Intellectual disability *

Intellectual disability is described in the DSM - 5 as a developmental disorder, referring to a neurodevelopmental disorder with its onset during the developmental period. Intellectual disability is characterised by intellectual and adaptive functioning deficits in conceptual, social and practical domains.

Adaptive functioning refers to the ability to adapt to the needs of everyday living and requires conceptual, social and practical skills. With an intellectual disability, a person’s lower intellectual functioning leads to limitations in adaptive functioning.

DSM-51 has seen a shift in focus from reliance on IQ, or intelligence scores alone to diagnose severity of intellectual disability, to an emphasis on levels of adaptive functioning. Although IQ scores have been removed from the main definition criteria, they are still included in the text description of intellectual disability.  Thus intelligence testing is not excluded from the assessment procedure, but it is not over emphasised as the defining

There are four levels of severity of intellectual disability each with different support needs

Mild intellectual disability

Intelligence quotient (IQ) is generally between 50 and 69 and accounts for about 80% of people with intellectual disability. Development during early life is slower than in typically developing children and developmental milestones are delayed (see Appendix B for an overview of developmental stages). Children and adults are able to communicate well and learn occupational basic skills. Their ability to learn to read and master computer skills is on par with learners of 9-11 years. As adults, they are able to gain employment on the open labour market but may need intermittent support.

Moderate intellectual disability

Intelligence quotient (IQ) is usually between 35 and 49 and accounts for about 10% of people with intellectual disability. Children and adults are slower in meeting intellectual developmental milestones than typically developing peers. Their ability to learn and think logically is impaired but they acquire adequate communication and academic skills. Adults will need varying degrees of support to live and work in the community. With supervision and regular support they can perform unskilled or semi-skilled work.

Severe intellectual disability  

Intelligence quotient (IQ) is usually between 20 and 34 and accounts for 3% - 4% of all cases. Every aspect of a child’s development in the early years is distinctly delayed. They have difficulty pronouncing words and have a restricted vocabulary. Through considerable practise and time, they may gain basic self-help skills but still need support at school, home and in the community, and have optimal functional capacities of a child of approximately 3-5 years.

Profound intellectual disability

Intelligence quotient (IQ) is usually below 20 and accounts for 1%-2% of all cases. These children and adults cannot take care of themselves and have little or no verbal language. Their capacity to express emotions may be poorly understood. Seizures, physical disabilities and reduced life expectancy are common. They have the optimal functional capacities of a child of less than 3 years.


* Adnams C & Johns R (eds) Understanding Intellectual Disability (WCFID 2016)