Today, the terms "intelligence test" and "IQ test" are often used interchangeably. Therefore, many people assume incorrectly that Binet came up with the idea of an intelligence quotient (IQ), a single number for expressing the overall result on an intelligence test. This distinction actually goes to German psychologist William Stern. In fact, Binet resisted the idea of reducing a person's intelligence to a single number. When Stern introduced the concept of IQ in 1912, Binet was no longer alive to complain. But his coauthor, Simon, later called the IQ concept a betrayal of their original ideas.
Nevertheless, Stern's concept caught on quickly; it involved some seemingly small but critical changes in the way Binet's test results were used. Binet had talked about the mental level of children who took his test. Stern recast this as mental age, which implied a more precise measurement scale. Then, Stern proposed that mental age could be divided by chronological age to yield a handy numerical score. In 1916, the American psychologist Lewis Terman suggested multiplying this score by 100 to get rid of fractions. For example, consider a seven-year-old child with a mental age of six. To calculate this child's IQ, an examiner would divide six by seven, then multiply the answer by 100. The child's IQ would be 86.
Most people would agree that a five-year-old with a mental age of three has a more serious delay than a 15-year-old with a mental age of 13. Using Binet's method, both children would simply be regarded as being two years behind. Using the IQ method, however, the differences in severity would be more obvious. The 15-year-old would have an IQ of 87 (in the normal intelligence range), while the five-year-old would have an IQ of only 60 (in the mentally retarded range). The 15-year-old would need to have a mental age of nine to get an IQ score that low. As this example shows, the use of IQs helped to equalize the scores for children who had roughly the same degree of mental retardation or normal intelligence, but who were of different ages.
Expressing intelligence as a single number also had other effects however. For one thing, it encouraged people to look at intelligence as a single entity, along the lines of Spearman's General Intelligence. For another thing, it gave researchers a number they could use in correlational studies. A flood of studies followed in which researchers looked at the association between "intelligence" (as measured by IQ tests) and an endless list of other variables. Yet many people had—and still have—grave doubts about whether something as complex as intelligence could really be boiled down into something as simple as a numerical score.
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