© 2003 Paul Cooijmans
One of the main questions in high-range IQ testing is: Do high-range tests measure g, the general factor in mental tests? As long as this has not been answered, I will use the term q for the (possible) general factor in high-range tests. On this web page I will mention the differences I find between q and g.
A key fact in this are the known sex differences in intelligence, discussed by Arthur Jensen in his book The g factor, chapter "Sex differences in g". If we combine the known facts with the observation that high-range testees are for about 80% males, the following can be said:
Males have on average considerably higher spatial ability than females. Also it is known spatial ability correlates lower with g than do verbal and numerical ability. Considering this, it is clear to me that this lower correlation is a direct result of the sex difference in spatial ability. A trait that is not equally divided over the sexes will, because of that, have a lower correlation with the general factor.
Males show greater variance in IQ than females, so that at the high end we find more males than females. This fits with the observation that there are more male than female high-range testees. But it also implies that, at the high end, the correlation between spatial ability and g - here called q - will rise again. Because the sex difference in spatial ability loses its reducing effect here, simply because there are fewer and fewer females, the higher you get. This implication - the rise of the correlation between spatial ability and q as you go up - will have to be verified, even though it is an inevitable effect.
From the previous two paragraphs it follows that q is not identical to g; It contains more spatial ability. And this piece of reasoning can be repeated for the other ability types that show a sex difference, although the other differences are smaller than the difference in spatial ability. Numerical ability shows a small difference favouring males, and some types of verbal ability show a difference favouring females.
One way of describing the difference between q and g would be to say q is "male g", whereas g itself is a sex-combined factor. And because of the difference in variance between the sexes, g can only express itself and therefore be measurable over a limited range, around IQ 100. The further above 100, the less g will there be in the variance of test scores, and the more q.