The best way to understand why I.Q.s rise, Flynn argues, is to look at one of the most widely used I.Q. tests, the so-called WISC (for Wechsler Intelligence Scale for Children). The WISC is composed of ten subtests, each of which measures a different aspect of I.Q. Flynn points out that scores in some of the categories—those measuring general knowledge, say, or vocabulary or the ability to do basic arithmetic—have risen only modestly over time. The big gains on the WISC are largely in the category known as “similarities,” where you get questions such as “In what way are ‘dogs’ and ‘rabbits’ alike?” Today, we tend to give what, for the purposes of I.Q. tests, is the right answer: dogs and rabbits are both mammals. A nineteenth-century American would have said that “you use dogs to hunt rabbits.”Blogger over at Mindhacks explains why this is relevant to the debate:
Flynn shows what happens when we recognize that I.Q. is not a freestanding number but a value attached to a specific time and a specific test. When an I.Q. test is created, he reminds us, it is calibrated or “normed” so that the test-takers in the fiftieth percentile—those exactly at the median—are assigned a score of 100. But since I.Q.s are always rising, the only way to keep that hundred-point benchmark is periodically to make the tests more difficult—to “renorm” them. The original WISC was normed in the late nineteen-forties. It was then renormed in the early nineteen-seventies, as the WISC-R; renormed a third time in the late eighties, as the WISC III; and renormed again a few years ago, as the WISC IV—with each version just a little harder than its predecessor. The notion that anyone “has” an I.Q. of a certain number, then, is meaningless unless you know which WISC he took, and when he took it, since there’s a substantial difference between getting a 130 on the WISC IV and getting a 130 on the much easier WISC.
IQ is designed so it always has a mean of 100 and a standard deviation of 15. However, during the past decades people have been scoring better on IQ tests, something known as the Flynn effect, meaning the new versions have been re-adjusted to make sure the mean stays at 100.
This is important, because it means that comparing IQ from the 1950s is not a far comparison to IQs from the 2000s, because they use tests with different standards.
Some of the people who argued that certain races are more intelligent than others have failed to include these changes in their calculations, and, as Gladwell points out, when these are accounted for, many of these differences completely disappear.