Causes of Intellectual Disability

There are many and diverse causes of intellectual disability, and many of the causes of intellectual disability also cause other significant disabilities or health problems and can be associated with characteristic physical features. For example people with Down’s syndrome have a characteristic appearance and are more likely to have heart abnormalities diagnosed at birth. They are also likely to develop dementia as they get older. It is possible to identify the cause of the intellectual disability in about 30 to 40 % of people but the majority have no reason identified.

Classification of Intellectual Disability

People with intellectual disability vary in their abilities and needs for supports and getting to know someone is often the best way of finding out what they need and how best to provide this. However one of the characteristics that determine how much support as person needs is the degree of cognitive impairment that the person has. For this reason a very common way of classifying level of disability is based on IQ.  The range of IQ that defines each category is broad, and there is some overlap between categories. People with borderline intellectual functioning will have an IQ score between 71-84. About 6% of the general population fall into this range. Usually they will have no evident problems in their day to day life but may struggle when challenged. Although they may need supports they are often not eligible for these because their IQ score is too high. People with a mild intellectual disability have an IQ score between 50-55 to approximately 70 This is the commonest level of intellectual disability and about 2 % of the general population will have an IQ in this range. Many have no problems out of the ordinary and can lead reasonably independent lives, although they may need support in managing complex situations and in crisis. These people may have problems in reading, writing and handling numbers and money, and may find getting employment and housing difficult. Many do not receive formal services and often live with the support of their family and friends. People with a moderate intellectual disability will have an IQ score between 35-40 to 50-55. Typically a person with Down’s syndrome will fall into this range, which makes up about 0.2% of the general population. People with this level of impairment are often able to attend to their personal hygiene and to get dressed, and perhaps to make a simple meal but will require supports with most domestic activities and to access community facilities. Often a cause for the intellectual disability can be identified and there is increased risk of other medical problems such as epilepsy. Severe intellectual disability (IQ 20-25 to 35-40) and profound intellectual disability (IQ below 20) are often grouped together because it can be difficult to separate IQ scores at this level. They form about 0.08% of the general population. People with this level of cognitive impairment are usually unable to attend to even basic needs and may need support with toileting, washing and dressing. They may have very limited ability to communicate and many have no language. The cause of intellectual disability is often identifiable and structural brain abnormalities are often present. People in this category often have mobility problems, epilepsy and other medical issues.

Key Learning Points:


An IQ score can be useful as an

approximate global indicator of

intelligence, but it may not be a direct

indicator of a person’s abilities in real life.


Intellectual disability has many different

causes. Some causes have a range of other

effects including physical health and



Classifying the level of disability can serve

as a rough guide to the level of supports a

person may need and may indicate

whether they are likely to have additional

problems. However, the level of disability

cannot describe what each person will be like

and what specific supports they might need.

Home Home Select Module Select Module About VDDS About VDDS Provide Feedback Provide Feedback Continue Learning
Module 1 - Introduction to Mental Health Problems in People with a Dual Disability