Intelligence quotient (IQ) and measuring intelligence

• TYPE OF PSYCHOLOGY: Cognitive; Developmental; Neuropsychology; Psychometrics

Early references to intelligence in literature can be found as far back as the ancient Greeks and Chinese. The measurement of intelligence began in its most rudimentary form when scientists attempted to identify those characteristics thought to contribute to the concept of intelligence. While initially, intelligence was viewed as being composed of various physical characteristics, such as head circumference and muscle strength, modern measures now focus on specific areas dealing with measurable human characteristics including abstract reasoning, memory, and spatial abilities.

Introduction

Because the concept of intelligence is considered to be hypothetical, those wishing to measure intelligence had to develop theories to explain their ideas of what constitutes intelligence. Once areas were identified, tasks could then be developed to measure these areas. For example, because memory was often considered to be an indicator of intelligence, tasks were developed to measure this area. Auditory memory was often measured by having an individual repeat a series of numbers or words back to the examiner, while visual memory tasks sometimes required an individual to look at a picture for a specific period of time and remember later what he or she saw.

The concept of the Intelligence Quotient (IQ) was created after intelligence tests were developed. It is the score derived from the test administration that describes a person's overall abilities. Early test developers utilized a ratio IQ score that was obtained by dividing an individual's mental age score obtained from the test administration by that person's chronological age. Because this often resulted in a number with a decimal point, the result was then multiplied by 100. Thus, an individual with a physical age of twenty years, who obtained a mental age score of twenty-five years, would have a ratio IQ of 125 (IQ = 25/20 X 100 = 125). Intelligence tests no longer use ratio IQ scores, which poses problems with interpretation, especially between individuals of different ages. Instead, a standard score is derived from testing that has a mean of 100 and a standard deviation of 15. This score is obtained by comparing the individual's score to those scores obtained in the norming sample. This technique allows for comparisons to be made between individuals of differing ages and over time.

Intelligence

While definitions of intelligence vary widely, most include some form of abstract reasoning and problem-solving abilities. Also frequently found in definitions are the concepts of spatial ability, memory, speed and accuracy, and learning and planning. Current intelligence tests generally consist of numerous subtests which are believed to tap into these areas. The scores obtained on the subtests combine to provide an intelligence quotient.

IQ scores in individuals older than eight years of age tend to be very consistent, suggesting that people generally cannot change their scores dramatically by studying or taking courses. Scores are determined through the interaction of both genetic and environmental factors. While genetic factors may set boundaries for an individual's abilities, environmental factors can determine where an individual will fall within those boundaries. For example, an individual born with superior intellectual abilities and raised in an enriching environment is likely to score well on an IQ test. Conversely, a person with similar genetic make-up but raised in an impoverished environment may never display his or her true potential. Studies performed on twins who were raised together and twins who were raised apart support this interaction of genetics and environment. Twins who were raised together showed less variation in their IQ scores than twins raised apart.

IQ scores are normally distributed and can be graphed on a normal or bell-shaped curve. The average IQ score is 100, meaning that 50% of scores fall above and below this score. The standard deviation, which indicates how far a score is from the mean, is 15. A person who obtains an IQ score of 85 would be considered one standard below the mean, while a score of 130 would be two standard deviations above. As scores deviate further from the mean, there are fewer scores. For example, 68 percent of all IQ scores fall within one standard deviation on either side of the mean. However, only 26 percent of scores fall between one and two standard deviations and only 4 percent fall between two and three standard deviations.

IQ scores are also categorized by ranges. While different intelligence tests tend to have slightly different classification names, the ranges themselves are consistent. For example, classifications used on instruments developed by prolific test developer Dr. David Wechsler include the following ranges: Average 90–109, High Average 110–119, Superior 120–129, Very Superior 130 and above, Low Average 80–89, Borderline 70–79, and Extremely Low 69 and below. These ranges are helpful when attempting to interpret test results.

The history of intellectual assessment

While at some level, humans have always attempted to assess the abilities of others, whether it be for self-preservation or scientific discovery, it was not until the beginning of the twentieth century that the first modern intelligence test was developed. In 1905, Alfred Binet and Theodore Simon were recruited by the French government to develop an instrument to determine which children should be provided an education and which should be dismissed from school. The Binet-Simon test included numerous tasks that measured verbal skills and provided a mental age score. It became quite popular and, in 1916, was brought to the United States by Lewis Terman, a professor at Stanford University. After revisions designed to have the instrument reflect American culture, Terman introduced the Stanford-Binet Intelligence Scale. The measure was widely accepted and the idea of intellectual assessment gained a strong foothold in American society. The fifth edition of the Stanford-Binet has been in use since 2003.

In 1939, David Wechsler published the first of his many intelligence tests, the Wechsler-Bellevue Intelligence Scale. While the Stanford-Binet provided a single overall score, Wechsler felt that intelligence included both verbal and performance (nonverbal) components. Thus, he developed an instrument that provided three separate IQ scores: Verbal IQ, Performance IQ, and Full Scale IQ, the combination of both verbal and performance. The instrument was very popular, and Wechsler went on to develop other instruments, including the Wechsler Adult Intelligence Scale (WAIS), the Wechsler Intelligence Scale for Children (WISC), and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI). These instruments became the most widely used instruments of their kind and are available today in their most current revisions: WAIS-IV, WISC-V, and the WPPSI-III.

In the 2010s and 2020s, trends in the development of intelligence tests have focused on the growing popularity of CHC Theory, based on the works of Raymond Cattell, John Horn, and John Carroll. This theory is well documented and supported empirically and consists of three levels or strata: Stratum I - Narrow Abilities, Stratum II - Broad Abilities, and Stratum III - General Ability. The most popular intelligence instrument, based on CHC Theory, is the Woodcock-Johnson IV, Tests of Cognitive Ability (WJ-IV Cog). As the name states, this instrument uses the term “cognitive abilities” which is steadily gaining favor over the term intelligence. The WJ-IV Cog consists of 18 individual subtests that make up the Narrow Abilities. The individual subtests combine with other subtests to provide Broad Ability scores that are called Cluster scores. Finally, the Cluster scores combine to provide the General Ability score, which is often viewed as an intelligence quotient.

IQ test construction

While early IQ tests were often developed with little consideration for statistical properties and standardization, today's instruments represent the capstone of test development. They are very valid, meaning that they measure what they report to measure, and are quite reliable in that they measure consistently over time. When an IQ test is developed, the first step is to identify the theoretical background to be used as its basis. For example, the WJ-IV Cog adheres to CHC Theory. Numerous theories of intelligence exist, and test developers often modify theories or expand on them for test development purposes.

Once a theory has been adopted, experts develop subtests and individual items to measure the components of the theory. Items go through extensive review and modification by the developer until a specific set of subtests and items are chosen for inclusion in the instrument. The test is then administered to a representative sample of our national population. This sample generally includes thousands of individuals who are selected on the basis of such characteristics as age, sex, socioeconomic status, education level, and geographic location. Information obtained from this sample, known as the norming sample, is then evaluated to establish the statistical qualities of the instrument. The information is also used to evaluate test items and make modifications where necessary. Once the test is fully developed, studies are conducted to compare the new instrument with already published instruments currently thought to be valid for the purpose of intellectual assessment. If scores on the newly developed instrument are similar to scores on the existing instruments, the authors can state that their instrument is valid for the measurement of intelligence. Future research, performed by practitioners who use the instrument along with researchers in universities and other institutions, will further establish the usefulness of the new test instrument.

Uses of IQ tests

Intelligence tests are used for a broad range of purposes in our society because they are capable of predicting success in a wide variety of areas and fields. Probably the most prolific use of IQ tests can be found in the preschool through twelfth grade educational setting. With the passage of such laws as The Education of All Handicapped Children Act in 1979 and the Individuals with Disabilities Education Improvement Act of 2004, schools use IQ tests to identify students with educational needs and then provide programming for them. These laws not only state that all children must be educated to the fullest of their abilities, but also that they be classified for funding purposes. IQ tests help determine, for example, which students have learning disabilities or which may be intellectually disabled. IQ tests are used to determine initial eligibility for services progress made in a remedial program, and eligibility for future services.

Intelligence tests are also used in a host of other settings, including the workplace, mental health facilities, the armed services, and research environments. Employers have increasingly used IQ assessments as part of the employment application process. Research has shown a correlation between IQ scores and job performance, and that applicants with higher IQs generally perform better than those with lower IQs. Employers also use test instruments to make promotional decisions as well as changes in placements. IQ tests have long been used in the profession of mental health for classification and treatment purposes, and in the armed services to classify large numbers of individuals and assign training options. Finally, researchers in a wide variety of fields use intelligence tests in their studies for numerous purposes, including subject placement, response to treatment, and classification.

By the 2020s, decades of research had found racial and class bias in IQ tests. Research also showed that an individual's IQ score could change over time. These findings have prompted states and school districts to begin to move away from using IQ tests to assess and categorize students' learning abilities. Parents and advocates who are against the use of IQ tests argue that school districts and states should do more to limit or avoid using tests that may misjudge students' potential.

Bibliography

Carr, Sarah. "The Outdated Tests Far Too Many Schools Still Use to Judge a Kid's Ability." Slate, 28 Mar. 2024, slate.com/human-interest/2024/03/iq-cognitive-testing-special-ed-critiques-placement.html. Accessed 31 Oct. 2024.

Conway, Andrew R. A., and Kristof Kovacs. “New and Emerging Models of Human Intelligence.” Wiley Interdisciplinary Reviews: Cognitive Science, vol. 6, no. 5, 2015, pp. 419–426.

Flanagan, D. P., and P. L. Harrison. Contemporary Intellectual Assessment: Theories, Tests, and Issues. 2nd ed., Guilford, 2005.

Flynn, James R., Richard Haier, and Robert Sternberg. “What Role do IQ Tests Play in Measuring Intelligence?” Fifteen Eighty Four, Cambridge UP, 1 June 2016, cambridgeblog.org/2016/06/what-role-do-iq-tests-play-in-measuring-intelligence/. Accessed 31 Oct. 2024.

Goldstein, Sam, Dana Princiotta, and Jack A. Naglieri, editors. Handbook of Intelligence: Evolutionary Theory, Historical Perspective, and Current Concepts. Springer, 2015.

Gregory, R. J. Foundations of Intellectual Assessment. Allyn, 1998.

Gregory, R. J. Psychological Testing: History, Principles, and Applications. 5th ed., Allyn, 2006.

Mackintosh, N. T. IQ and Human Intelligence. 2nd ed., Oxford UP, 2011.

Sattler, J. M. Assessment of Children: Cognitive Foundations. 5th ed., Sattler, 2008.