Scientific interest in the concept of intelligence began with the research and writings of the Englishman Francis Galton in the latter half of the nineteenth century. Stimulated by his belief in the hereditary basis of individual differences in mental giftedness, Galton devised a number of simple sensorimotor tests (movement speed, muscular strength, touch and pain sensitivity, weight discrimination, reaction time, etc.) that, collectively, he believed would measure human intelligence. This approach to the measurement of intelligence was consistent with the doctrine of mental association-ism espoused by many philosophers of the time. Associationism held that the contents of the mind are the results of combinations and elaborations of elementary sense impressions ("Nothing is in the mind which was not first in the senses"). Unfortunately for Galton, his tests did not prove useful in predicting performance in schoolwork or on other tasks that presumably involve intelligence.

Galton's efforts to measure the concept of intelligence were soon superseded by the work of two Frenchmen—Alfred Binet and Theodore Simon. Binet and Simon were commissioned by the Paris school system to devise a method for distinguishing between children who could benefit from formal educational instruction and those who could not. Applying themselves to the problem, Binet and Simon constructed the first practical intelligence test. Unlike the simple sensorimotor tests administered by Galton and his associates, the tests on the Binet-Simon Intelligence Scale consisted of school-type tasks that increasing numbers of children could perform at successive age levels. Examples are memory for digits and sentences, and identifying objects and body parts.

Early intelligence tests such as the Binet-Simon scale and an American adaptation and extension, the Stanford-Binet Intelligence Scale, were designed primarily for the assessment of intelligence in children. Numerous definitions of the intelligence measured by these tests were offered, including Binet's "the ability to judge well, to comprehend well, to reason well" and Terman's "the ability to carry on abstract thinking." Whatever definition may have been preferred, the tests were used primarily to determine the ability of children to learn school-type tasks and for grade-placement purposes. The

Stanford-Binet Intelligence Scale was an age scale on which a child's performance was converted to a mental age and then to an intelligence quotient, by dividing the mental age by the chronological age and multiplying by 100.

Some of the tests on the Stanford-Binet were appropriate for adults, but it was not considered to be an adequate measure of intelligence in adults. During World War I, tests were devised specifically for measuring the intelligence of adults. In contrast to individual tests such as the Stanford-Binet, which was administered to one examinee at the time, these group tests—the Army Examinations Alpha and Beta—could be administered simultaneously to a large group of individuals. Group testing subsequently became even more extensive than individual testing. Between the end of World War I and the beginning of World War II many educational and psychological tests—both group and individual—were constructed and administered. The most famous of the new individual tests of intelligence was the Wechsler-Bellevue Intelligence Scale.

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