I'm always amazed that, in discussions of standardized tests and how well different demographics do on them, no one mentions the disaffected kids who don't try very hard, or stop answering questions halfway through, or just give random answers.

Kids who didn't grow up expecting a professional/academic career, or who think it's unrealistic to expect one because of their school experience so far, don't think they have a stake in doing well on standardized tests. They may even have a perverse interest in doing badly.

If you already feel insecure, and don't want to face getting poor results on an aptitude test on top of that, the smart move is not to try. One of my brothers did this. He got a fabulously low score on the SAT because he just didn't read any of the questions.

Doing really badly on a test can even be the smart move socially, if you're afraid you're dumb. If anyone ever sees the scores, you want to be able to say you didn't try, so you look cool instead of stupid. For this to work, it really helps if your score is improbably low.

Some people also panic in the middle of doing a test because they feel like they're doing really badly. The temptation is to end the torture by just not answering any more questions, or answering them at random.

And then there are all the people who just aren't trying that hard on the test because they don't expect it to affect their lives, and everyone they know says the test is bullshit.

It seems to me that these factors must cause a lot of the disparity in average SAT, ACT, and even IQ test performance between different demographics (on top of other factors that are more commonly discussed).

I suspect you would see a massive change in the distribution of standardized test scores if kids in poor neighborhoods got $500 for an above-average score.