• Sir Francis Galton, a pioneer in the measurement of individual differences in late nineteenth-century England, was particularly concerned with sensory responses (visual and auditory acuity and reaction times) and their relationship to differences in ability.

• Several individual tests have been used to test intelligence.

  • The Binet-Simon intelligence scale, developed by French psychologists Alfred Binet and Theodore Simon, was administered to children to evaluate their performance ( mental age) at a given chronological age. The mental age/chronological age measure, called a mental quotient, was used to evaluate a child's learning potential.

  • Lewis Terman of Stanford University revised the Binet scale in 1916. The revised scale, called the Stanford-Binet intelligence scale, although it retained the concept of mental and chronological ages, introduced the concept of the intelligence quotient (IQ) arrived at by the following widely used formula, which allows comparison between children of different ages.

    
    

    
    

    The 1986 revision of the test, the latest of several, varies the calculation so that the test is useful for adults as well as for children. An individual's score for correct answers is compared to a table of scores of test takers of the same age (with the average score always scaled to 100). Scores between 90 and 110 are labeled as “normal,” above 130 as “superior,” and below 70 as mentally deficient, or “retarded.” The distribution of IQ scores approximates a normal (bell-shaped) curve (Figure 1 ).
    
    

    Figure 1 The Normal IQ Distribution

  • David Wechsler developed the Wechsler Adult Intelligence Scale (WAIS) in 1939, revised as the WAIS-R. Wechsler also developed the Wechsler Intelligence Scale for Children (WISC), revised as the WISC-R. The revised forms of these scales are still widely used. They contains two sub-scales, verbal and performance, which provide a verbal IQ and a performance IQ; the subscales are combined for the total IQ. Test score combinations may reveal other strengths and weaknesses to a skilled examiner.

Tests of aptitude and achievement. Group tests (such as the California Achievement Tests and the SAT, the Scholastic Assessment Test) are often used to measure aptitude, the capacity to learn (including both verbal and performance aptitudes) and achievement, what has been learned.

Ranges of intelligence scores. The two extremes of levels of intellectual functioning are known as developmentally disabled and gifted.

• Those identified as mentally retarded (sometimes described as developmentally disabled) have IQ scores of 70 or below. Severity of disability and corresponding IQ scores are mild (50 to 70), moderate (35 to 50), severe (20 to 35), and profound (below 20). Some, but not all, of the causes of mental retardation are known and include Down syndrome, a genetic disorder; phenylketonuria, a metabolic disorder; and developmental disability due to anoxia (lack of oxygen) during gestation.

• The gifted usually fall within the upper 2% to 3% of the IQ score distribution (between 130 and 145). Louis Terman's well-known longitudinal study of the gifted, which will not be complete until 2010, found that gifted children are generally superior to average-IQ peers in health, achievement, and adjustment to life stresses. Currently, gifted children are identified not only by IQ but also by superior potential in any of six areas: general intelligence, specific aptitudes (math, for example), creativity, leadership, performing arts, and athletics.

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