Dr Manju Antil (Counselling Psychologist and Psychotherapist)

Binet revised, expanded, and refined his first scale in 1908 and 1911. Its scoring developed into a system in which credit for items passed was given in terms of years and months so that a mental level could be calculated to represent the quality of the performance. In 1911, a German psychologist named William Stern proposed that the mental level attained on the Binet-Simon scale, now labelled a "mental age score," be divided by the subject's chronological age to obtain a "mental quotient" that would more accurately represent ability at different ages. To eliminate the decimal, the mental quotient was multiplied by 100 and soon became known as the intelligence quotient or IQ. 

This now-familiar score, a true ratio IQ was popularized in the most famous revision of the Binet-Simon scale, the Stanford-Binet Intelligence Scale, published in 1916 by Lewis Terman. Despite several problems with the ratio IQ, its use would last for several decades until a better way of integrating age into the scoring of intelligence tests (described in Chapter 3) was devised by David Wechsler (Kaufman, 2000; Wechsler, 1939). Binet’s basic idea—namely, that to be average, below average, or above average in intelligence means that one performs at, below, or above the level typical for one’s age group on intelligence tests—has survived and become one of the primary ways in which intelligence is assessed. 

While Binet was developing his scales in France, in England, Charles Spearman (a former student of Wundt’s and follower of Galton) had been trying to empirically prove that hypothesis concerning the link between intelligence and sensory acuity. In the process, he developed and expanded the use of correlational methods pioneered by Galton and Karl Pearson and provided the conceptual foundation for factor analysis, a technique for reducing a large number of variables to a smaller set of factors that would become central to the advancement of testing and trait theory. 

Spearman also devised a theory of intelligence that emphasized a general intelligence factor (or g) present in all intellectual activities (Spearman, 1904a, 1904b). He had been able to gather moderate support for Galton’s notions by correlating teachers’ ratings and grades with measures of sensory acuity, but he soon realized that the tasks assembled in the Binet-Simon scale provided a far more useful and reliable way of assessing intelligence than the tools he had been using. Even though Spearman and Binet differed widely in their views about the nature of intelligence, their combined contributions were unsurpassed in propelling the development of psychological testing in the 20th century.