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Monday, March 27, 2006

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'g' and its measures

I am one of those awful people who mention IQ in public from time to time so I thought a few quick words of background on it might not go astray:

IQ tests exist because 'g' (general intelligence) exists. It just is a fact that people who are good at solving one sort of problem tend to be good at solving lots of other different problems. Some problems, however, are particularly good at detecting people who are generally good at solving problems. Researchers speak of such "good-predictor" problems as ones that "load highly on 'g'". And, pesky though it may be, the problems that load most highly on 'g' (i.e. the ones that are the purest measure of general intelligence) are also the ones that differentiate blacks and whites most strongly. As Charles Murray explains:

"As long ago as 1927, Charles Spearman, the pioneer psychometrician who discovered 'g', proposed a hypothesis to explain the pattern: the size of the black-white difference would be "most marked in just those [subtests] which are known to be saturated with g." In other words, Spearman conjectured that the black-white difference would be greatest on tests that were the purest measures of intelligence, as opposed to tests of knowledge or memory.

A concrete example illustrates how Spearman's hypothesis works. Two items in the Wechsler and Stanford-Binet IQ tests are known as "forward digit span" and "backward digit span." In the forward version, the subject repeats a random sequence of one-digit numbers given by the examiner, starting with two digits and adding another with each iteration. The subject's score is the number of digits that he can repeat without error on two consecutive trials. Digits-backward works exactly the same way except that the digits must be repeated in the opposite order.

Digits-backward is much more 'g'-loaded than digits-forward. Try it yourself and you will see why. Digits-forward is a straightforward matter of short-term memory. Digits-backward makes your brain work much harder. The black-white difference in digits-backward is about twice as large as the difference in digits-forward. It is a clean example of an effect that resists cultural explanation. It cannot be explained by differential educational attainment, income, or any other socioeconomic factor. Parenting style is irrelevant. Reluctance to "act white" is irrelevant. Motivation is irrelevant. There is no way that any of these variables could systematically encourage black performance in digits-forward while depressing it in digits-backward in the same test at the same time with the same examiner in the same setting".

The Lynn/Flynn effect

What I have put up so far up helps us to understand something else: The Lynn effect -- which is usually called the Flynn effect -- after its main publicist -- James Flynn. It was however discovered by Richard Lynn. What it refers to is the fact that average IQ test scores rose steadily thoroughout the 20th century. The process may however have reached an asymptote (peaked) now.

For a long time, I accepted that the rising scores represented a real increase in 'g' (underlying general intelligence). Two of my three very minor contributions to the academic literature on IQ were based on that view (See here and here). There were good reasons for that view. Nutrition does have a (usually small) effect on IQ and nutrition did improve over the 20th century. IQ can be influenced in some ways by general health -- and medical care did improve over the 20th century -- with improved perinatal care being an obvious candidate for positive effects. And signs that people were in fact better off physically were there -- an increase in average brain size particularly. And brain size does correlate to a degree (correlation of around .40) with IQ.

BUT: It seems that I was wrong. Although scores on all sorts of IQ subtests (puzzle categories) rose, they did not rise evenly. And the scores that rose least were for those problems that loaded most highly on 'g' (See e.g. here). The implication is that scores on a perfect measure of 'g' would not have risen at all.

So how do we explain the Lynn effect? There is no general agreement but I find the commonest explanation to be pretty persuasive -- that it reflects increasing test sophistication. Kids now spend MANY more years in the educational system than they once did and although there is probably little to show for that overall, kids DO get a lot of practice in passing tests of various sorts. And practice may not make perfect but it would be surprising if test-taking skills and strategies (such as guessing when you are not certain) were not improved by many years of extra practice at taking such tests.

(For more postings from me, see EDUCATION WATCH, GREENIE WATCH, POLITICAL CORRECTNESS WATCH, GUN WATCH, SOCIALIZED MEDICINE, AUSTRALIAN POLITICS and DISSECTING LEFTISM. My Home Page. Email me (John Ray) here.)

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posted by JR at 3:53 PM