Is Intellectual Disability a Mental Illness?

Intellectual disability is classified as a mental disorder in most classification schemes. However it not a diagnosis in the medical sense that can tell us what treatment someone needs to make them better or how their illness might progress. Most mental illnesses or disorders are identified by the fact they share a common cause, however this cannot be applied to intellectual disability as we know there are a number of different causes. The term disability is used to indicate that the person has difficulties in undertaking normal everyday functions because of an underlying impairment. If intellectual disability is understood in this way it can be seen as the consequence of some kind of brain pathology that is caused by an underlying disorder. For example the symptoms of Down’s syndrome include short stature, typical facial appearance, cardiac abnormalities and cognitive impairment (leading to intellectual disability). In other words one way of thinking about the population of people with intellectual disability is that they consist of people with a range of other diagnoses such as cerebral palsy or Fragile X Syndrome  that give rise to the cognitive impairment and intellectual disability. Understanding this helps to explain why the population of people with intellectual disability are so diverse and have such different needs. It also highlights the common misconception that behaviour problems cannot be attributed to the intellectual disability itself. If this was the case then the implication would be that the person’s intellectual disability should be treated to manage the behaviour.   In summary, although intellectual disability is classified as a mental disorder it does not really fit into this concept. ID is a concept that identifies a group of people that may need additional supports rather than a diagnosis of a mental disorder that can explain symptoms and determine treatment. Key Learning Points: 1. It is not straightforward to decide what constitutes a mental health problem and there is no definition of mental illness that captures the range of problems for which the term is used.


Various terms are used to describe mental

health problems including mental illness, mental

disorder, psychiatric disorder, personality disorder

and behaviour disorder.


Intellectual disability is classified as a mental

disorder but is best understood as a bureaucratic

category that identifies who is eligible for services.


The population of people with intellectual

disability have a large number of underlying

diagnoses that share cognitive impairment as a

symptom, and this results in intellectual disability.

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Module 1 - Introduction to Mental Health Problems in People with a Dual Disability