The SAT is said to correlate about as well with IQ as two different IQ tests correlate with one another. In his book Real Education (pg 69-70), Charles Murray claims that only 17 year olds capable of getting an 1180+ on the post 1995 SAT (Critical Reading + Math) are true college material. Although 35% of American 17 year olds take the SAT, he estimates that only 10% of 17 year would score 1180+ if all of them took it.. In other words, 1180+ is equivalent to an IQ of 120+.
In his book Coming Apart (pg 375) he estimates an SAT score of 1400 is equivalent to an IQ of 135. From these two data points, we can create the following formula for converting SAT into IQ equivalents:
IQ = 39.545 + 0.068(SAT score)
Note, this formula only applies to the post 1995 SAT. Before 1995 the SAT was much harder.
This formula works well for individuals, but should never be used to estimate the median IQ of a college. This is because elite colleges select for SATs, so the estimated median IQ will be artificially inflated by a selection bias.
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.
Since the earliest days of intelligence testing, it’s been known that criminals average IQ’s around 90 (about 10 points below the North American average of 100). What’s interesting is that criminals also score 10 points below their non-delinquent siblings who grow up in the same home. Comparing criminals to their siblings is useful because it controls for so many of the socio-economic factors that are so often assumed to confound research on both crime and IQ.
Given that intelligence and morality are two different concepts, some may be surprised by the fact that criminals tend to have lower IQ’s. Indeed criminal geniuses such as the fictional Hannibal Lecter are a pop culture fixture. In the book, The g Factor by Arthur Jensen, three hypotheses are discussed:
1) Criminals have lower IQ’s because they’re not smart enough to achieve success through conventional avenues and thus must stoop to unethical methods.
2) Criminals have lower IQ’s because crimes are often impulsive, and so people who think through the long-term consequences (for themselves and the victims) are more likely to avoid crime.
3) Criminals have lower IQ’s because they’re not smart enough to understand why their crimes are wrong.
Pumpkin Person would like to propose a fourth hypothesis. Criminals have lower IQ’s because some of the same factor(s) that stunted their intellectual development, also stunted their moral development, since both are a function of the brain.
A popular hypothesis among laymen is that criminals actually don’t have lower IQ’s, they just appear to because all the smart ones get away, but this idea was dismissed by Charles Murray and Richard Herrnstein in the book The Bell Curve. They argued that there are not enough unsolved crimes for there to be a secret group of high IQ criminals and that among convicted criminals, the duller one commit more crimes, and used both these facts to dismiss the high IQ criminal theory.
However there’s also this saying: Behind every great fortune, there’s a great crime. Perhaps the smartest criminals conceal their crimes so well, we don’t even know one’s been committed.
Brain Size 