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time, sensitivity to pain, and weight discrimination, using the term “mental test.” Although Galton and Cattell’s measurements correlated poorly with reasoning ability, they drew attention to the systematic study of measuring cognitive and behavioral differences among individuals. At about the same time, French psychologist Alfred Binet was hired by the French government to identify children who would not benefit from a traditional school setting and those who would benefit from special education. He thought intelligence could be measured by sampling performance of tasks that involved memory, comprehension, and judgment. He collaborated with Theodore Simon to create the Binet-Simon scale, which he meant to be used only for class placement.

Alfred Binet’s Measurement of Judgment

Binet thought that as we age, we become more sophisticated in the ways we know about the world and that, therefore, most 6-year-olds answer questions differently from 8-year-olds. As a result of their responses to test items, children were assigned a mental age or mental level reflecting the age at which typical children give those same responses. Although mental age differentiates between abilities of children, it can be misleading when a 6-year-old and an 8-year-old, for example, have mental ages 2 years below their actual (chronological) ages. The younger child would be proportionally further behind peers than the older child. German psychologist William Stern suggested using the ratio of mental age (MA) to chronological age (CA) to determine the child’s level of intelligence.

Mental Age and the Intelligence Quotient

In adapting Binet’s test for Americans, Lewis Terman developed the Stanford-Binet Intelligence Scale reporting results as an IQ, intelligence quotient, which is the child’s mental age divided by his/her chronological age, multiplied by 100; or MA/CA × 100. A 10-year-old who answers questions typical of most 12-year-olds has an IQ score of 120. Another 10-year-old youngster who answers questions typical of an 8-year-old scores 80. With the development of intelligence tests for adults, the ratio IQ becomes meaningless and has been replaced by the deviation IQ determined as a result of the standardizing process for a particular test. For the fifth edition of the Stanford-Binet Intelligence Scale for Adults, the test has been standardized with a representative sample of test takers up to age 90. Fluid reasoning, visual-spatial processing, working memory, and quantitative reasoning seem to peak in the 30s, whereas knowledge seems to peak in the 50s.

The newest version assesses each of five ability areas, such as knowledge, fluid reasoning, and quantitative reasoning, both nonverbally and verbally. By combining these subtest scores, one IQ score is determined.

The Wechsler Intelligence Scales

David Wechsler developed another set of age-based intelligence tests: the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) for preschool children, the Wechsler Intelligence Scale for Children (WISC) for ages 6 to 16, and the Wechsler Adult Intelligence Scale (WAIS) for older adolescents and adults. The latest edition, the WAIS-III, has a verbal scale including items on comprehension, vocabulary, information, similarities, arithmetic, and digit span; and a performance scale including items dealing with object assembly, block design, picture completion, picture arrangement, and digit symbols. Wechsler based his measures on deviation IQs or how spread out the scores were from the mean of 100 (Figure 15.1). Since intelligence has a bell curve distribution, 68% of the population will have an IQ between 85 and 115. These test takers are considered to be low normal through high normal. Test takers who fall two deviations below the mean have a score of 70, typically considered the borderline for mental retardation, while test takers two standard deviations above the mean have scores of 130, sometimes considered intellectually gifted, and those three standard deviations above the mean have scores of 145, sometimes considered geniuses. The