On page 63 of Charles Murray’s book Coming Apart, he displays data on the mean IQ’s of people with various education levels, as of age 25. Although the data is only for white Americans, the white IQ distribution is fairly similar to that of Americans as a whole. He shows data for both 1982-1989 and 2005-2009. Since the data is virtually identical for both eras, I’ll describe the more recent stats:
White Americans with no degree (about 10 years of completed education?): Average IQ 87
White Americans with high school diploma/GED (about 12 years education?): Average IQ 99
White Americans with an Associate degree (about 14 years education?): Average IQ 104
White Americans with a Bachelor’s degree (about 16 years education?): Average IQ 113
White Americans with a Master’s degree (about 18 years education?): Average IQ 117
White Americans with PhD, LLD, MD, DDS (about 20 years education?): Average IQ 124
The relationship between IQ and years of education seems pretty linear. Extrapolating from the data, it seems that white native born adult Americans who have zero years of completed education would have an average IQ of 54, those who only completed grade one would average IQ 58, those who completed only up to second grade would average IQ 61 etc. In other words, average IQ increases by about 3.54 points for each year you move up the education ladder. These numbers sound plausible, because not completing ANY education in modern America would suggest a pretty serious disability.
It’s interesting to ask whether IQ causes people to get more education or does education raise IQ? Certainly if you have a high IQ, you’re likely to find school easy and rewarding, and thus are more likely to successfully pursue more and more education. In addition, high IQ people are smart enough to realize that staying in school will increase the odds of getting a good and high paying job.
However staying in school also affects IQ. On page 615 of The Bell Curve by Herrnstein and Murray, they find that even after controlling for earlier IQ, each year of education independently adds 1.65 IQ points to later IQ. However one should not conclude that school makes you smarter. According to Arthur Jensen, the preponderance of evidence suggests that general intelligence is a physiological variable that can not being improved by psychological or cultural influences. However IQ tests are not perfect measures of intelligence, so getting a lot of education allows you to artificially boost your score. School teaches you to concentrate on complex mental tasks and gives you the confidence to try your best. It also exposes you to the general knowledge and vocabulary that many IQ tests probe.
Does this mean we should give bonus points to high school drop outs and deduct points from PhDs to level the playing field? Perhaps not, because even though getting a PhD makes you perform better on an IQ test, the people who get PhDs also tend to be smarter to begin with. In other words, if everyone had the same education, performances on intelligence tests would vary much less from person to person, but the rank order of scores would likely remain almost the same. Since IQ is a measure of your cognitive rank within the population, and not your absolute performance, the unfair performance boost that comes from staying in school has limited effect.