I wonder if the hypothesis in the OP applies to all Presidents or just to 20th century presidents?
If a little bit of time passing can put a good polish on Kennedy how much more so for Washington or Lincoln or Teddy Roosevelt? Are all are heros liars and dissemblers?
Does the idea of scizophrenia in politicians apply equally throughout all of history or is it a phenomenon that rears its head when the atmosphere of the day calls for it?
This may have been discussed...