What surprises me most about these graphs is how relatively flat they are! Let's pretend for a moment that the distribution of SAT scores across income strata

What surprises me most about these graphs is how relatively flat they are! Let's pretend for a moment that the distribution of SAT scores across income strata would be identical if all kids were forced to take the test. Given that a much greater percentage of wealthy kids take the SAT and the probability one takes it likely increases along with his expected score, you would think the correlation would be the inverse of the NYT graph. If a wealthy child is an idiot, his parents will encourage him to take the test in hope that some college somewhere will accept him. But, a dumb poor kid probably dropped out of school before he had a chance to even take the SAT. So, the positive correlation in the NYT article suggests a much greater disparity of test taking ability (and all that it might imply) than just the slope of the graph naively implies.