Alfred Binet and Theodore Simon, commissioned by the French government to devise a test that would differentiate between children who could not learn because of low ability and those who would not learn because of poor motivation, created the first intelligence test in 1905 (Thorndike, 2005). Binet did not really have a theory of mental ability, but he thought that intelligence could be best expressed in and assessed by complex cognitive tasks. The first intelligence test consisted of items that assessed memory, knowledge, and reasoning skills arranged in a sequence of increasing difficulty and grouped into age levels, or when the “average” child could accomplish them. This first test has undergone numerous revisions and is now known as the Stanford-Binet Intelligence Scale, Fifth Edition (Roid, 2003).

Currently, the most widely-used intelligence tests are the Wechsler Scales. Designed by David Wechsler (1939), a clinical psychologist who used the test to help identify cognitive strengths and weaknesses in his clients and assist in diagnoses, the tests were based on his definition of intelligence as “the aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with his environment” (p. 3). Because the test seemed to be a valid measure of general intelligence, its use expanded to other populations.

There are three separate Wechsler scales:

  • Wechsler Preschool and Primary Scale of Intelligence, 3rd ed. (WPPSI-III) (Wechsler, 2002)
  • Wechsler Intelligence Scale for Children, 4th ed. (WISC-IV) (Wechsler, 2003)
  • Wechsler Adult Intelligence Scale, 3rd ed. (WAIS-III) (Weschler, 1997)

The WISC-IV is designed for use with children and adolescents ages 6 to 16 years 11 months and will be the focus of this section. Sample items from several of the 10 core subtests are presented below:

  • Similarities.In what way are wool and cotton alike?
  • Vocabulary.What does corrupt mean?
  • Comprehension.Why do people buy fire insurance?
  • Digit Span.I am going to say some numbers. Listen carefully and when I am through, say the numbers right after me: 734186
  • Coding.In the top row, each figure is shown with a number. Fill in the number that goes with each figure in the second row
  • Matrix Reasoning. For each item the child looks at an incomplete matrix and selects the missing portion from five response options.
  • Picture Concepts. The child identifies objects that share some common property.

Some of the items in this test require children and adolescents to arrange materials rather than talk with the test administrator. This procedure allows non-English-speaking children, as well as youth with speech or language disorders, to demonstrate their intellectual capabilities. Some professionals find the division between verbal and performance items very useful for assessment purposes (Hildebrand & Ledbetter, 2001). The items make up four index scores—Verbal Comprehension, Perceptual Organization, Freedom from Distractability, and Processing Speed—that are combined to give an overall ability score. When the four index scores are added together, they produce a total ability score, or intelligence quotient (IQ). On the Wechsler scales, the mean total IQ is set at 100 with a standard deviation of 15. Most other major intelligence tests have set the same mean so it is easy to compare the IQ score from one intelligence test to another.

How to interpret an IQ score.

An IQ score represents how far the adolescent’s raw score (number of items answered correctly) differs from the typical performance of same-aged individuals. To develop the norms for the WISC-IV, thousands of children and adolescents were given the test; their performance created a bell-shaped curve.   The highest point of the curve represents the average performance of a particular age group and accounts for the scores of the largest number of adolescents. Adolescents who are represented to the right of the highest point answered more items correctly, and adolescents who are represented to the left of the highest point answered fewer items correctly. The bell-shaped, or normal, curve has at its center the mean, or average, score on this intelligence test (100) and a standard deviation of 15, which shows how spread out the scores are from the mean. also displays the intellectual labels that are applied to the various ranges of scores. An IQ of 100 can be interpreted to mean that an adolescent scored higher than 50% percent of adolescents who took this test and is in the “average” range.

The stability of IQ.

Stability refers to the consistency of performance over time. In other words, is an IQ score achieved at age 10 related to IQ at age 20? Research on the stability of IQ shows that the older the individual is at the first test-taking session, the better that score predicts IQ at later points in time. Correlations between IQ scores achieved after age 6 and those taken later are quite high (e.g., .70s–.80s). IQ scores obtained during preschool are less predictive of later scores (Humphreys & Davey, 1988). In addition, the less time between test-taking sessions produces stronger relationships between IQ scores. IQ tests taken several years apart yield IQ scores that are more similar than those from tests taken 20 years apart.