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Over half a million Australians (3% of the population) have an intellectual disability and a majority (61%) of those people have a severe or profound limitation in 'core' activities of daily living (AIHW  2007). The term intellectual disability encompasses any set of conditions, resulting from genetic, neurological, nutritional, social, traumatic or other factors, prior to birth, at birth, or up to age 18 that affect intellectual development’ (World Health Organisation 2011 pg 5).

The impact of the disability on the person may range from minimal to severe. It is widely recognized that other factors such the presence of other disabilities (physical, sensory, psychiatric) and social support (family, friends, the community) play an important role in how each person with an intellectual disability functions in their daily life.

Some of the causes of intellectual disability include:

  • Brain injury or infection before, during or after birth
  • Genetic conditions/disorders (e.g.  Down Syndrome and Fragile X Syndrome the two most common genetic causes of intellectual disability)
  • Premature birth (long before the expected birth date)
  • Problems during pregnancy, labour and birth (e.g. lack of oxygen)
  • Growth or nutrition problems
  • Health problems during childhood (e.g. meningitis)
  • A range of medical disorders

The NSW Intellectual Disability Rights Service (IDRS) and Centre for Developmental Disability Health (Vic) define intellectual disability as a disability that occurs in the developmental period that is:

  • Characterised by limitation in intellectual functioning (meaning general mental capacity)  and
  • Significant difficulty in at least 2 areas of adaptive behaviour which covers many everyday Conceptual (e.g. language, literacy, numbers, planning, self-direction), Social (e.g. interpersonal skills, communication) and Practical skills (e.g. activities of daily living, self care, work skills, healthcare, travel, use of community resources).


Whilst every person is unique, many people with an intellectual disability may ‘have difficulty understanding, learning, remembering new things and in applying that learning in new situations’ (WHO report on Disability 2011, p305).  This may include difficulty processing information, grasping abstract concepts (e.g. money and time), planning and organisation and understanding the subtleties of interpersonal interactions and so may sometimes behave inappropriately in social situations.

The Western Australian Ministerial Advisory Council on Disability 2006 also identified decision making and identifying/ choosing options at key life transition points and adjusting to changing circumstances and unfamiliar environments as challenges for a person with an intellectual disability. They suggested greater support was needed during times of change, in particular key life transition points from home to school and from school to adult life (e.g. work, post-school study, participation in meaningful activities).

Historically, intellectual disability was defined on the basis of an IQ score (mild, moderate, severe and profound) which remained unchanged. However the Intellectual Disability Rights Service (IDRS) suggests intellectual disability should be considered in terms of the ‘support’ a person needs and not an unchanging characteristic of the person. The capacity of each person or the effect of their disability is influenced not only by their intellectual disability but also the environment and the support they receive. Changing the environment and the support to meet the person’s needs can greatly increase the person’s capacity and reduce the effect of their disability.

Diagnosis and assessment is helpful at any age to better understand and meet the person’s needs, particularly if the person has complex or challenging needs. The NSW Council for Intellectual Disability website lists NSW Diagnosis And Assessment Services with expertise in intellectual disability.


  • The NSW Council for Intellectual Disability - 2011
  • World Health Organisation report on Disability - 2011
  • The Centre for Developmental Disability Health Victoria (CDDHV) - Dr Jane Tracy
  • The NSW Intellectual Disability Rights Service (IDRS) -2011
  • Australian Institute of Health and Welfare- 2007a

The CDDHV have developed a number of Fact Sheets on topics and issues related to developmental Disability.

Last Modified: 18/10/2017 4:37 PM