In a previous post, I cited data showing that white American high school dropouts average IQ 87 (13 points below the U.S. mean). But is dropping out of high school the product or the cause of low IQ? In a fascinating 1968 Swedish study, a scholar named K. Harnqvist compared the IQ’s of males who were tested at both age 18 and age 13. He found that if two boys, for example, were both of the same social class, and both had the same IQ at age 13, but one boy completed all four years of high school and the other completed none of them, the latter boy scored nearly 8 points lower at age 18. In other words, each year of missed schooling causes IQ to drop by 1.8 points. Herrnstein and Murray independently found a similar effect on page 615 of The Bell Curve (1.65 IQ points per school year).
It’s very unlikely that school makes you smarter, but it is possible that school makes you more test savvy and more motivated to do well on mental tests. Many IQ tests require complex focused thinking. Someone who dropped out of school at 13 and has been working in an outdoors fun type job is likely to find such mental effort excruciatingly boring, annoying, and intimidating, while someone who stayed in school her whole life and works in academia is likely to be used to sitting still and focusing and may enjoy the challenge and have the confidence and intellectual ego to stay motivated.
Another possibility is that schooling has no impact on IQ, and the above studies showing it does are just statistical artifacts. For example, even controlling for IQ at age 13 does not completely control for intelligence, because IQ tests are imperfect measures and actual academic success may be capturing indicators of intelligence that even the IQ test missed.
In many ways IQ tests are delayed measures of intelligence, especially IQ tests that rely on general knowledge and vocabulary. Bright children do learn more words and facts over time, so some IQ tests use tests of knowledge as a proxy for learning speed. But what happens when a once quick learning child is no longer the quickest learner in class? Tests of crystallized knowledge will continue to show him being the brightest, because they’re testing knowledge he learned when he was the brightest, but unless he takes an entirely fluid test of novel problem solving, his current intellectual functioning might be overestimated. The kid may intuitively realize his intelligence (relative to his peers) is dropping long before the IQ tester does, and decide to drop out of school. Then years later when he scores lower on another (more fluid) test, people will incorrectly blame the decline on his decision to drop out of school.
Thus the only way to absolutely prove that dropping out of high school lowers IQ is to force an entirely random sample of kids to drop out of school and force another entirely random sample of kids to stay in school, and then compare their IQ’s years later. But such a study would be too unethical to actually do.