What is an Intellectual Disability?
An intellectual disability is a lifelong condition characterised by significant limitations for the individual both in intellectual functioning (reasoning, learning, problem solving) and in adaptive behaviour. They are classified as having significantly below-average intelligence (IQ) or mental ability (e.g., reasoning, learning, problem solving) and a lack of social and practical skills necessary for day-to-day living (adaptive functioning). The onset of intellectual and adaptive functioning difficulties should be evident before the individual is 18 years old. Significantly below average intellectual ability refers to an IQ score of 70 or less as measured by standardised tests.
The IQ test we administer for adults to assess an intellectual disability is called the Wechsler Adult Intelligence Scale - Fourth Edition, Australian and New Zealand Language Adapted Edition (WAIS-IV A&NZ; age range: 16 to 90 years, 11 months). The WAIS-IV covers four cognitive domains: verbal comprehension; perceptual reasoning, working memory and processing speed. Scores in these four areas are then combined to give a Full-Scale IQ Percentile, which is one way of assessing an individual’s level of general intellectual functioning.
For the assessment of intellectual disability, we also assess the individual’s adaptive functioning using the Adaptive Behaviour Assessment System 3rd Edition (ABAS-3). These scales measure typical everyday functioning across conceptual, social and practical domains (e.g., communication, self-care, memory, social skills, problem solving and work/study skills.)