Intelligence is precisely and exactly what IQ tests measure.

A related misconception that people have is the claim that IQ is not a measure of general intelligence.  Some people believe in the concept of intelligence; they know that some people are more intelligent than others.  But they do not believe that IQ tests accurately measure individuals’ intelligence, once again, because IQ test scores typically show average differences between different groups and they believe that individuals from different groups on average must be equally intelligent.

Contrary to this view, intelligence researchers unanimously agree that intelligence is exactly what IQ tests measure, in the same way that your weight is exactly what your bathroom scale measures.  To maintain that intelligence is real and some people are more intelligent than others, yet IQ tests do not accurately measure intelligence is akin to claiming that weight is real and some people are heavier than others, but the bathroom scale does not accurately measure weight.  It simply does not make any sense.

In my last post, I say that Raven’s Progressive Matrices is the single best IQ test currently available, and that is true.  But there is actually a better way to measure someone’s general intelligence than Raven’s, and that is to administer a series of different cognitive tests.  The best way to assess someone’s level of general intelligence is to administer a large number of cognitive tests like vocabulary, verbal comprehension, arithmetic, digit span (to measure the ability to repeat a sequence of digits after it is given, sometimes exactly as it is given, sometimes backwards), spatiovisual rotation (to measure the ability to imagine what a three-dimensional object would look like if it is rotated in space), etc.

Across individuals, performance on all these cognitive tests are highly positively correlated.  In other words, people who do well in verbal comprehension tests tend also to do well on arithmetic tests, and they have better ability to visualize a three-dimensional object from a different angle or to repeat a sequence of digits that is given to them backwards.  Contrary to popular belief, people who are good with concrete tasks are also good with abstract tasks; people who are good with numbers are also good with words.

For example, in a classic paper published in 1904, Charles Spearman shows that students’ relative school performance in mathematics is highly correlated with their performance in classics (r = .87), French (r = .83), English (r = .78), pitch discrimination (r = .66) and music (r = .63).  (The “r” is a measure of statistical association between two variables, known in statistics as the correlation coefficient.  It varies from -1, when the two variables are perfectly negatively correlated, through 0 when they are completely unrelated to each other, to +1 when they are perfectly positively correlated.  As you can see, all of the correlations reported by Spearman are very highly positive.)  In fact, the students’ relative performance in music is more highly correlated with their mathematical ability than with their pitch discrimination (r = .40)!

As I note in my next post, the correlation between true blood pressure and blood pressure measured by the sphygmomanometer is about .50.  It means that using one’s relative performance in mathematics to measure one’s relative performance in music ability is more accurate than using the sphygmomanometer to measure blood pressure.  That is how high all measures of cognitive abilities are intercorrelated.

What psychometricians (whose job it is to measure intelligence accurately and to devise tests to do so) do then is to subject individual scores on all these cognitive tests to a statistical technique called factor analysis.  What factor analysis does is to analyze the correlations between all pairs of cognitive tests and then measure an individual’s latent cognitive ability that underlie their performance on all of the cognitive tests.  This latent cognitive ability is general intelligence.  Factor analysis also eliminates all random measurement errors that are inevitably associated with any individual cognitive test as a measure of intelligence.  So it can measure general intelligence purely, without any random measurement errors.

The IQ score thus obtained is a pure measure of intelligence.  It measures someone’s ability to think and reason in various contexts and situations, such as numerical manipulations like arithmetic, verbal comprehension like reading, and mental visualization like spatiovisual rotation.  Believe it or not, all these cognitive abilities have something in common, and that something is general intelligence.  So intelligence is precisely and exactly what IQ tests measure.  Intelligence is what allows us to perform on all kinds of cognitive tests.

Follow me on Twitter:  @SatoshiKanazawa

Read more about the surprising facts about intelligence, what it is (and what it is not), how it affects you in virtually every sphere of life, and how more intelligent people are different from less intelligent people, in The Intelligence Paradox:  Why the Intelligent Choice Isn’t Always the Smart One.