What Does IQ Stand For?

IQ stands for “intelligence quotient” and is a composite score compiled from various tests meant to test intellectual ability. Under the common understanding, a person’s IQ score is a measure of the magnitude of their intelligence and is normally expressed as a single integer number. IQ tests consist of a battery of exercises meant to measure participants’ psychological capacities, such as memory, recall, executive functioning, and perceptual reasoning.

IQ tests have a long history and there have been different kinds of IQ tests that have been structured in different ways. Nowadays, the most commonly administered IQ test in the world is the Wechsler Adult Intelligence Scale, currently in its 4th edition (WAIS-IV). The WAIS-IV consists of 10 core subtests and 5 supplemental tests. These tests measure participants’ specific abilities on tasks like vocabulary recall, reading comprehension, visuospatial reasoning, mathematical ability, working memory, and mental processing speed. The results from these 15 different tests are then averaged and compiled into four distinct scores, each corresponding to some general class of psychological capacities. The single digit IQ score of a person is the sum of these 4 scores.

The distribution of IQ scores is understood to take the form of a normal distribution, with the majority of incidences occurring near the middle of that distribution. The median raw score of an IQ test is defined as 100, where intervals of 15 points correspond to one standard deviation of the distribution. This standard means that, by definition, two-thirds of the population falls within the 85-115 IQ range. About 2.5% fall within the 130+ range and about 2.5% fall lower than 70. Generally, a score lower than 70 points is the threshold for a diagnosis of mental disability.

The normal distribution of IQ scores. Credit:: WikiCommons CC BY-SA 3.0

IQ scores have been found to be moderately to highly correlated with other factors, such as income, mortality, social status, and parental IQ. IQ scores are used for educational placement, assessment of intellectual disability, and evaluating potential employees. It has been found that most people do not show any significant amount of variation in IQ over their life after the age of 10.

What Does An IQ Score Measure?

Under the common understanding, a person’s IQ score is a measure of how intelligent they are. Unfortunately, the concept of “intelligence” has no standard definition, and different psychologists operationalize the term in different ways. Unlike properties like length or distance, intelligence cannot be concretely measured and must be investigated indirectly.

Several proposed definitions have been offered, most construing intelligence as related to a person’s ability to effectively navigate a wide range of environments and process the most amount of information in the most efficient way. The APA has defined intelligence as the constellation of mental patterns characterized as an “ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, to engage in various forms of reasoning, to overcome obstacles by taking thought.”1

Although there is disagreement about the essence of intelligence, arguably the most influential and widely accepted model of intelligence in the Cattel-Horn-Carroll (CHC) theory of intelligence. CHC describes human cognition as a hierarchical three-tiered structure, with broader, more general psychological capacities located near the top, and narrower more specific abilities located near the bottom. A person’s intelligence corresponds to how well they can navigate this three-tiered structure and the strength of the individual capacities.

The three-tiered structure of intelligence under the CCC model. Source: T. Bates via WikiCommons CC BY-SA 3.0

At the top of this hierarchy (stratum III) is the most general factor of intelligence, sometimes referred to as just “general-factor (g-factor).” The g-factor represents and summarizes various correlations between the person more specific capacities. It is thought that differences in this g-factor account for about 45-50% of the difference between individuals on the test measuring cognitive ability. IQ tests are often interpreted as giving the participants standing on the scale of g-factors.

The next level (stratum II) corresponds to more general cognitive faculties, that each presides over narrower specific abilities. Some general categories include:

Comprehensive knowledge (Gc): Acquired knowledge and ability to communicate that knowledge

Fluid reasoning (Gf): General reasoning, concept-formation, incorporating new concepts or experiences

Reading & writing ability (Grw): Reading comprehension, vocabulary, writing skill

Short-term memory (Gsm): Ability to hold and manipulate information in immediate memory

Long-term memory (Glr): Ability to store and retrieve information later.

Visual processing (Gv): Memorizing, recognizing, and manipulating visual perceptions

Auditory processing (Ga): Ability o discriminate sound and analyze speech and other auditory stimuli

Processing speed (Gs): Ability to automatically perform cognitive tasks that require attention

Quantitative knowledge (Gq): Understanding number and other quantity-related concepts

Under each of these broad categories are specific capacities related to that domain of cognitive functioning. With this understanding of the structure of intelligence, IQ tests can be thought of as gauging the strength and connections of these psychological capacities.


As stated previously, the WAIS-IV is the world standard IQ test. The WAIS-IV 15 individual tests that give scores in 4 different categories. These categories are considered as representing the major components of intelligence.

Verbal Comprehension Index

The first score index is verbal comprehension. Essentially, questions in the verbal comprehension section gauge a participant’s ability to read a text, extract its central meaning, apply that meaning to other cases, and reason by analogy. Participants are also tested on the general level of knowledge, abstract reasoning skills, and level of vocabulary.

Perceptual Reasoning 

Questions in the perceptual reasoning section are meant to measure a person’s ability to process visual stimuli and spatial reasoning skills. The kinds of tasks measured in this section include: matching a picture of an object with a corresponding word, solving a visual geometric puzzle that requires the mental manipulation of shapes, non-verbal problem solving, pattern recognition, and quantitative reasoning.

Working Memory

the working memory index is a measure of a person’s ability to hold, manipulate, and synthesize multiple pieces of information at the same time. “Working memory” in this context is analogous to computer RAM; it is a measure of how many different tasks you can do at once. Assessments in this section include listening to and recalling sequences of numbers. performing mental math computations, focus and attention, and the ability to recall recently encoded information.

Processing Speed

If working memory is analogous to computer RAM, the processing speed is analogous to the CPU. The processing speed index tells you how quick a person’s “mental reflexes” are; that is, how quickly they can uptake, process, and regurgitate information. Test in this section includes searching for patterns in data or registering grapho-motor (writing) speed from seeing a picture of an object. The processing speed index tells you how quickly a person can integrate and change between different psychological functions.

An example of a question on the WAIS-IV perceptual reasoning section. the participant is asked to determine which shape comes next in the sequence. Source: “Raven Matrix” via WikiCommons CC BY-SA 3.0

IQ Scores And Related Issues

The exact relationship between IQ, environment, and genetics is not known. It is widely accepted that both genetics and environment play a role in determining IQ, but the exact proportions and mechanisms of influence are not well understood. It is possible that one of the two may have a dominant influence, but it is likely that both do in different ways.

IQ scores have been found to correlate with a number of other factors, like school performance, job performance, income, crime, and general health. All other things being equal, high IQ is linked to higher life satisfaction and high social status. This is not to say that those with high IQ are more likely to be happy, just that IQ and life satisfaction share a positive correlation.

There have been many criticisms of the concept of IQ throughout its history, mainly on the grounds that IQ tests leave out an important element of human experience and abilities that contribute to social success. The late paleontologist Stephen Jay Gould criticized the idea that intelligence could be ranked with a number and ordered in a linear fashion. Other have criticized IQ tests for leaving out assessments of other kinds of intelligence, like emotional or social intelligence, that produce success in society. Further, IQ tests have been criticized with respect to their historical use in promoting scientific racism. In the US, biased IQ tests have often been used to keep blacks and other minorities segregated from mainstream educational and political life.

IQ tests have also been criticized on the grounds that they are biased towards a particular style of abstract reasoning most prominent in Western countries. Alternatively, it is thought that IQ assessments are based on too narrow an understanding of cognitive functioning. Other intelligence assessments are based on a different operationalization of intelligence.  Psychologist Lee Vygotsky introduced a proximal zone of development theory that measures intelligence as a process of psychological development from one level of ability to another.

Regardless of these criticisms, there is agreement that IQ tests are not the end-all-be-all of social determinates. Having a high IQ does not guarantee success in life and having a low IQ likewise does not guarantee failure. There are many other important psychological characteristics like empathy, moral intelligence, and motivation that are extremely important to psychological and social flourishing. IQ is just one of the pieces of the puzzle.