For about six years, I tutored high-school students in the Boston area, mostly for standardized tests like the SAT and ACT. In most cases, these were high-achieving students who were looking squeeze an extra percentile or two out of their scores. As the instruction was one-on-one, this usually meant traveling to the family’s house, and working with the student at his or her kitchen table. These were generally very nice tables, in very nice kitchens, in very nice houses, themselves in very nice neighborhoods. Instructions as to how to find to the bathroom were often shockingly involved. Suffice to say, most — though not quite all — of these kids were stinking rich.

To left-leaning critics of the test prep-industry — and to many of those within the industry itself — this sort of observation often leads to worries that the tests are merely testing the students’ economic privilege, rather than their actual abilities and knowledge. It seems all the more so when most of your students are sharp-and-rich and those who aren’t are far more likely to be dull-but-rich than sharp-but-poor.

As Charles Murray points out, that’s not an illogical conclusion so much as one based on poorly-chosen assumptions. Yes, rich kids do disproportionately well on standard tests, but that’s less because their parents are rich than because their parents are disproportionately sharp, and sharp people are disproportionately wealthy and have disproportionately sharp kids: