Illustration of Spearman's two-factor intelligence theory.

I, Schaefer, made this image in Photoshop, based on a similar illustration appearing on page 214 of Arthur Jensen's Bias in Mental Testing. The small ovals represent tests of cognitive ability, while the red circle represents psychometric g. The blue portions of each test show the amount of test variance attributed to s, or the specific test factor, by Charles Spearman in his two-factor theory of cognitive abilities. The purple areas show the amount of test variance attributed to g. An ideal IQ test would be entirely purple, with no blue. If a test were to be entirely blue, with no purple, it might have measured some other mental aspect (e.g., personality or sensory abilities) but would be unrelated to general intelligence.

This model of intelligence is no longer used by psychometricians as it ignores group factors in test scores. This chart should be of purely historical interest.