In 1912 the German psychologist Wilhelm Stern proposed dividing the mental age of a child by his chronological age to establish an overall indicator of intelligence. David Wechsler later developed the IQ tests most widely used today. The critics of IQ testing were quick to point out that IQ is an abstract concept that may have no bearing on the real world.
Intelligence is what the tests test.
The statement is fundamentally true, but the same could be said of all tests. However, no one would seriously argue that people who fail the driving test, on average, drive as well as those who have perfect scores. If a person scores below average in one type of mental-abilities test, he is likely to score below average in another type. Conversely, if he is above average in one, he is likely to score similarly high in another. Those who do well in reading, for instance, usually do well in math.