@JasonR - It's not that the concepts are hard in themselves. For example, the concept of negative numbers isn't hard and practically everybody knows it. However, if you try to explain the concept of negative numbers to someone who has never heard about it before, how fast they grasp it will be significantly determined by how smart they are.
Very few things are inherently hard. Most things are fairly simple. Given enough time, I believe I can explain the special theory to anyone.
Smart is probably not the right word or the problem here. The bigger problem is just how enormously ignorant people are.
For example, one third can't find north on a map!
I'm not making this up.
"On a more practical level, given a map of a hypothetical place and told they could escape an approaching hurricane by evacuating to the northwest, one-third would travel in the wrong direction."
This level of ignorance is mind-blowing. It shows that the middle of the bell curve when it comes to actual knowledge is just slightly above that of being able to tell left from right on a map.
Believe you me, if you ask questions like "if you put $100 in the bank at 5% interest, how much money will you have in the bank one year from now" to a random person, it doesn't get much better. A very significant fraction will simply not know how to answer this question.
It's really tempting to be an elitist in such an environment. Elitism is really nothing more than believing that smarter/better informed people make better decisions; alternatively, that stupid/uninformed people make dumb decisions. However, elitism per se has nothing to do with how dumb people are or with the problem of teaching ERE. It's just an argument for which people should make the decisions.
The general problem isn't that people aren't "intelligent" in the IQ sense. The problem is that 1) People don't think---they just regurgitate results; and 2) People don't know much of anything beyond their chosen profession.
So if I try to explain things like "So if your expense level is $9,000/year and you can invest your money at 3%, then if you have $300,000 you can get $9,000/year adjusted for inflation for an eternity and thus you don't need to work for income since you got everything covered". And you know what the typical answer is ... even from people who are supposedly capable of calculating with percentages? Such as people who graduated 8th grade.
"But don't you need like a million dollars to retire?"
(because that's the number they heard from some expert)
No, the ability to think for yourself is extremely rare(!)
The INTJ personality does seem to come with this ability though. Thinking critically is a natural tendency for INTJs---not just a school assignment. It's not that it doesn't come with other types, but it's very expressed in the INTJ personality.