Just saw a documentary on stupidity. Pretty thin (and a little stupid), but there was one good point in it: people form schemas of the world, belief systems, which they use to filter out information that disagrees and process information that does agree. In other words, they see only what they want to see.
The intent is to prevent cognitive disonance. Even if we've had all your money stolen by a man in a gorilla suit, sold our children for magic seeds, and woken up with a sheep, we will try to rationalize your behaviour so as not to appear dumb, especially to ourselves. Police investigators encountered an extreme example of this in a man who got taken by the Nigerian scam and continued to believe, even after the investigators laid out the whole scam to him, that the con men were acting in good faith, and that things just went wrong. Anything rather than admit that we made a mistake. Genuine personal paradigm shifts are rare. That Zen ideal, 'New Mind', where the person approaches the world without preconceptions, is a rare thing.
There was a case of a man who had a brain injury which damaged the emotional part of his brain. He could reason perfectly, but he could not make a decision. Rationality may play a part in our decision making process, but only if we have an emotional attachment to rationality itself. Evidence may also play a part, but only if we value the weight of evidence over existing beliefs. I enjoy James Randi ranting and storming about pseudo-science, con artists, and irrationality, but he's preaching to the converted. To the True Believer it's like that Far Side cartoon about what the dog hears: "Blah blah blah Rover blah blah..." Rational empiricism demonstrates it validity only after one has accepted and practiced it, and even then it may take time. The original choice to accept it is a leap of faith, because at the outset there is no more 'proof' that reason and evidence will give you the answers than any other competing view. And it requires a faith in our own judgement, which requires encouragement to develop.
Unless kids grow up in an environment where learning is important, their natural curiosity will die out. In the conformity that rules elementary and high schools, too much knowledge makes you stand out. We all know the names: geek, nerd, bookworm, weirdo, and so on. The bullies usually consider themselves dumb and don't like anyone who shows them up. And, of course, who are the school heroes, but the jocks. It's as if we have to grow through the earlier stages of civilization before we reach adulthood. Homo Sapiens, meet Neanderthal Man.
So we end up with a population that has no curiosity, and doesn't like to think. Of course, we all think for ourselves--and yet, everyone thinks the same thing... Hmmm. We believe very strongly in our opinions, but is that because they are really our own opinions, or because they are the opinions given to us that we dare not part with, because we actually suspect that we are too dumb to come up with something on our own to replace it? It reminds me of an ad I saw years ago in a sleazy men's magazine: "What kind of man reads this? The kind of man who has firm opinions on current talking points!" Oh... my... God... Sort of like the latest fashions from Vogue. "Paris says ugly this year, and who cares?" Rush Limbaugh said it all. "Don't think, I'll think for you." His fans actually call themselves ditto-heads. The mind boggles--except, of course, when you don't have one.
And so the stupidity goes on.