30-2 When and why were intelligence tests created, and how do today’s tests differ from early intelligence tests?

Some societies concern themselves with promoting the collective welfare of the family, community, and society. Other societies emphasize individual opportunity. Plato, a pioneer of the individualist tradition, wrote more than 2000 years ago in The Republic that “no two persons are born exactly alike; but each differs from the other in natural endowments, one being suited for one occupation and the other for another.” As heirs to Plato’s individualism, people in Western societies have pondered how and why individuals differ in mental ability.

Western attempts to assess such differences began in earnest with English scientist Francis Galton (1822–1911), who was fascinated with measuring human traits. When his cousin Charles Darwin proposed that nature selects successful traits through the survival of the fittest, Galton wondered if it might be possible to measure “natural ability” and to encourage those of high ability to mate with one another. At the 1884 London Health Exhibition, more than 10,000 visitors received his assessment of their “intellectual strengths” based on such things as reaction time, sensory acuity, muscular power, and body proportions. But alas, on these measures, well-regarded adults and students did not outscore others. Nor did the measures correlate with each other.

Although Galton’s quest for a simple intelligence measure failed, he gave us some statistical techniques that we still use (as well as the phrase nature and nurture). And his persistent belief in the inheritance of genius—reflected in his book, Hereditary Genius—illustrates an important lesson from both the history of intelligence research and the history of science: Although science itself strives for objectivity, individual scientists are affected by their own assumptions and attitudes.

Modern intelligence testing traces its birth to early twentieth-century France, where a new law required all children to attend school. French officials knew that some children, including many newcomers to Paris, would struggle and need special classes. But how could the schools make fair judgments about children’s learning potential? Teachers might assess children who had little prior education as slow learners. Or they might sort children into classes on the basis of their social backgrounds. To minimize such bias, France’s minister of public education gave Alfred Binet and others, including Théodore Simon, the task of studying this problem.

In 1905, Binet and Simon first presented their work under the archaic title, “New Methods for Diagnosing the Idiot, the Imbecile, and the Moron” (Nicolas & Levine, 2012). They began by assuming that all children follow the same course of intellectual development but that some develop more rapidly. On tests, therefore, a “dull” child should score much like a typical younger child, and a “bright” child like a typical older child. Thus, their goal became measuring each child’s mental age, the level of performance typically associated with a certain chronological age. The average 9-year-old, then, has a mental age of 9. Children with below-average mental ages, such as 9-year-olds who perform at the level of typical 7-year-olds, would struggle with age-appropriate schoolwork. A 9-year-old who performs at the level of typical 11-year-olds should find schoolwork easy.

To measure mental age, Binet and Simon theorized that mental aptitude, like athletic aptitude, is a general capacity that shows up in various ways. They tested a variety of reasoning and problem-solving questions on Binet’s two daughters, and then on “bright” and “backward” Parisian schoolchildren. The items they developed eventually predicted how well French children would handle their schoolwork.

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Binet and Simon made no assumptions concerning why a particular child was slow, average, or precocious. Binet personally leaned toward an environmental explanation. To raise the capacities of low-scoring children, he recommended “mental orthopedics” that would help develop their attention span and self-discipline. He believed his intelligence test did not measure inborn intelligence as a scale measures weight. Rather, it had a single practical purpose: to identify French schoolchildren needing special attention. Binet hoped his test would be used to improve children’s education, but he also feared it would be used to label children and limit their opportunities (Gould, 1981).

Binet’s fears were realized soon after his death in 1911, when others adapted his tests for use as a numerical measure of inherited intelligence. This began when Stanford University professor Lewis Terman (1877–1956) found that the Paris-developed questions and age norms worked poorly with California schoolchildren. Adapting some of Binet’s original items, adding others, and establishing new age norms, Terman extended the upper end of the test’s range from teenagers to “superior adults.” He also gave his revision the name it retains today—the Stanford-Binet.

From such tests, German psychologist William Stern derived the famous intelligence quotient, or IQ. The IQ is simply a person’s mental age divided by chronological age and multiplied by 100 to get rid of the decimal point:

Thus, an average child, whose mental and chronological ages are the same, has an IQ of 100. But an 8-year-old who answers questions as would a typical 10-year-old has an IQ of 125.

The original IQ formula worked fairly well for children but not for adults. (Should a 40-year-old who does as well on the test as an average 20-year-old be assigned an IQ of only 50?) Most current intelligence tests, including the Stanford-Binet, no longer compute an IQ in this manner (though the term IQ still lingers as a shorthand expression for “intelligence test score”). Instead, they represent the test-taker’s performance relative to the average performance of others the same age. This average performance is arbitrarily assigned a score of 100, and about two-thirds of all test-takers fall between 85 and 115.

Terman (1916, p. 4) promoted the widespread use of intelligence testing to “take account of the inequalities of children in original endowment” by assessing their “vocational fitness.” In sympathy with Francis Galton’s eugenics—the much-criticized nineteenth-century movement that proposed measuring human traits and using the results to encourage only smart and fit people to reproduce—Terman envisioned that the use of intelligence tests would “ultimately result in curtailing the reproduction of feeble-mindedness and in the elimination of an enormous amount of crime, pauperism, and industrial inefficiency” (p. 7).

With Terman’s help, the U.S. government developed new tests to evaluate both newly arriving immigrants and World War I army recruits—the world’s first mass administration of an intelligence test. To some psychologists, the results indicated the inferiority of people not sharing their Anglo-Saxon heritage. Such findings were part of the cultural climate that led to a 1924 immigration law that reduced Southern and Eastern European immigration quotas to less than a fifth of those for Northern and Western Europe.

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Binet probably would have been horrified that his test had been adapted and used to draw such conclusions. Indeed, such sweeping judgments became an embarrassment to most of those who championed testing. Even Terman came to appreciate that test scores reflected not only people’s innate mental abilities but also their education, native language, and familiarity with the culture assumed by the test. Abuses of the early intelligence tests serve to remind us that science can be value-laden. Behind a screen of scientific objectivity, ideology sometimes lurks.

Psychologist David Wechsler created what is now the most widely used individual intelligence test, the Wechsler Adult Intelligence Scale (WAIS), together with a version for school-age children (the Wechsler Intelligence Scale for Children [WISC]), and another for preschool children (Evers et al., 2012). The latest (2008) edition of the WAIS consists of 15 subtests, including these:

The WAIS yields not only an overall intelligence score, as does the Stanford-Binet, but also separate scores for verbal comprehension, perceptual organization, working memory, and processing speed. Striking differences among these scores can provide clues to cognitive strengths or weaknesses. For example, a low verbal comprehension score combined with high scores on other subtests could indicate a reading or language disability. Other comparisons can help a psychologist or psychiatrist establish a rehabilitation plan for a stroke patient. In such ways, these tests help realize Binet’s aim: to identify opportunities for improvement and strengths that teachers and others can build upon. Such uses are possible, of course, only when we can trust the test results.