18.4: Intelligence

Learning Objectives

1. Explain how intelligence is measured and the meaning of IQ scores.
2. Identify key contributors to intelligence testing and describe their tests (e.g., Binet, Terman, Wechsler).
3. Discuss the Flynn effect and its implications for intelligence testing over time.
4. Evaluate the uses and limitations of IQ tests, including concerns about cultural bias and variability in definitions of intelligence.
5. Interpret the normal distribution of IQ scores and its relevance to identifying intellectual disability and giftedness.

Measuring Intelligence: Standardization and the Intelligence Quotient

The goal of most intelligence tests is to measure “g”, the general intelligence factor. Good intelligence tests are reliable, meaning that they are consistent over time, and also demonstrate validity, which means that they accurately measure intelligence rather than some other attribute. Because intelligence is such an important part of individual differences, psychologists have invested substantial effort in creating and improving measures of intelligence, and these tests are now considered the most accurate of all psychological assessments.

Intelligence changes with age. A 3-year-old who could accurately multiply 183 by 39 would certainly be intelligent, but a 25-year-old who could not do so would be seen as unintelligent. Thus, understanding intelligence requires that we know the norms or standards in a given population of people at a given age. The standardization of a test involves administering it to a large number of people across different age groups and computing the average score on the test at each age level.

Once the standardization has been accomplished, we have a picture of the average abilities of people at different ages. We can calculate a person’s mental age, which represents the intellectual age at which a person is performing. If we compare a person's mental age to their chronological age, the result is the Intelligence Quotient (IQ), a measure of intelligence adjusted for age. A simple way to calculate IQ is by using the following formula:

IQ = mental age ÷ chronological age × 100.

Thus, a 10-year-old child who does as well as the average 10-year-old child has an IQ of 100 (10 ÷ 10 × 100), whereas an 8-year-old child who does as well as the average 10-year-old child would have an IQ of 125 (10 ÷ 8 × 100). Most modern intelligence tests are based on the relative position of a person’s score among people of the same age, rather than on the basis of this formula, but the idea of intelligence “ratio” or “quotient” provides a good description of the score’s meaning.

The Flynn Effect

It is important that intelligence tests be standardized regularly, as the overall level of intelligence in a population may change over time. The Flynn effect refers to the observation that scores on intelligence tests worldwide have increased substantially over the past decades (Flynn, 1999). Although the increase varies somewhat from country to country, the average increase is about 3 IQ points every 10 years. There are several explanations for the Flynn effect, including improved nutrition, increased access to information, and greater familiarity with multiple-choice tests (Neisser, 1998). But whether people are actually getting smarter is debatable (Neisser,1997). ³³

The Value of IQ Testing

The value of IQ testing is most evident in educational or clinical settings. Children who appear to be experiencing learning difficulties or severe behavioral problems can be tested to determine whether their difficulties can be partly attributed to an IQ score that is significantly different from the mean for their age group. Without IQ testing—or another measure of intelligence—children and adults needing extra support might not be identified effectively. People also use IQ testing results to seek disability benefits from the Social Security Administration.

While IQ tests have sometimes been used as arguments in support of insidious purposes, such as the eugenics movement, which was the science of improving a human population by controlled breeding to increase desirable heritable characteristics. However, the value of this test is important to help those in need.³⁴

Intelligence Tests and Those Who Created Them

Alfred Binet & Théodore Simon - Stanford- Binet Intelligence Test

From 1904 to 1905, the French psychologist Alfred Binet (1857–1914) and his colleague Théodore Simon (1872–1961) began working on behalf of the French government to develop a measure that would identify children who would not be successful with the regular school curriculum. The goal was to help teachers better educate these students (Aiken, 1994).

Binet and Simon developed what most psychologists today regard as the first intelligence test, which consisted of a wide variety of questions that included the ability to name objects, define words, draw pictures, complete sentences, compare items, and construct sentences. Binet and Simon (Binet, Simon, & Town, 1915; Siegler, 1992) believed that the questions they asked the children all assessed the basic abilities to understand, reason, and make judgments.

Figure \(\PageIndex{1}\): Alfred Binet (b). This page is from a 1908 version of the Binet-Simon Intelligence Scale. The children being tested were asked which face, in each pair, was prettier. Images are in the public domain.

Soon after Binet and Simon introduced their test, the American psychologist Lewis Terman, at Stanford University (1877–1956), developed an American version of Binet’s test, which became known as the Stanford-Binet Intelligence Test. The Stanford-Binet is a measure of general intelligence that comprises a wide variety of tasks, including vocabulary, memory for pictures, naming familiar objects, repeating sentences, and following commands.³⁶

David Wechsler- Wechsler-Bellevue Intelligence Scale

In 1939, David Wechsler, a psychologist who spent part of his career working with World War I veterans, developed a new intelligence quotient (IQ) test in the United States. Wechsler combined several subtests from other intelligence tests used between 1880 and World War I. These subtests assessed a variety of verbal and nonverbal skills, as Wechsler believed that intelligence encompassed “the global capacity of a person to act purposefully, to think rationally, and to deal effectively with his environment” (Wechsler, 1958, p. 7). He named the test the Wechsler-Bellevue Intelligence Scale (Wechsler, 1981). This combination of subtests became one of the most extensively used intelligence tests in the history of psychology.

Figure \(\PageIndex{2}\): David Wechsler. Image by Comet Photo AG (Zürich) is licensed under CC BY-SA 4.0.

Today, there are three intelligence tests credited to Wechsler, the Wechsler Adult Intelligence Scale-fourth edition (WAIS-IV), the Wechsler Intelligence Scale for Children (WISC-V), and the Wechsler Preschool and Primary Scale of Intelligence—Revised (WPPSI-III) (Wechsler, 2002). These tests are widely used in schools and communities throughout the United States and are periodically normed and standardized as a means of recalibration.

Bias of IQ Testing

Intelligence tests and psychological definitions of intelligence have been heavily criticized since the 1970s for being biased in favor of Anglo-American, middle-class respondents and for being inadequate tools for measuring non-academic types of intelligence or talent. Intelligence changes with experience, and intelligence quotients or scores do not reflect that ability to change. What is considered intelligent varies culturally as well, and most intelligence tests do not account for this variation. For example, in the West, being smart is associated with being quick. A person who answers a question the fastest is often perceived as the smartest, but in some cultures, being smart is associated with considering an idea thoroughly before providing an answer. A well-thought-out, contemplative answer is the best answer.³⁸

A Spectrum of Intellectual Development

The results of studies assessing the measurement of intelligence show that IQ is distributed in the population in the form of a Normal Distribution (or bell curve), which is the pattern of scores usually observed in a variable that clusters around its average. In a normal distribution, the majority of scores cluster around the middle, with fewer scores occurring at the extremes. The normal distribution of intelligence indicates that, on IQ tests and most other measures, the majority of people cluster around the average (in this case, where IQ = 100). At the same time, fewer individuals are either exceptionally intelligent or exceptionally dull (see below).

Figure \(\PageIndex{3}\): The majority of people have an IQ score between 85 and 115. Image by CNX Psychology is licensed under CC BY 4.0.

Distribution of IQ Scores in the General Population

This means that about 2% of people score above an IQ of 130, often considered the threshold for giftedness, and about the same percentage score below an IQ of 70, often considered the threshold for an intellectual disability.

References, Contributors and Attributions

33. Introduction to Psychology - Measures of Intelligence references Psychology by OpenStax CNX, licensed under CC BY 4.0 (modified by Dawn Rymond)

34. Introduction to Psychology - Measures of Intelligence references Psychology by OpenStax CNX, licensed under CC BY 4.0 (modified by Dawn Rymond)

36. Introduction to Psychology - Measures of Intelligence references Psychology by OpenStax CNX, licensed under CC BY 4.0

38. Sociology: Brief Edition – Agents of Socialization by Steven E. Barkan is licensed under CC BY-NC-SA 3.0

Introduction to Psychology - Measures of Intelligence references Psychology by OpenStax CNX, licensed under CC BY 4.0

Aiken, L. R. (1994). Assessment of intellectual functioning. Springer Publishing Company.

Binet, A., Simon, T., & Town, C. H. (1915). The development of intelligence in children (E. S. Kite, Trans.). Williams & Wilkins. (Original work published 1905)

Flynn, J. R. (1999). Searching for justice: The discovery of IQ gains over time. American Psychologist, 54(1), 5–20. https://doi.org/10.1037/0003-066X.54.1.5

Neisser, U. (1997). Rising scores on intelligence tests. American Scientist, 85(5), 440–447.

Neisser, U. (1998). The rising curve: Long-term gains in IQ and related measures. In U. Neisser (Ed.), The rising curve: Long-term gains in IQ and related measures (pp. 1–22). American Psychological Association. https://doi.org/10.1037/10270-001

Siegler, R. S. (1992). Emerging minds: The process of change in children's thinking. Oxford University Press.

Wechsler, D. (1958). The measurement and appraisal of adult intelligence (4th ed.). Williams & Wilkins.

Wechsler, D. (1981). WAIS-R manual: Wechsler Adult Intelligence Scale-Revised. Psychological Corporation.

Wechsler, D. (2002). WPPSI-III: Wechsler Preschool and Primary Scale of Intelligence—Third Edition. The Psychological Corporation.