Figure 1. Starting from early primary school years, researchers and clinicians assess intelligence through standardized test batteries (i.e., IQ tests), as these tests are intended to be suitable for a wide age range.

Intelligence is among the oldest and longest studied topics in all of psychology. The development of assessments to measure this concept is at the core of the development of psychological science itself.

When might an IQ test be used? What do we learn from the results, and how might people use this information? While there are certainly many benefits to intelligence testing, it is important to also note the limitations and controversies surrounding these tests. For example, IQ tests have sometimes been used as arguments in support of insidious purposes, such as the eugenics movement (Severson, 2011). The infamous Supreme Court Case, Buck vs. Bell, legalized the forced sterilization of some people deemed “feeble-minded” through this type of testing, resulting in about 65,000 sterilizations (Buck vs. Bell, 274 U.S. 200; Ko, 2016). Today, only professionals trained in psychology can administer IQ tests, and the purchase of most tests requires an advanced degree in psychology. Other professionals in the field, such as social workers and psychiatrists, cannot administer IQ tests.

In psychology, human intelligence is commonly assessed by IQ scores that are determined by IQ tests. However, there are critics of IQ tests who, while they do not dispute the stability of IQ test scores or the fact that they predict certain forms of achievement rather effectively, do on the other hand argue that to base a concept of intelligence on IQ test scores alone is to ignore many important aspects of mental ability.

For instance, individual differences in general intelligence are one of the strongest predictors of occupational attainment, social mobility, and job performance. People with higher general intelligence in childhood or early adulthood also tend to have better overall physical health, and have a longer life expectancy. Further, children with higher childhood IQ have a lower risk of developing dementia and of being diagnosed with schizophrenia spectrum disorder, major depression, or any anxiety disorder in adulthood. Understanding how differential IQ levels become associated with these life outcomes is crucial not only from a scientific point of view, but also for developing public policy and determining effective interventions.

Types of IQ Tests and Tasks

There are a wide variety of IQ tests that use slightly different tasks and measures to calculate an overall IQ score. The most commonly used test series is the Wechsler Adult Intelligence Scale (WAIS) and its counterpart, the Wechsler Intelligence Scale for Children (WISC). Other commonly used tests include the original and updated version of Stanford-Binet, and the Kaufman Assessment Battery for Children. We will discuss all of these.

Stanford-Binet Intelligence Test

The person best known for formally pioneering the measurement of intellectual ability is Alfred Binet. Binet was particularly interested in the development of intelligence, a fascination that led him to observe children carefully in the classroom setting. Along with his colleague Theodore Simon, Binet created a test of children’s intellectual capacity. Simon and Binet created individual test items that should be answerable by children of given ages. For instance, a child who is three should be able to point to her mouth and eyes, a child who is nine should be able to name the months of the year in order, and a twelve-year-old ought to be able to name sixty words in three minutes. Their assessment became the first IQ test.

IQ or intelligence quotient is a name given to the score of the Binet-Simon test. The score is derived by dividing a child’s mental age (the score from the test) by their chronological age to create an overall quotient. These days, the phrase IQ does not apply specifically to the Binet-Simon test and is used to generally denote intelligence or a score on any intelligence test. In the early 1900s, the Binet-Simon test was adapted by a Stanford professor named Lewis Terman to create what is, perhaps, the most famous intelligence test in the world, the Stanford-Binet (Terman, 1916). The major advantage of this new test was that it was standardized. Based on a large sample of children, Terman was able to plot the scores in a normal distribution, shaped like a bell curve (see Fig. 1). To understand a normal distribution, think about the height of people. Most people are average in height, with relatively fewer being tall or short, and fewer still being extremely tall or extremely short. Terman (1916) laid out intelligence scores in exactly the same way, allowing for easy and reliable categorizations and comparisons between individuals.

Figure 1. The bell curve showing the normal distribution of IQ, with 100 being the average score.

Weschler Intelligence Scales

A look at another modern intelligence test—the Wechsler Adult Intelligence Scale (WAIS)—can provide clues to a definition of intelligence itself. Motivated by several criticisms of the Stanford-Binet test, psychologist David Wechsler sought to create a superior measure of intelligence. He was critical of the way that the Stanford-Binet relied so heavily on verbal ability and was also suspicious of using a single score to capture all intelligence. To address these issues, Wechsler created a test that tapped a wide range of intellectual abilities. The the Wechsler Adult Intelligence Scale (WAIS) assesses people’s ability to remember, compute, understand language, reason well, and process information quickly (Wechsler, 1955). David Wechsler’s approach to testing intellectual ability was based on the fundamental idea that there are, in essence, many aspects to intelligence. The Stanford-Binet test reflected mostly verbal abilities, while the Wechsler test also reflected nonverbal abilities. The Stanford-Binet has also been revised several times and is now similar to the Wechsler in several aspects, but the Wechsler continues to be the most popular test in the United States.

The current version of the test, the WAIS-IV, which was released in 2008, comprises 10 core subtests and five supplemental subtests, with the 10 core subtests yielding scaled scores that sum to derive the Full Scale IQ. With the WAIS-IV, the verbal/performance IQ scores from previous versions were removed and replaced by the index scores. The General Ability Index (GAI) was included, which consists of the similarities, vocabulary, and information subtests from the verbal comprehension index and the block design, matrix reasoning and visual puzzles subtests from the perceptual reasoning index. The General Ability Index (GAI) is clinically useful because it can be used as a measure of cognitive abilities that are less vulnerable to impairments of processing speed and working memory.

Figure 2. The WAIS uses a variety of components to determine a person’s IQ score, including verbal, memory, perceptual, and processing skills.

Index Scores and Scales

There are four index scores representing major components of intelligence:

• Verbal Comprehension Index (VCI)
• Perceptual Reasoning Index (PRI)
• Working Memory Index (WMI)
• Processing Speed Index (PSI)

Two broad scores, which can be used to summarize general intellectual ability, can also be derived:

• Full Scale IQ (FSIQ), based on the total combined performance of the VCI, PRI, WMI, and PSI
• General Ability Index (GAI), based only on the six subtests that the VCI and PRI comprise.

Tasks grouped by index

Index	Task	Core?	Description	Proposed abilities measured
Verbal Comprehension	Similarities		Describe how two words or concepts are similar.	Abstract verbal reasoning; semantic knowledge
	Vocabulary		Name objects in pictures or define words presented to them.	Semantic knowledge; verbal comprehension and expression
	Information		General knowledge questions	Degree of general information acquired from culture
	Comprehension		Questions about social situations or common concepts	Ability to express abstract social conventions, rules, and expressions
Perceptual Reasoning	Block Design		Put together red-and-white blocks in a pattern according to a displayed model; this is timed, and some of the more difficult puzzles award bonuses for speed	Visual spatial processing and problem solving; visual motor construction
	Matrix Reasoning		View an array of pictures with one missing square, and select the picture that fits the array from five options	Nonverbal abstract problem solving, inductive reasoning
	Visual Puzzles		View a puzzle in a stimulus book and choose from among pieces of which three could construct the puzzle	Visual spatial reasoning
	Picture Completion		Select the missing part of a picture	Ability to quickly perceive visual details
	Figure Weights		View a stimulus book that pictures shapes on a scale (or scales) with one empty side and select the choice that keeps the scale balanced	Quantitative reasoning
Working Memory	Digit Span		Listen to sequences of numbers orally and to repeat them as heard, in reverse order, and in ascending order	Working memory, attention, encoding, auditory processing
	Arithmetic		Orally administered arithmetic word problems; timed	Quantitative reasoning, concentration, mental manipulation
	Letter-Number Sequencing		Recall a series of numbers in increasing order and letters in alphabetical order	Working memory, attention, mental control
Processing Speed	Symbol Search		View rows of symbols and target symbols, and mark whether or not the target symbols appear in each row	Processing speed
	Coding		Transcribe a digit-symbol code using a key. The task is time-limited	Processing speed, associative memory, graphomotor speed
	Cancellation		Scan arrangements of shapes and mark specific target shapes within a limited amount of time	Processing speed

The Wechsler Intelligence Scale for Children (WISC)

Figure 2. The (WISC), developed by David Wechsler, is an individually administered intelligence test for children between the ages of six and 16. The Fifth Edition (WISC-V; Wechsler, 2014) is the most recent version.

The Wechsler Intelligence Scale for Children (WISC) is an individually administered intelligence test for children between the ages of 6 and 16. The Fifth Edition is the most recent version.

The WISC-V generates a Full Scale IQ that represents a child’s general intellectual ability. It also provides five primary index scores: Verbal Comprehension Index, Visual Spatial Index, Fluid Reasoning Index, Working Memory Index, and Processing Speed Index. Five ancillary composite scores can be derived from various combinations of primary or primary and secondary subtests.

Five complementary subtests yield three complementary composite scores to measure related cognitive abilities relevant to assessment and identification of specific learning disabilities, particularly dyslexia and dyscalculia. Variation in testing procedures and goals can reduce time of assessment to 15–20 minutes for the assessment of a single primary index, or increase testing time to three or more hours for a complete assessment, including all primary, ancillary, and complementary indices.

Kaufman Assessment Battery for Children

The Kaufman Assessment Battery for Children (KABC) is a clinical instrument (psychological diagnostic test) for assessing cognitive development. Its construction incorporates several recent developments in both psychological theory and statistical methodology. The test was developed by Alan S. Kaufman and Nadeen L. Kaufman in 1983 and revised in 2004. The Kaufman Assessment Battery for Children (KABC) also gives special attention to certain emerging testing needs, such as use with handicapped groups, application to problems of learning disabilities, and appropriateness for cultural and linguistic minorities. The authors rightly caution, however, that success in meeting these special needs must be judged through practical use over time. The KABC-II helps to identify an individual’s strengths and weaknesses in cognitive ability and mental processing. The information provided by the KABC-II can facilitate clinical and educational planning, treatment planning, and placement decisions.