The G Factor: The Science of Mental Ability

By Arthur R. Jensen | Go to book overview

At this level of analysis, this general factor will forever appear unitary, although it is actually the result of two separate processes, A and B.

To show that the general factor involves individual differences in two independent processes, A and B, and is therefore not fundamentally unitary would require that individual differences in A and B be measured separately and that A and B are each independently correlated with the general factor of the psychometric tests. The more difficult condition to satisfy (which has been the basis of contention on this issue) is that it must be assumed that the empirical g factor scores derived from the tests are "pure" g uncontaminated by any non-g "impurities" that constitute some of the variance in the measures of processes A or B. Because it is virtually impossible to prove definitively that the g factor scores are "pure" in this sense, the issue retreats from the scientific arena, and it then becomes a purely metaphysical question whether g is or is not unitary. However, the fact that g has all the characteristics of a polygenic trait (with a substantial component of nongenetic variance) and is correlated with a number of complexly determined aspects of brain anatomy and physiology, as indicated in Chapter 6, makes it highly probable that g, though unitary at a psychometric level of analysis, is not unitary at a biological level.


NOTES
1.
The defining criteria of a ratio scale are that it has: (1) a true zero point and (2) units of measurement that represent equal intervals or increments of the thing or dimension being measured. An ordinary thermometer (whether centigrade or Fahrenheit), for example, is an equal-interval scale, but it is not a ratio scale, because it lacks a true zero point. In the case of the centigrade thermometer, the C point is arbitrarily set at the freezing point of distilled water at sea level; 100° C is set at the boiling point. The true zero point is 273° below C, which is the absolute limit of "coldness." The Kelvin thermometer, which is a ratio scale, measures temperature from its absolute zero point, so that the freezing point of water is 273° K. Ratios between the measurements are meaningless on any scale that is not a true ratio scale. For example, 100° C does not represent twice as much heat as 50° C, because on the absolute Kelvin scale these temperatures are 373°K/323°K = 1.15, not 2. Similarly, an IQ of 100 does not represent twice as much "intelligence" as an IQ of 50. Statements often seen in the popular press, such as "children have developed half of their adult level of intelligence by the age of four," are wholly nonsensical unless "intelligence" can be measured on one and the same equal-interval ratio scale at age four and at maturity. At present there is no psychometric test that has the ratio scale properties that could justify such a statement. The actual shape of the mental growth curve (beyond saying that it is an increasing monotonic function of age, from infancy to maturity) cannot be known without ratio scale measurements of mental ability. However, a true growth curve of increasing ability could be plotted from measurements based on time (e.g., "X milliseconds to process one bit of information"). The question then is whether the ratio scale measurements based on time adequately represent a construct of scientific or practical importance as indicated by significant relations to other phenomena of interest, such as psychometric g.
2.
Johnson et al. ( 1985) analyzed virtually all of Galton's data by means of the

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