Intelligence

Intelligence as One Thing

In 1904, Charles Spearman published an important analysis on the nature of mental abilities. For some time, researchers had understood that tests of mental abilities tended to correlate positively. In other words, people who did well on one test of mental ability tended to do well on others; this finding was termed positive manifold. Through a statistical method called factor analysis, Spearman identified a single factor that accounted for this positive manifold. He termed this factor the general factor or simply g. Spearman indicated that individual differences in cognitive ability measured in a battery of tests could be decomposed into two broad parts: the shared variance (g) and the specific variance of the tests (s).

Spearman maintained that g was a statistical property of a test battery and emerged only in individual differences. In other words, g is based on correlations between tests. Correlations cannot be calculated in a single individual, so a person does not have g. This statistical understanding does not automatically exclude a psychological variable operating at the level of the individual to explain g. At various times, researchers have proposed speed of processing, nerve conduction velocity, executive control of attention, working memory capacity, fluid reasoning, and others as psychological underpinnings of a general mental ability represented by g.

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