General intelligence

Spearman's early research

The concept of "general intelligence" first arose from Spearman's 1904 paper "'General Intelligence', Objectively Determined and Measured", where he applied his new statistical methods for correcting correlations to tests of ability to propose a two-factor theory of intelligence. Based on the observation that tests of ability typically positively correlate with each other, he proposed that these tests all measure the same thing—general intelligence—and that individual tests measure a combination of two factors: 'general intelligence' (g), common to all tests, and a 'specific ability' (s), specific to one test. This concept of "general intelligence" was supposed to provide an undisputed definition of intelligence which could be described as "objectively determined and measured". There were several corollaries of his theory, such as the claim that it is "possible to rank order the measures in terms of their g-to-s ratio". The most important was Spearman's law of tetrad differences, demonstrated by Spearman in 1924. It states that the pairwise products of two sets of correlations are equal–that is, their difference is zero. For four traits labeled 1, 2, 3, 4, this is r₁₃⋅r₂₄-r₂₃r₁₄=0. This is equivalent to the prediction that for a correlation matrix statistically removing the common factor "g" would yield a matrix of partial correlations that are all 0.