Measuring intelligence

Almost all of the theories discussed above have in common the use of fairly complex tasks for gauging intelligence in both children and adults. Some of these tasks have been explicitly discussed--for example, those requiring recognition of analogies, classification of similar terms, extrapolation of number series, performance of transitive inferences, and the like. How did theorists of intelligence come to use these tasks in preference to the myriad other possibilities? That question is answered through the discussion of the measurement of intelligence.

Although the kinds of complex tasks that have been discussed above fall into a single tradition for the measurement of intelligence, the field actually has two major traditions. The tradition that has been discussed most prominently and has been most influential is that of the French psychologist Alfred Binet. But an earlier tradition, and one that still shows some influence upon the field, is that of the English scientist Sir Francis Galton.

The publication in 1859 of Charles Darwin's The Origin of Species had a profound effect on many lines of scientific endeavour. The book suggested that the capabilities of humans are, in some sense, continuous with those of lower animals and, hence, can be understood through scientific investigation. One person who was strongly influenced by Darwin's thinking was his cousin Sir Francis Galton. For seven years--from 1884 to 1890--Galton maintained a laboratory at the South Kensington Museum in London, where, for a small fee, visitors could have themselves measured on a variety of psychophysical tasks, such as weight discrimination and sensitivity to musical pitch. But Galton believed that these tests measured more than just psychophysical abilities. He believed that psychophysical abilities were the basis of intelligence and, hence, that his tasks were measures of intelligence. The earliest formal intelligence test, therefore, required a person to perform such simple tasks as deciding which of two weights was heavier or showing how forcefully he could squeeze his hand. The Galtonian tradition was taken to the United States by the psychologist James McKeen Cattell. One of Cattell's students, Clark Wissler, collected data showing that scores on Galtonian types of tasks were not good predictors of grades in college, or even of each other. Cattell continued his work in psychometric research, however, and with Edward L. Thorndike developed the principal facility in the United States for mental testing and measurement.

The IQ test

The more influential tradition of mental testing was developed by Binet and his collaborator, Theodore Simon, in France. In 1904 the minister of public instruction in Paris named a commission to study or create tests that would insure that mentally retarded children received an adequate education. The minister was also concerned that certain children were being placed in classes for the retarded not because they were retarded but because they had behaviour problems, and teachers did not want them in their classrooms. Even before Wissler's research, Binet, who was charged with developing the new test, had flatly rejected the Galtonian tradition, believing that Galton's tests measured fairly trivial abilities. He proposed instead that tests of intelligence should measure skills such as judgment, comprehension, and reasoning--the same kinds of skills measured on most intelligence tests today. Binet's early test was taken to the United States by a Stanford University psychologist, Lewis Terman, whose version came to be called the Stanford-Binet test. This test has been revised frequently and continues in use.

The Stanford-Binet test, and others like it, have traditionally yielded at the very least an overall score referred to as an intelligence quotient, or IQ. Some tests, such as the Wechsler Adult Intelligence Scale (Revised) and the Wechsler Intelligence Scale for Children (Revised) yield an overall IQ as well as separate IQs for verbal and performance subtests. An example of a verbal subtest would be vocabulary, whereas an example of a performance subtest would be picture arrangement, the latter requiring an examinee to arrange a set of pictures into a sequence so that they tell a comprehensible story.

IQ was originally computed as the ratio of mental age to chronological (physical) age, multiplied by 100. Thus, if a child of 10 had a mental age of 12 (that is, performed on the test at the level of an average 12-year-old), the child was assigned an IQ of (12/10) x 100, or 120. If the 10-year-old had a mental age of eight, the child's IQ would be (8/10) x 100, or 80. A score of 100, where the mental age equals the chronological age, is average.

As discussed above, the concept of mental age has fallen into disrepute, and few tests continue to involve the computation of mental ages. Many tests still yield an IQ, but they are most often computed on the basis of statistical distributions. The scores are assigned on the basis of what percentage of people of a given group would be expected to have a certain IQ.

Contents of this article:

Introduction
Cognitive-contextual theories and biologic theories
Hemispheric studies
Development of intelligence
Post-Piaget theories and the environmental viewpoint
Measuring intelligence
The distribution of IQ scores and the malleability of intelligence