Definition of Intellectual Disability
According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-V), intellectual disability is a developmental disorder that occurs before the age of 18 years old and is characterised by significantly below-average intellectual functioning and deficits in adaptive functioning (e.g. communication, social participation, education, play/leisure and activities of daily living) (American Psychiatric Association, 2013; Parekh, 2017).
Causes of Intellectual Disability
The causes of intellectual disability are categorised on the basis of when they occur in the developmental period, i.e. before birth, during birth, or after birth. Some of the identified causes include:
- Before birth: genetic causes, exposure to maternal infection, alcohol, or drugs
- During birth: prematurity, low birth weight, and various labour/delivery-related events
• During childhood: complications from infectious diseases, accidental injury to the brain, and poverty
Types of Intellectual Disability
Individuals with intellectual disabilities are often placed into levels to reflect the severity of their intellectual and adaptive functioning impairment (APA, 2013). These levels include:
- Mild (Intelligence Quotient (IQ)) Score Range: 50 – 70)
- Moderate (Intelligence Quotient (IQ)) Score Range: 35 – 49)
- Severe (Intelligence Quotient (IQ)) Score Range: 20 – 34)
- Profound (Intelligence Quotient (IQ)) Score Range: Less than 20)
For an individual to be diagnosed with an intellectual disability, the following three criteria must be met:
Criteria 1 – deficits in intellectual functioning (intelligence)
This includes various mental abilities:
- Problem solving
- Abstract thinking
- Academic learning (ability to learn in school via traditional teaching methods)
- Experiential learning (the ability to learn through experience, trial and error, and observation)
Criteria 2 – deficits in adaptive functioning
Adaptive functioning refers to a collection of skills in communication, social, conceptual, personal independence, school or work functioning that enables the individual to cope with common life demands, and function independently, effectively, and as expected in an age-appropriate manner (American Psychiatric Association, 2013).
These limitations occur during the developmental period. This means problems with intellectual or adaptive functioning were evident during childhood or adolescence.
American Psychiatric Association (2013). Diagnostic and Statistical Manual of Mental Disorders (5th ed.).
Parekh, R. (2017). What is Intellectual Disability?