The internationally accepted diagnostic criteria for the condition of intellectual disability is found in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5):
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:
a. 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.
b. 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.
c. Onset of intellectual and adaptive deficits during the developmental period.
Where the term ‘intellectual disability’ is defined – in the Crimes Act 1958 (Vic), Sentencing Act 1991 (Vic) and the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (Vic)) – it relies on the definition in section 3 of the Disability Act 2006 (Vic) (‘Disability Act’).
Section 3 of the Disability Act states that:
Intellectual disability in relation to a person over the age of five years, means the concurrent existence of:
a. significant sub-average general intellectual functioning; and
b. significant deficits in adaptive behaviour,
c. each of which became manifest before the age of 18 years.
Sections 6(3) and 6(4) of the Disability Act provide further clarification as to the technical requirements in relation to standardised measurements of intelligence.