Going back to WW2
FDR - smart
Truman - average
Eisenhower - average
JFK - very smart
LBJ - hmmm, street smart
Nixon - smart
Ford - ???
Carter - very smart
Reagan - somewhere between average and kinda dumb
Bush Sr - very smart
Clinton - very smart
George W Bush - average to above average
Obama - very smart
If anyone can find any kind of correlation whatsoever please share.
Also please note this graded on a bit of a curve considering circumstance. Even a president I rank 'average', I still figure would have a much more sophisticated understanding of things than your actual average person just based on their life experiences.
PS: Herbert Hoover - absolutely brilliant.