G theory

From Academic Kids

The title of this article is incorrect because of technical limitations. The correct title is g theory.

g theory, in the field of psychology (see also psychometrics), is the general factor common to the scores of all intelligence tests.


History of g

Missing image
An illustration of Spearman's two-factor intelligence theory. Each small oval is a hypothetical mental test. The blue areas show the variance attributed to s, and the purple areas the variance attributed to g.

g was first identified by Charles Spearman. He proposed a model where all variation in intelligence test scores can be explained by two factors. The first is the factor specific to an individual mental task: the individual abilities that would make a person more skilled at one cognitive task than another. The second is g, a general factor that governs performance on all cognitive tasks. Spearman's theory proved too simple, however, as it ignored group factors in test scores that arise through factor analysis.

Mental testing and g

The abstraction of g stems from the observation that scores on all forms of cognitive tests correlate positively with one another. g can be derived as the principle factor from cognitive test scores using the method of principal components analysis or factor analysis.

Tests of cognitive ability derive most of their validity from the extent to which they measure g. If quantifiable measures of the performance of a task correlate highly with g, it is said to be g-loaded. Creators of IQ tests, whose goals are generally to create highly reliable and valid tests, have thus made their tests as g-loaded as possible. Historically, this has meant dampening the influence of group factors by testing as wide a range of mental tasks as possible. However, tests such as Raven's Progressive Matrices are considered to be the most g-loaded in existence, even though Raven's is quite homogeneous in the types of tasks comprising it.

Elementary cognitive tasks (ECTs) also correlate strongly with g. ECTs are, as the name suggests, simple tasks that apparently require very little intelligence, but still correlate strongly with more exhaustive intelligence tests. Determining whether a light is red or blue and determining whether there are four or five squares drawn on a computer screen would both be examples of ECTs. The answers to such questions are usually provided by quickly pressing buttons. Often, in addition to buttons for the two options provided, a third button is held down from the start of the test. When the stimulus is given to the subject, he removes his hand from the starting button to the button of the correct answer. This allows the examiner to determine how much time was spent thinking about the answer to the question (reaction time, usually measured in small fractions of second), and how much time was spent on physical hand movement to the correct button (movement time). Reaction time correlates strongly with g, while movement time correlates less strongly.

ECT testing has allowed quantitative examination of hypotheses concerning test bias, subject motivation, and group differences. By virtue of their simplicity, ECTs provide a link between classical IQ testing and biological inquiries such as fMRI studies.

Biological correlates of g

g has a large number of biological correlates, and they are rapidly being discovered by new research. Strong correlates include mass of the prefrontal lobe, overall brain mass, and glucose metabolization rate within the brain. g correlates less strongly, but significantly, with overall body size. There is conflicting evidence regarding the correlation between g and peripheral nerve conduction velocity, with some reports of significant positive correlations, and others of no or even negative correlations.

Current research suggests that broad-sense heritability of g is between 0.5 and 0.8, and narrow-sense heritability approximately 0.3, but the causal pathways are currently unknown. The heritability of most test performance is primarily a result of g.

Brain size has long been known to correlate with g (Jensen, 1998). Recently, an MRI study on twins (Thompson et al., 2001) showed that frontal gray matter volume was highly significantly correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002). g has been observed in mice as well as humans (Matzel et al 2003).

g is probably limited by the channel capacity of short-term memory. Mental power, or the capacity C of short-term memory (measured in bits of information), is the product of the individual mental speed Ck of information processing (in bit/s) (see the external link below to the paper by Lehrl and Fischer (1990)), and the duration time D (in s) of information in short-term working memory, meaning the duration of memory span. Hence:

C (bit) = Ck(bit/s) × D (s).

Social correlates of g

g positively correlates with measures of success (academic achievement, job performance, career prestige) and negatively correlates with various social pathologies (school dropout, illegitimate childbearing, poverty). Differences in g between ethnic groups (see race and intelligence) have sparked public controversy.

The Flynn effect and g

Several studies have shown increases in g from the Flynn effect. Another sign of this is the change in cranial vault size and shape in the US during the last 150 years.

Challenges to g

The late Stephen Jay Gould voiced his objections to the concept of g, as well as intelligence testing in general, in his book The Mismeasure of Man.


  • Jensen, R.A. (1998) The g Factor. Praeger, Connecticut, USA.
  • Matzel, L.D., Han, Y.R., Grossman, H., Karnik, M.S., Patel, D., Scott, N., Specht, S.M., Gandhi, C.C. (2003) Individual differences in the expression of a "general" learning ability in mice. Journal of Neuroscience, 23(16):6423-33.
  • Posthuma, D., De Geus, E.J., Baare, W.F., Hulshoff Pol, H.E., Kahn, R.S., Boomsma, D.I. (2002) The association between brain volume and intelligence is of genetic origin. Nature Neuroscience, 5(2):83-4.
  • Thompson, P.M. et al. (2001). Genetic influences on brain structure. Nature Neuroscience, 4(12):1253-1258.

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