Gifted Issues Discussion homepage Forums General Discussion SAT not an affluence test

All high-quality academic tests look as if they’re affluence tests. It’s inevitable. Parental IQ is correlated with children’s IQ everywhere. In all advanced societies, income is correlated with IQ. Scores on academic achievement tests are always correlated with the test-takers’ IQ. Those three correlations guarantee that every standardized academic-achievement test shows higher average test scores as parental income increases.

But those correlations also mean that a lot of the apparent income effect is actually owed to parental IQ. The SAT doesn’t have IQ information on the parents. But the widely used National Longitudinal Survey of Youth contains thousands of cases with data on family income, the mother’s IQ, and her children’s performance on the math and reading tests of the Peabody Individual Achievement Test battery, which test the same skills as the math and reading tests of the SAT.

For the SAT, shifting to more than $200,000 of family income from less than $20,000 moved the average score on the combined math and reading tests to the 74th percentile from the 31st—a jump of 43 percentiles. The same income shift moved the average PIAT score to the 82nd percentile from the 30th—a jump of 52 percentiles.

Now let’s look at the income effect in the PIAT when the mother’s IQ is statistically held constant at the national average of 100. Going to a $200,000 family income from a $1,000 family income raises the score only to the 76th percentile from the 50th—an increase of 26 percentiles. More important, almost all of the effect occurs for people making less than $125,000. Going to $200,000 from $125,000 moves the PIAT score only to the 76th percentile from the 73rd—a trivial change. Beyond $200,000, PIAT scores go down as income increases.