Intelligence is the highest attribute of human beings. Different psychologists have defined intelligence differently. Lewis Term an explains intelligence as, an ability to think on an abstract level.

David Wechsler defines intelligence as, ‘the aggregate or global capacity of an individual to act purposefully, to think rationally and to deal effectively with his environment’. Many psychologists have contributed to the measurement of Intelligence.

Sir Francis Galton thought that he could determine intelligence by measuring the size of the human skull. He administered a battery of tests to measure variables such as head size, reaction time, visual acuity, etc. However, these tests did not prove useful to measure intelligence.

Raymond Cattell, used the term ‘mental test’ for the first time. Like his mentor, Sir Galton, Cattell also believed that intelligence is best measured by sensory tasks. However, be emphasized that test administration must be standardized.

In 1905, Alfied Binet in collaboration with Theodor Simon, published the First Scale of Intelligence. This scale was revised in 1908 and 1911. In 1916, Lewis Terman revised the scale, i.e., adapted few items, established new age norms etc. This came to be called ‘Stanford Binet Test’. Binet also introduced the concept of Mental Age. It is defined as the age at which the person successfully performs on all items of the test prepared for that age level.

In 1917, Robert Yerkes and his colleagues developed the Army Alpha (verbal test) and Army Beta (performance test) intelligence tests. These two tests were used to recruit soldiers. In 1939, during World War II also, to recruit army personnel, the Army General Classification Test was used.

In 1939, David Wechsler published the Wechsler-Bellevue Intelligence Test. In 1955, the test was revised and then called Wechsler Adult Intelligence Scale (WAIS). It has a verbal scale and performance scale. Wechsler also developed a test to measure intelligence of children, i.e., Wechsler Intelligence Scale for Children (WISC).

Mental Age – Alfred Binet introduced the concept of Mental Age. It is defined as the age at which the person successfully performs on all items of the test prepared for that age level. Mental Age need not correspond to Chronological Age. It Mental Age (MA) is the same as Chronological Age (CA), the person has average intelligence.

Intelligence Quotient – In 1912, William Stern introduced the concept of Intelligence Quotient (IQ). Term an, refined the formula for calculating IQ which is as stated below IQ = \(\cfrac{MA}{CA}\) × 100 IQ made it possible to compare the intelligence of individuals of different age CA groups.