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Intellectual disability matters because it is defined by deficits in BOTH intellectual functioning and adaptive functioning together, not by a low IQ (intelligence quotient, a standardised score of general cognitive ability) score alone. It is diagnosed as significant limitations in reasoning, problem-solving, and learning, alongside limitations in adaptive skills such as communication, self-care, and independent living, with onset during the developmental period. For example, imagine an adult with an IQ score of 65 who nonetheless manages a job, cooks, and lives independently: adaptive functioning this strong may mean the diagnostic threshold is not met. In practice, IQ alone can never confirm or exclude the diagnosis.

Severity, mild, moderate, severe, or profound, is now classified primarily by the level of support a person needs across conceptual, social, and practical domains, rather than by IQ score bands alone. Onset must occur during the developmental period, broadly before age eighteen, which separates intellectual disability from a later acquired cognitive decline such as dementia (a progressive decline in memory and thinking). Take two adults with the same IQ score of 60: one needs daily support with money and safety (higher support needs), the other manages independently with occasional help (lower support needs). In practice, this support-based approach means two people with identical IQ scores can carry different severity classifications.

• A man with intellectual disability becomes withdrawn and stops eating, and the team attributes this to his disability. What error is this?
• Which condition is the commonest inherited cause of intellectual disability?
• Compared with the general population, what is the rate of epilepsy in people with intellectual disability?

• Dementia, acquired decline after a period of normal functioning, unlike intellectual disability's onset during the developmental period.
• Specific learning disorder, one narrow skill deficit with otherwise normal general ability, whereas intellectual disability involves global deficits in both intellectual and adaptive functioning.
• Autism spectrum disorder, social-communication and repetitive-behaviour difficulty that can occur with normal intelligence, unlike intellectual disability's defining global cognitive deficit, though the two often co-occur.
• Intellectual disability, severity is set by support needs across domains, rather than by IQ score band alone.

• Confirmation biasA systematic error that skews a study's results away from the truth in a particular direction.Read more → is seeking evidence that supports a held belief, which is not the error named here.
• Down syndrome is the commonest genetic cause but is usually sporadic rather than inherited.
• Anchoring is over-weighting the first piece of information received.
• The availability heuristic is judging likelihood by ease of recall.
• Base-rate neglect concerns ignoring background prevalenceThe proportion of a population who have a condition at a given point or period in time.Read more →.

• Extract the defining phenomenology, duration, severity and functional-impact clues.
• Match the pattern to the most specific diagnostic or assessment construct.
• Actively test the closest look-alike against the missing or contradictory clue.
• Choose the precise term rather than a broader category that is merely compatible.