IQ

IQ stands for intelligence quotient, which is a type of test score obtained from certain intelligence tests. IQ is often used simply as a synonym for general intelligence. This entry describes the meaning and use of the intelligence quotient within the history of intelligence test development.

In France at the beginning of the 20th century, Alfred Binet and Theodore Simon were asked by the French government to identify French children with [Page 495]mental retardation so that they could be given an appropriate education. Binet and Simon developed the first modern intelligence test, the 1905 Binet-Simon intelligence scale. This 30-item test was organized so that successive items were more difficult than previous items. By standardizing the scale on groups of children at each age, Binet and Simon derived normative scores for different ages. In this way, a child's mental ability could be compared with average performance for that age. Various degrees of mental retardation could be identified at each age level. Binet and Simon used the term mental level to describe performance on this scale, though the term mental age became more popular.

Binet-Simon test items were arranged according to an age scale. Different types of test items were grouped together by the age at which most children were successful with those items. Passing most or all of the items at an age level resulted in a score of that mental level. Children who answered items at higher age levels earned additional credit in months above the baseline mental age. Binet and Simon revised this scale in 1908 and 1911 by adding items and increasing the age range.

Lewis M. Terman, a professor at Stanford University, conducted a major revision of the Binet-Simon test in 1916, after Binet's death. Terman renamed it the Stanford Revision and Extension of the Binet-Simon Scale, which came to be known as the Stanford-Binet Intelligence Scale. Scoring was modified to incorporate the concept of the mental quotient as suggested by William Stern, a German psychologist, in 1912. The mental quotient equaled the mental age divided by the chronological age.

Terman expanded this concept by multiplying the mental quotient by 100 to obtain the intelligence quotient, or IQ. A child whose mental age and chronological age were the same had an IQ score of 100. The new ratio score transformed all intelligence test results to the same scale. Binet and Simon had been able to compare a single child to other children of the same age by comparing mental ages, but Terman's introduction of the IQ score allowed the comparison of children of different ages.

Technical difficulties in the use of the IQ score ratio soon emerged. The standard deviation, or the amount of variability, of IQ scores differed with age. This meant that any score other than 100 could mean different things at different ages. For example, the standard deviation for 6-year-old children might be larger than the standard deviation for 8-year-old children. Therefore, a 6-year-old child with an IQ score of 110 would be less advanced compared with same-age peers than would an 8-year-old child with an IQ score of 110. The 8-year-old with the identical IQ score would actually be more different from the average 8-year-old because of lower variability among 8-year-olds.

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