Charles Spearman and the
Measurement of Intelligence
A real breakthrough occurred around 1900 when there was a debate about the nature of human intelligence. A large battery of so-called psychometric tests, mostly of the questionnaire and puzzle type, had been devised in order to “look into the mind” and “measure” intelligence. Most of these tests are highly correlated with one another, implying that part of the information they contain is the same. In other words, the multiple measurement express a common factor of intelligence.
Around 1904 the British psychometrician Charles Spearman defined the Single-Factor theory of intelligence. The search for a common factor in the results of a battery of tests is called factor analysis. It led to the definition of the Intelligence Quotient or IQ as a general factor of human intelligence, which is still much in use (and sometimes abuse) today. A limitation of early factor analysis was that it only accounted for one and only one common factor.