This might be a partial explanation for why affirmations appear to work for some people. Perhaps using language to tell yourself that you are a different person (happier, more successful, etc.) causes you to become more like the words.
We know that the brain is bidirectional. If it's happy, it can make you smile. But if you force yourself to smile when you are not happy, it can make you happier.
When I was in college, which was my first social experience outside the tiny town where I grew up, I noticed that a lot of people were asking me the same question: How are you? So I decided that my answer to that question, regardless of the truth, would be always be something along the lines of great, spectacular, excellent or sensational. It's the one situation in which there is no social penalty for saying out loud that you are incredible.
How are you?
My reasoning was that over time I might program myself through repetition to become better than I was. I have no idea if it works, but I know I enjoy telling people I'm fabulous.
It would be easy to test this sort of thing. Just take a random group of kids and teach them to say good things to themselves, or even aloud, about their intelligence, on a regular basis. Then compare their test scores with a control group.
If this method improved test scores, do you think schools would be allowed to teach it? I'm guessing no, because it would seem like witchcraft to the fundamentalists.