Speculation about one’s abilities has existed at least since the time of the ancient Greeks. Although there was no one number which neatly conveyed one’s mental faculties, words such as “phronenis” (e.g.- practical wisdom or judgement), “episteme” (e.g.- understanding or knowledge), and “gnome” (e.g.- wisdom, insight, or emotional intelligence) indicate that even Aristotle and Plato were concerned with cognition and how it relates to human life.
Fast forward to the 19th and 20th century, French physician Paul Broca and English statistician Sir Francis Galton were a few of the first scientists to try and systematically measure intelligence. Although their practice of measuring bumps in the skull to determine ability (a-la phrenology) turned out to be utterly wrong, they spurred a critical wave of psychologists, medical doctors, and scientists who became consumed with refining a unified definition of intelligence.
When the French Ministry of Education requested that French psychologists Alfred Binet and Theodore Simon develop a “test” which allowed schools to distinguish “mentally retarded” children from “those normally intelligent, yet lazy,” the two psychologists complied. The Simon-Binet IQ test was the first, official systematic measure of an intelligence quotient and was widely circulated in both Europe and America to place children in special needs classrooms.
Yet the most recent, standardized psychological test measuring intelligence came from the work of David Wechsler, an American psychologist born in Romania. Wechsler abolished the idea of a single number to represent intelligence (the “quotient” component of an IQ score) and initially divided the measurement of intelligence into two components: verbal and performance-based. In this model, Verbal IQ represented one’s ability to use verbal information while Performance IQ represented non-verbal abilities. A summation of both VIQ and PIQ provided a full-scale measure of intelligence (FSIQ). Redefining the way we assess IQ stemmed from his work with patients at the Bellevue Clinic in New York City. Weschler wanted to learn more about his patients and their cognitive abilities but found the Simon-Binet IQ measures to be too broad and unable to capture meaningful subcomponents of patients’ thinking and reasoning abilities. He provided a new definition of IQ which is still used today: “Intelligence is the aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with his environment” (1944).
The Wechsler tests (he created both adult and child versions to account for variability in cognitive factors due to maturation) are still the most commonly used cognitive testing battery available today and are considered the gold standard for basic cognitive testing. If you, a colleague, or a child has ever had any formal “IQ” testing, it was probably a Wechsler test. Today, the concept of IQ actually references four domains on the Weschler tests: verbal comprehension, perceptual reasoning, working memory, and processing speed, which altogether create the entire IQ score.
Verbal Comprehension measures verbal concept formation, verbal reasoning, and acquired knowledge from one’s culture. Perceptual Reasoning assesses spatial processing, perceptual and fluid reasoning, and visual-motor integration. The Working Memory index evaluates attentional capacity, concentration, mental control, and reasoning with novel problems. The Processing Speed section evaluates attention, short-term visual memory, and visual-motor coordination. Fundamentally, an individual may be exceptionally high in one of the four indices but especially low in another one of the four areas. Therefore, the single number we tend to think of when someone mentions “IQ” (which comes from the full-scale IQ, the summation of all four of these intelligence factors) may be a gross misrepresentation of one’s true abilities. When the four domains measured are closely related to one another, the FSIQ score will likely be a valid representation of the examinee’s abilities. However, all of these areas are susceptible to emotional dysregulation, exhaustion, and the effects of physiological issues such as hunger or pain. For example, being upset during a Wechsler test may divert one’s attention from the task at hand, artificially dragging down the Processing Speed domain of the IQ score. If the four domain scores vary widely, providing an overall IQ is not even warranted as it is not psychometrically sound. However, many health professionals do not master the psychometric components of assessment and provide this overall IQ measure to the client anyway. Providing scores on the four domains which comprise intelligence is often more useful than providing one score, since a large number of individuals will have some notable variability across the four domains of intelligence.
Therefore, while the Weschler assessment (WAIS-IV for adults or WISC-5 for children) has been shown in hundreds of studies to validly and reliably measure the intelligence factors it claims, the scores represent a snapshot in time: a transient state rather than trait intelligence. This is imperative for anyone taking an intelligence test to understand, lest the results be misinterpreted or misused by one’s physician, teacher, or employer. IQ may be best understood as a hypothesis about one’s abilities, which at minimum, represent a person’s cognitive functioning at the time in which the test was taken.