and knowledge. A great part of their writings was simply communicating this thinking. Similarly, Porphyry writes how his teacher Plotinus did not write down any of his teaching until after teaching for ten years. In other words, the teaching was the understanding of Plotinus, given expression by the words he chose to express it at any given time.