Howard Gardner has also convinced us that the word intelligence carries with it undue affect and political baggage. It is still a useful word, but we shall subsequently employ the more neutral term cognitive ability as often as possible to refer to the concept that we have hitherto called intelligence, just as we will use IQ as a generic synonym for intelligence test score. Since cognitive ability is an uneuphonious phrase, we lapse often so as to make the text readable. But at least we hope that it will help you think of intelligence as just a noun, not an accolade.

We have said that we will be drawing most heavily on data from the classical tradition. That implies that we also accept certain conclusions undergirding that tradition. To draw the strands of our perspective together and to set the stage for the rest of the book, let us set them down explicitly. Here are six conclusions regarding tests of cognitive ability, drawn from the classical tradition, that are by now beyond significant technical dispute:

1. There is such a thing as a general factor of cognitive ability on which human beings differ.
2. All standardized tests of academic aptitude or achievement measure this general factor to some degree, but IQ tests expressly designed for that purpose measure it most accurately.
3. IQ scores match, to a first degree, whatever it is that people mean when they use the word intelligent or smart in ordinary language.
4. IQ scores are stable, although not perfectly so, over much of a person’s life.
5. Properly administered IQ tests are not demonstrably biased against social, economic, ethnic, or racial groups.
6. Cognitive ability is substantially heritable, apparently no less than 40 percent and no more than 80 percent.

Charles Murray, “The Bell Curve Explained”, American Enterprise Institute, 2017-05-20.

Suppose that the question at issue regards individuals: “Given two 11 year olds, one with an IQ of 110 and one with an IQ of 90, what can you tell us about the differences between those two children?” The answer must be phrased very tentatively. On many important topics, the answer must be, “We can tell you nothing with any confidence.” It is well worth a guidance counselor’s time to know what these individual scores are, but only in combination with a variety of other information about the child’s personality, talents, and background. The individual’s IQ score all by itself is a useful tool but a limited one.

Suppose instead that the question at issue is: “Given two sixth-grade classes, one for which the average IQ is 110 and the other for which it is 90, what can you tell us about the difference between those two classes and their average prospects for the future?” Now there is a great deal to be said, and it can be said with considerable confidence — not about any one person in either class but about average outcomes that are important to the school, educational policy in general, and society writ large. The data accumulated under the classical tradition are extremely rich in this regard, as will become evident in subsequent chapters.

[…]

We agree emphatically with Howard Gardner, however, that the concept of intelligence has taken on a much higher place in the pantheon of human virtues than it deserves. One of the most insidious but also widespread errors regarding IQ, especially among people who have high IQs, is the assumption that another person’s intelligence can be inferred from casual interactions. Many people conclude that if they see someone who is sensitive, humorous, and talks fluently, the person must surely have an above-average IQ.

This identification of IQ with attractive human qualities in general is unfortunate and wrong. Statistically, there is often a modest correlation with such qualities. But modest correlations are of little use in sizing up other individuals one by one. For example, a person can have a terrific sense of humor without giving you a clue about where he is within thirty points on the IQ scale. Or a plumber with a measured IQ of 100 — only an average IQ — can know a great deal about the functioning of plumbing systems. He may be able to diagnose problems, discuss them articulately, make shrewd decisions about how to fix them, and, while he is working, make some pithy remarks about the president’s recent speech.

At the same time, high intelligence has earmarks that correspond to a first approximation to the commonly understood meaning of smart. In our experience, people do not use smart to mean (necessarily) that a person is prudent or knowledgeable but rather to refer to qualities of mental quickness and complexity that do in fact show up in high test scores. To return to our examples: Many witty people do not have unusually high test scores, but someone who regularly tosses off impromptu complex puns probably does (which does not necessarily mean that such puns are very funny, we hasten to add). If the plumber runs into a problem he has never seen before and diagnoses its source through inferences from what he does know, he probably has an IQ of more than 100 after all. In this, language tends to reflect real differences: In everyday language, people who are called very smart tend to have high IQs.

All of this is another way of making a point so important that we will italicize it now and repeat elsewhere: Measures of intelligence have reliable statistical relationships with important social phenomena, but they are a limited tool for deciding what to make of any given individual. Repeat it we must, for one of the problems of writing about intelligence is how to remind readers often enough how little an IQ score tells about whether the human being next to you is someone whom you will admire or cherish. This thing we know as IQ is important but not a synonym for human excellence.

Charles Murray, “The Bell Curve Explained”, American Enterprise Institute, 2017-05-20.