Basic Neurosciences · Paper A
Classification of intellectual disability severity by IQ range
Clinicians need a shared language to describe how much support a person with intellectual disability requires, and that need is why severity levels exist alongside, not instead of, an IQ score. For example, two people with the same IQ score can need very different levels of daily support depending on their adaptive functioning. The classification connects a statistical concept, an IQ score standardised against a population mean of 100, to a clinical concept, how independently someone can live.
Historically, ICD-10 defined four severity bands for intellectual disability by IQ range: mild, approximately 50-69, moderate, approximately 35-49, severe, approximately 20-34, and profound, below 20. DSM-5 and ICD-11 both moved away from relying on IQ score alone, defining severity instead primarily by adaptive functioning across conceptual, social and practical domains, because IQ alone does not capture how much day-to-day support a person needs. The traditional IQ bands are still widely taught and quoted for orientation, but an exam question testing current classification should prioritise adaptive functioning as the defining feature.
The stem describes a person's IQ score and/or their level of independence in daily activities, such as self-care, communication or safety awareness, and asks which severity level, or which classification principle, applies.
Distinguish intellectual disability, a neurodevelopmental condition with onset during the developmental period affecting both intellectual and adaptive functioning, from a specific learning disorder, which affects one particular academic skill, such as reading, while general intellectual functioning stays within the average range. Distinguish severity graded by adaptive functioning, the current DSM-5/ICD-11 standard, from severity graded by IQ band alone, the older, ICD-10-style approach.
Trapassuming a specific IQ number alone determines the DSM-5 severity level. Resolution: DSM-5 defines severity primarily by adaptive functioning, conceptual, social and practical skills, using IQ mainly to confirm the diagnosis, not to grade severity precisely. Trap: assuming the ICD-10 IQ bands are still the primary current classification method. Resolution: these bands remain widely taught for orientation, but ICD-11 and DSM-5 both prioritise functional support needs over a fixed IQ cutoff.
When a question gives both an IQ number and a description of daily functioning, weight the functioning description more heavily for current DSM-5/ICD-11 classification; use the IQ number mainly to sanity-check that it is broadly consistent with the described severity.