The general principles developed during the latter part of the twentieth century and continued into the twenty-first century guiding support for people with intellectual disabilities remain those of social inclusion and the provision of services to enable people to make, as far is possible, their own choices and to participate as full citizens in society. These are articulated in national policy documents, such as the White Paper for England, ‘Valuing people⁽¹⁾ and also at an international level in the UN Declaration on the rights of people with disability.⁽²⁾ However, given that people with intellectual disabilities represent a highly complex and heterogenous group with very varied needs, in order for such objectives to be achieved, a range of community based support and interagency and interdisciplinary collaboration is required. It is acknowledged that people with intellectual disabilities experience considerable health inequalities with the presence of additional disabilities due to the presence of physical and sensory impairments and co-morbid physical and mental ill-health, much of which goes unrecognized, and also the occurrence of behaviours that impact on their lives and the lives of those supporting them.^(3,4,5) In the twenty-first century, few would now challenge the objectives of social inclusion and community support. The tasks for Government and society are to provide special educational support in childhood and also support to the families of children with intellectual disabilities, and the necessary range of services to meet the social and health needs of this diverse group of people in their adult life. This includes enabling adults with intellectual disabilities to gain meaningful support or full employment and to exercise their rights as citizens and to participate fully in society. To achieve such objectives there is a need to be able to characterize the nature and level of need, to establish the presence and significance of co-morbid illnesses and/or challenging behaviours, and to organize and provide support and services to meet such identified needs.

This complexity of need has meant that no single ‘label’, such as ‘intellectual disability’, can adequately describe this group of people. What individuals have in common is a difficulty in the acquisition of basic living, educational, and social skills that is apparent early in life, together with evidence of a significant intellectual impairment. However, for some this may be of such severity that, for example, meaningful language is never acquired and there are very substantial care needs. For others, there is the presence of subtle signs of early developmental delay, and evidence of learning difficulties that only becomes clearly apparent at school when there is an expectation that more sophisticated skills will be acquired. The nature and extent of disability and of any functional impairments in general, distinguishing those people with intellectual disabilities from those with specific learning difficulties, such as dyslexia.

In infancy and early childhood, the reason for any apparent developmental delay needs to be established. This is primarily the responsibility of paediatric and clinical genetic services. Such information helps parents understand the reasons for their child’s difficulties and may guide, in a limited way, an understanding of future needs and potential risks. Later in childhood, the nature and extent of a child’s learning difficulties and a statement of special educational needs is the main task and later still, the main focus may be the assessment of longer-term social care needs. Throughout life, there may also be questions about a child’s or adult’s behaviour or mental state or the nature and extent of physical or sensory impairments and disabilities. The role of assessment is essentially to determine need and to inform the types of intervention and treatments, whether educational, medical, psychological, or social, which are likely to be effective and of benefit to the person concerned. Systems of classification provide useful frameworks for such assessments.

The term ‘classification’ is unfortunate as it carries with it the stigma associated with previous legislation (e.g. Mental Deficiency Act, 1913) and the associated history of institutionalization consequent upon the eugenics movement at the beginning of the twentieth century.
However, systems of classification are an important way of organizing information and thereby enabling the reliable passing of that information to others and providing a framework to guide intervention. Whilst there are clear strengths to this process, any system of classification has serious limitations. It will tend to focus on a few particular characteristics to the potential exclusion of others, and none can impart a truly comprehensive picture. Methods of classification have inevitably changed over time in an attempt to better clarify the key issues and to minimize stigma that might be associated with any given label. However, the central principle of any system of classification is to bring order to knowledge in a manner that may then enable further advances or the instigation of interventions that previous research has shown to be effective. There is no single universal system—the system of classification used depends on the reasons for its use. These may be as diverse as being predominately administrative or for the purposes of guiding intervention and the use or not of specific treatments.

Classification systems also differ with respect to whether they are dimensional or categorical in nature. Intellectual disability illustrates this difference in that measures such as those obtained from IQ tests are clearly dimensional and continuous whereas labels such as ‘intellectual disability’ or the identification of particular syndromes are categorical. More recently such obvious categorical distinctions have begun to break down as the genetic basis for syndromes are more clearly elucidated. For example, in fragile X syndrome there is variation in the extent of the number of repeat sequences in the FMR-1 mutation, both within carrier and affected individuals that influence whether or not an intellectual disability is likely to be present.⁽⁶⁾ Various different systems of classification are examined below and the relationship between assessment and classification is considered.