There are many degrees of intellectual disability. The definitions and accepted diagnostic test for determining whether or not a person has an intellectual disability are explained along with key legislation. It is vital that each person’s case be assessed individually, and that the views of the person who has an intellectual disability always be sought, taken into account and, wherever possible, acted upon. All people with Autism Spectrum Disorders can now be considered for disability services.

Intellectual disability: Definitions and criteria

Last updated

30 June 2019

Defining intellectual disability

‘Intellectual disability’ is the term used in Australia to describe individuals who have impaired cognitive functioning evidenced in childhood. The same condition is described as ‘learning disability’ in the United Kingdom and, until recently, ‘mental retardation’ in the United States, where it is now called ‘Intellectual Disability (Intellectual Developmental Disorder)’.

There are several areas in Victorian and federal legal processes where a definition of ‘intellectual disability’ may be required. These include criminal justice, guardianship and administration, discrimination, and access to disability support services. There are several definitions of intellectual disability, and which one applies depends on the particular area of legislation and jurisdiction.

Victorian definition of intellectual disability

The Disability Act 2006 (Vic) (‘Disability Act’) (s 6(3)) sets out the test for determining whether or not a person has an intellectual disability. Sections 6(4)–(9) set out other important provisions relating to people who have an intellectual disability.

The legal definition of ‘intellectual disability’ in Victoria is in item (b) of the definition of ‘disabilities’ in section 3 of the Disability Act:

Intellectual disability in relation to a person over the age of five years, means the concurrent existence of:

  1. significant sub-average general intellectual functioning; and
  2. significant deficits in adaptive behaviour,
  3. each of which became manifest before the age of 18 years.

See the definition of ‘disability’ under ‘Disability Act’ in Chapter 8.1: Understanding disability and the law.

The definition of intellectual disability in the Disability Act was, until very recently, relevant when deciding on access to support services for people with a disability in Victoria. However, this is no longer the case since the roll-out of the National Disability Insurance Scheme (NDIS).

The definition of intellectual disability in the Disability Act is now most relevant to people with an intellectual disability in Victoria who are involved in the criminal justice system, who may be eligible to receive a ‘justice plan’. For information about justice plans, contact the Victorian Government Department of Health and Human Services (DHHS) (see ‘Contacts’ at the end of this chapter).

International legal definition of intellectual disability and criteria

Current internationally accepted diagnostic criteria for the condition of intellectual disability are more explicit.

For example, the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) – the internationally used manual for the diagnosis of mental impairments – states:

Intellectual Disability (Intellectual Developmental Disorder) is a disorder with onset during the develop­mental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains. The following three criteria must be met:

  1. Deficits in intellectual functions, such as reasoning, problem-solving, planning, abstract thinking, judgment, academic learning and learning from experience, and practical understanding confirmed by both clinical assessment and individualized, standardized intelligence testing.
  2. Deficits in adaptive functioning that result in failure to meet developmental and sociocultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more activities of daily life, such as communication, social participation, and independent living, and across multiple environments, such as home, school, work, and recreation.
  3. Onset of intellectual and adaptive deficits during the developmental period.

These DSM-5 diagnostic criteria may be useful in arguing that a person has an intellectual disability.

National legal definition of intellectual disability and criteria

Until recently, disability services in Victoria were provided or funded by the DHHS and eligibility to receive services was determined by the criteria set out in the Disability Act. However, from mid-2013 to mid-2019, the state disability services system transitioned to the national system, the NDIS.

The NDIS was established by the National Disability Insurance Scheme Act 2013 (Cth) (‘NDIS Act’) and is administered by the National Disability Insurance Agency (NDIA) (see ‘Contacts’ at the end of this chapter). Now, people who meet the NDIS eligibility requirements receive disability services through the NDIS – and no longer under the Disability Act. (However, there are currently some exceptions to this, as the transition to the NDIS is not fully complete in some areas of Victoria and some eligible people have not yet received their NDIS plans or services.)

Note that intellectual disability is not treated as a separate category but is included with various disabilities listed under ‘disability requirements’ in the NDIS Act. Also note that the ‘NDIS Operational Guidelines’ contain several lists of conditions that may or may not meet the eligibility requirements for a person to become a NDIS participant. Intellectual disability is included in list A, as does Autism.

Conditions likely to meet the disability requirements in the NDIS Act

The conditions that are likely to meet the disability requirements in section 24 of the NDIS Act include:

  • Intellectual disability diagnosed and assessed as moderate, severe or profound in accordance with current DSM-5 criteria (e.g. an IQ of 55 or less and severe deficits in adaptive functioning).
  • Autism diagnosed by a specialist multi-disciplinary team, paediatrician, psychiatrist or clinical psychologist who is experienced in the assessment of pervasive developmental disorders, and assessed using the current DSM-5 diagnostic criteria as having a level 2 severity (requiring substantial support) or level 3 severity (requiring very substantial support).

It should be noted that although the DSM-5 does not emphasise IQ results in the way the preceding
DSM-4 did, it does in part define intellectual disability in terms of current intellectual deficits of two or more standard deviations below the population mean. This generally translates to performance in the lowest three per cent of a person’s age and cultural group.

The DSM-5 also defines intellectual disability as an IQ of 70 or below (which the DSM-5 states should be measured with an individualised, standardised, culturally appropriate, psychometrically sound measure). An IQ of 70 is much higher than the IQ of 55 in list A of the ‘NDIS Operational Guidelines’.

Intellectual disability vs mental illness

In the past, there has been general confusion between intellectual disability and mental illness.

The Disability Act (and the Intellectually Disabled Persons’ Services Act 1986 (Vic) (repealed) prior to 1 July 2007) and the Mental Health Act 2014 (Vic) make a clear distinction between the two.

Some people may have both an intellectual disability or a developmental disability and a mental illness. This is called ‘dual disability’. Another term, ‘dual diagnosis’, means a person has both a mental illness and a substance-use disorder.

Other forms of cognitive impairment

Until 1 July 2007, when the Disability Act came into effect, no specific legislation dealt with the provision of services to people with other forms of cognitive impairment (e.g. acquired brain injury, which is cognitive impairment caused by accidents or drug use, or organic brain injury) and some Autism Spectrum Disorders. People with these disabilities could access services funded under the Disability Services Act 1991 (Vic) [repealed] and the Disability Services Act 1986 (Cth) (‘DSA (Cth)’). The Disability Act extended the coverage to include people with some other forms of cognitive impairment (e.g. acquired brain injury). The NDIS Act includes all these types of cognitive disability.

Autism Spectrum Disorders

Autism, also known as Autism Spectrum Disorders (ASDs), was initially not included in the Disability Act. However, the Victorian Government developed a ‘State Autism Plan’ and in 2009, ASDs were acknowledged and included in the Disability Act as a ‘neurological impairment’. This meant that all people with an ASD could be considered for disability services. (See ‘Disability Act’ in Chapter 8.1: Understanding disability and the law, for the definition of ‘disability’, where item 1a of the definition includes the term ‘neurological’.) ASD is included in list A of the ‘NDIS Operational Guidelines’.

It is estimated that 70 per cent of people who have an ASD also have an intellectual disability. Some of the remaining 30 per cent, although they do not have an intellectual disability, may nonetheless have a disability that requires services to assist them. For example, a child who has Asperger’s Syndrome or high functioning Autism (on the mildest end of the autism spectrum) may require speech therapy, psychological treatment and counselling.

Early intervention is extremely important for children with ASDs. The NDIS may provide funding for this crucial early intervention assistance. 

Diagnosis of an ASD must be made by a paediatrician in conjunction with a psychologist or psychiatrist. Some adults who have high-functioning Autism may not have an intellectual disability but nonetheless may be extremely disabled by their condition and require significant support to function and be part of their community. Many people with an ASD are also vulnerable to developing a mental illness.

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