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Intelligence testing is often reduced in casual speech to a single number — IQ (intelligence quotient). That single-number framing hides most of what an exam question is actually testing: what the number MEANS, how it is DISTRIBUTED, and what it CANNOT tell you. Hold the phrase "more than one number" through every section below.

An intelligence test is a standardised psychometric (measuring mental characteristics) assessment designed to measure general cognitive ability, most often expressed as an IQ score. The most widely used modern instrument is the Wechsler Adult Intelligence Scale (WAIS), which yields a Full-Scale IQ alongside several sub-index scores rather than one flat number.

A common mistake is treating IQ as a fixed, purely biological trait, unaffected by anything else. In reality, IQ scores are influenced by education, culture, test familiarity, mood, and physical health at the time of testing — a person tested while acutely unwell or severely depressed can score artificially low, which is exactly why intelligence testing is never interpreted in isolation from clinical context.

Imagine plotting the IQ scores of an entire population on a graph. They form a bell-shaped curve called the normal distribution, centred on a mean of 100, with a standard deviation (a measure of how spread out scores are around the mean) of 15. Roughly two-thirds of the population score between 85 and 115 (within one standard deviation), and roughly 95% score between 70 and 130 (within two standard deviations). A score below 70 (more than two standard deviations below the mean) is the threshold historically used to flag significantly below-average intellectual functioning.

Modern intelligence tests like the WAIS do not report just one score; they break performance into index scores such as verbal comprehension (understanding and using language), perceptual reasoning (non-verbal pattern and spatial reasoning), working memory (holding and manipulating information briefly in mind), and processing speed (how quickly simple mental tasks are completed). For example, a patient recovering from a head injury might show preserved verbal comprehension but markedly reduced processing speed — a pattern a single overall IQ number would completely hide. Different instruments are used at different ages: the Wechsler Intelligence Scale for Children (WISC) is used in childhood, and other batteries exist for specific purposes, such as pre-school assessment or non-verbal-only testing in someone with a language barrier or hearing impairment.

Intelligence testing measures general cognitive ability broadly; it is not the same as a screening tool for cognitive decline such as the MMSE (Mini-Mental State Examination) or MoCA (Montreal Cognitive Assessment), which are brief bedside screens looking for a CHANGE from a person's previous baseline, most often to flag dementia, rather than measuring stable overall intelligence. It is also not the same as an academic attainment test (which measures learned knowledge in a specific subject, such as reading or arithmetic, rather than general reasoning ability) — a child can have average IQ but a specific learning difficulty in reading, a pattern that attainment testing, not IQ testing alone, is designed to reveal.

IQ testing itself is not a diagnosis, but it is one of two required pillars in diagnosing intellectual disability (also called learning disability): DSM-5 requires BOTH a significantly below-average IQ score (traditionally below 70, i.e. more than two standard deviations below the mean) AND significant deficits in adaptive functioning (day-to-day skills such as communication, self-care, and independent living), with onset during the developmental period. Choosing the wrong instrument for the wrong population — for example, a purely verbal test for someone who does not speak the test's language fluently — produces a misleadingly low score that reflects the mismatch, not true ability, which is why the IQ score is never used alone.

Test administration itself must be carried out by a trained clinical or educational psychologist under standardised conditions, since the score is only meaningful if the test was given exactly as designed. Results feed directly into real decisions: educational statementing (an Education, Health and Care Plan in England) to secure extra school support, capacity assessments under the Mental Capacity Act, and forensic or vocational planning. NICE guidance on learning disabilities recommends comprehensive assessment, combining cognitive testing with adaptive-functioning assessment, before any diagnosis or support plan is finalised, rather than relying on the IQ number in isolation. The management implication is practical rather than pharmacological: the test profile informs support planning, reasonable adjustments, educational interventions, occupational recommendations, and psychological therapy adaptations, but it does not by itself prescribe a first-line medication or treatment. A low IQ result should therefore trigger a broader assessment-and-support plan, not a diagnosis or management decision made from the number alone.

Exam writers sometimes present a single IQ number in a vignette and expect the candidate to comment on functional ability, when in fact functional ability depends on the PROFILE across sub-scores and on adaptive functioning, not the single overall figure, and always needs to be interpreted alongside the person's education, culture, and clinical state at the time of testing.

IQ testing gives a profile across sub-scores, not one number, and only becomes a diagnosis of intellectual disability when a low score is paired with genuine deficits in day-to-day adaptive functioning.