Although a strict definition of intelligence has proven elusive, a number of psychologists have argued that it can be quantified, primarily through testing. In 1905, Alfred Binet and Theodore Simon devised a system for testing intelligence, with scoring based on standardized, average mental levels for various age groups. In 1916 the Binet-Simon Intelligence Scale was expanded and reworked by Lewis Terman at Stanford Univ., and later revisions called the Revised Stanford-Binet Intelligence Tests were published in 1937, 1960, and 1985. A highly successful series of tests, designed by psychologist David Wechsler, have been in wide use for years as diagnostic and evaluative instruments. Known in 1939 as the Wechsler-Bellevue Intelligence Scale, the Wechsler Adult Intelligence Scale is a standard tool for intelligence testing today. All of these tests are administered to one individual at a time by a psychometrician. While no consensus of opinion prevails about what such tests actually measure, their use in education has had great practical value in assigning children to suitable class groups and in predicting academic performance.

The Army Alpha Test, which was first administered to nearly 2 million new recruits in World War I, and the Otis Group Intelligence Scale, were forerunners of many other group tests that are administered economically and quickly to large numbers, and were thus effective for use in schools and industry. National, standardized group tests are administered for college and graduate school entrance, and for a number of civil service positions.

The work of Binet, Terman, and Wilhelm Stern paved the way for a method of classifying intelligence in terms of a standardized measure, with standardization ensured by the large number of individuals of various ages taking the test. German psychologist L. Wilhelm Stern was the first to coin the term intelligence quotient (IQ), a figure derived from the ratio of mental age to chronological age. Although Stern's method for determining IQ is no longer in common use, the term IQ is still used today to describe the results in several different tests. Today, an average IQ score is considered to be 100, with deviations based on this figure. Mentally retarded individuals usually score below 70 in IQ tests, and are classified according to functional ability through reference to a scale of low IQ scores.

One criticism of intelligence testing is that it is difficult to insure that test items are equally meaningful or difficult for members of different sociocultural groups. Testing is often considered validated in part, however, by the finding that the quantity measured by the tests can be closely correlated in American society with career and academic achievement. There has been a decline in interest in pure intelligence tests since the 1920s, with a corresponding increase in the number of mental tests that measure special aptitudes and personality factors (see psychological tests).

See R. J. Sternberg and R. K. Wagner, ed., Practical Intelligence (1986); R. Fancher, The Intelligence Men: Makers of the I.Q. Controversy (1987); P. Chapman, Lewis M. Terman, Applied Psychology, and the Intelligence Testing Movement, 1880–1930 (1988).