According to Jensen (1998), the g factor is the highest-order common factor that can be extracted in a hierarchical factor analysis from a large battery of diverse tests of an assortment of cognitive abilities. Spearman thought of g as "mental energy" that could be applied to mental tasks (Jensen, 1998).
According to Spearman, "individual differences in performance on any mental task result from two sources: differences in the amount of mental 'energy' that can be delivered to the specific 'engine' that mediates performance of the task, and differences in the efficiency of energy utilization by the 'engine.' The efficiency of the various 'engines' differs independently within the same person" (Jensen, 1998, p. 19).
As a construct, the g factor can be represented with varying degrees of convenience, efficiency, and validity by a wide variety of vehicles (psychometric tests, laboratory techniques, physiological indices) (Jensen, 1998).
To obtain what Jensen (1998) called the information-processing speed of the brain, you use a device that contains a black button surrounded by a semicircle of eight white buttons.
The person being tested sits with her hand on the button box. She presses the black button on the box and waits for a beep; then, after a short but unpredictable interval, one of the eight white buttons lights up. As soon as that happens, the subject takes her finger off the black button and presses whichever of the white buttons is lit. Through a series of these exercises, a computer records, in milliseconds, how long it took her finger to leave the black button (reaction time) and how long to press the lighted white button (movement time).
After more than 25 years, Jensen and his colleagues developed quite a few similar tests, all of which have in common the measurement in milliseconds of how...