What good is IQ?

The Truth

JF-Expert Member
Oct 21, 2007
What good is IQ?

A reader asks:

To clarify, while I believe natural selection explains a lot I have caveats about IQ as a tool for testing intelligence. If you can’t measure the coast of France with a single number how can you do it with human intelligence?

Easily. Human intelligence is a great deal less complex than the coast of France. :)

It’s fashionable nowadays to believe that intelligence is some complicated multifactor thing that can’t be captured in one number. However, one of the best-established facts in psychometry (the science of measuring mind) is that it is quite difficult to write a test of mental ability that is not at least 50% correlated with all other such tests. Or, to put it another way, no matter how you design ten tests for mental ability, at least about half the variance in the scores for any one of them statistically appears to be due to a “general intelligence” that shows up on the other nine tests as well.

Psychometricians call this general intelligence measure “g”. It turns out to predict important real-world success measures quite well — not just performance in school but income and job success as well. The fundamental weakness in multiple-factor theories of intelligence is that measures of intelligence other than g appear to predict very little about real-world outcomes. So you can call a lot of other things “intelligence” if you want to make people feel warm and fuzzy, but doing so simply isn’t very useful in the real world.

Some multifactor theorists, for example, like to describe accurate proprioception (an acute sense of body position and balance) as a kind of intelligence. Let’s say we call this “p”. The trouble with this is that there are very few situations in which a combination of high p and low g is actually useful — people need to be able to balance checkbooks more often than they need to walk high wires. Furthermore, g is easier to substitute for p than the other way around; a person with high g but low p can think up a way to not have to walk a high wire far better than a person with low g but high p can think up a way not to have to balance a checkbook. So g is in a strict functional sense more powerful than p. Similar arguments apply to most of the other kinds of specialized non-g ‘intelligence’ that have been proposed.

Once you know about g, you can rank mental-capability tests by how well their score correlates with g. IQ is valuable because a well-composed IQ test measures g quite effectively. For purposes of non-technical discussion, g and IQ can be considered the same, and pychometricians now accept that an IQ test which does not closely track g is defective.

A lot of ink has been spent by people who aren’t psychometricians on insisting that g is a meaningless statistical artifact. The most famous polemic on this topic was Stephen Jay Gould’s 1981 book The Mismeasure of Man, a book which was muddled, wrong, and in some respects rather dishonest. Gould was a believing Marxist; his detestation of g was part of what he perceived as a vitally important left-versus right kulturkampf. It is very unfortunate that he was such a persuasive writer.

Unfortunately for Gould, g is no statistical phantom. Recently g and IQ have been shown to correlate with measurable physiological variables such as the level of trace zinc in your hair and performance on various sorts of reaction-time tests. There are hints in the recent literature that g may be largely a measure of the default level of a particular neurotransmitter associated with states of mental alertness and speed of thought; it appears that calling people of subnormal intelligence “slow” may not be just a metaphor!

IQ is one of several large science-related issues on which political bias in the dominant media culture has lead it to present as fact a distorted or even reversed version of the actual science. In 1994, after Murray and Herrnstein’s The Bell Curve got a thoroughly undeserved trashing, fifty leading psychometricians and psychologists co-signed a summary of mainstream science on intelligence. It makes eye-opening reading.

The reasons many popular and journalistic accounts continue to insist that IQ testing is at best meaningless and at worst a sinister plot are twofold. First, this belief flatters half of the population. “My IQ may be below average, but that doesn’t matter because IQ is meaningless and I have high emotional intelligence!” is, understandably, a favorite evasion maneuver among dimwits. But that isn’t the worst of it. The real dynamite is not in individual differences but rather that the distribution of IQ (and hence of g) varies considerably across groups in ways that are politically explosive.

Men vs. women is the least of it. With other variables controlled, men and women in a population have the same mean IQ, but the dispersion differs. The female bell curve is slightly narrower, so women have fewer idiots and fewer geniuses among them. Where this gets touchy is that it may do a better job than cultural sexism of explaining why most of the highest achievers in most fields are male rather than female. Equal opportunity does not guarantee equal results, and lot of feminist theory goes out the window.

But male/female differences are insignificant compared to the real hot potato: differences in the mean IQ of racial and ethnic groups. These differences are real and they are large enough to have severe impact in the real world. In previous blog entries I’ve mentioned the one-standard-deviation advantage of Ashkenazic Jews over gentile whites; that’s roughly fifteen points of IQ. Pacific-rim Asians (Chinese, Japanese, Koreans etc.) are also brighter on average by a comparable margin. So, oddly enough, are ethnic Scots — though not their close kin the Irish. Go figure…

And the part that, if you are a decent human being and not a racist bigot, you have been dreading: American blacks average a standard deviation lower in IQ than American whites at about 85. And it gets worse: the average IQ of African blacks is lower still, not far above what is considered the threshold of mental retardation in the U.S. And yes, it’s genetic; g seems to be about 85% heritable, and recent studies of effects like regression towards the mean suggest strongly that most of the heritability is DNA rather than nurturance effects.

For anyone who believe that racial equality is an important goal, this is absolutely horrible news. Which is why a lot of well-intentioned people refuse to look at these facts, and will attempt to shout down anyone who speaks them in public. There have been several occasions on which leading psychometricians have had their books canceled or withdrawn by publishers who found the actual scientific evidence about IQ so appalling that they refused to print it.

Unfortunately, denial of the facts doesn’t make them go away. Far from being meaningless, IQ may be the single most important statistic about human beings, in the precise sense that differences in g probably drive individual and social outcomes more than any other single measurable attribute of human beings.

Mean IQ differences do not justify making assumptions about any individual. There are African black geniuses and Ashkenazic Jewish morons; humanity and ethics demand that we meet each individual human being as an individual, without prejudice. At the same time, group differences have a significance too great to ignore. In the U.S., blacks are 12% of the population but commit 50% of violent crimes; can anyone honestly think this is unconnected to the fact that they average 15 points of IQ lower than the general population? That stupid people are more violent is a fact independent of skin color.

And that is actually a valuable hint about how to get beyond racism. A black man with an IQ of 85 and a white man with an IQ of 85 are about equally likely to have the character traits of poor impulse control and violent behavior associated with criminality — and both are far more likely to have them than a white or black man with an IQ of 110. If we could stop being afraid of IQ and face up to it, that would give us an objective standard that would banish racism per se. IQ matters so much more than skin color that if we started paying serious attention to the former, we might be able to stop paying attention to the latter.

UPDATE: An excellent summary of science relating to g is here


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