Compiled by a Working Group of the British Psychological Society’s Division of Clinical
Psychology, Faculty for People with Intellectual Disabilities


This document has been produced as guidance for clinical psychologists who are
frequently asked to assess whether or not an individual has an intellectual disability. It will also be of use to commissioners, colleagues in intellectual disability services and families and individuals who are seeking clarification on this issue. The aim is to outline good practice in this area. It also considers the different contexts in which assessment of
intellectual disability may be relevant, including mental health and mental capacity
legislation, court proceedings, service entitlement and the family courts.

Intellectual disability is defined as a significant impairment in intellectual functioning and significant impairment in adaptive behaviour (social functioning), with each of these
impairments beginning prior to adulthood. In practice, a diagnosis of intellectual disability is sometimes made without reference to all three criteria and with debate over the meaning of scores on an assessment of intellectual functioning (usually known as an
IQ test). This has resulted in confusion for families, service users and care providers and
consequent difficulties in ensuring that individuals receive an appropriate service.

This document seeks to clarify the components of an assessment, considers the meaning of the ‘scores’ that are obtained and outlines the means by which psychologists reach their opinion in relation to whether or not an individual has an intellectual disability. It provides guidance on technical issues and notes the difficulties associated with assessing intellectual functioning for people who have an intellectual disability. It also notes the relevance of psychologists using their clinical judgement in interpreting complex information.

The document recommends specific measures that should be used and provides guidance on how findings should be presented. It also notes the importance of ensuring that assessment is undertaken by an appropriately qualified psychologist. A further
recommendation is that a judgement as to whether or not an individual has an intellectual disability should only be made when all three components of the assessment are carried out by an appropriately qualified professional, who is able to justify their opinion in accordance with this guidance. This would reduce confusion for individuals, families and services.