Intellectual Disabilities

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About the Author

Dr. Mark Lovell, Consultant Child and Adolescent Intellectual Disability Psychiatrist, Tees, Esk and Wear Valleys NHS Foundation Trust, and ACAMH Deputy Chair and Director of CPD and Training.

Acknowledgements: Thank you to Stavros Simitsis, Mary Busk and Dr Juliette Kennedy for assistance in the production of this topic guide.

This topic guide has been created to describe what an Intellectual Disability (ID) is, the historical influences, the terminology issues and the diagnostic criteria. It outlines how an ID is suspected and identified and also the areas to consider when creating a care plan. It includes an alphabetical framework to assist professionals in screening for and identifying an Intellectual Disability.

Identifying an Intellectual Disability is important to a young person, their family, carers and services e.g. education, health or social care. A variety of evidence should be considered when diagnosing or formulating, with a view to creating a person-centred plan to meet a person’s needs.

It is very important to be aspirational for everyone with an ID in terms of quality of life and life outcomes. Having an ID should not be seen as an impairment, nor a barrier to having a good life and a full life like everyone else.

 Key Points

  1. Intellectual Disabilities have other equivalent names which have changed over time
  2. Identification requires more than just an IQ score
  3. Alternative diagnoses/formulations should be considered
  4. Meeting needs is more important than just identifying an Intellectual Disability
  5. Diagnosing an Intellectual Disability may be essential for access to services or for legal protection.
  6. A structured approach to identifying an Intellectual Disability can assist in making a diagnosis and in considering each person as an individual.
  • Introduction

    What is an intellectual disability?


    Many terms have been used to describe an intellectual disability (ID) or medical conditions linked to an ID. Some terms that were originally designed to describe levels of intellectual disability or specific medical conditions, have unfortunately become part of common derogatory language used within society and so have become insults; e.g. moron, idiot, imbecile, mental retard, feeble-minded, mentally deficient, mental impairment, cretin, spastic and the list goes on. Therefore, the words used to describe ID keep changing in order to move away from the negative connotations attached to them.

    Different countries, health disciplines, educational settings, classification systems and advocacy groups use different terminology, though a common ground appears to be developing now.

    Within mental health and other settings, the term ‘learning disability’ has been established for a number of years and many services still use this term. However, ‘Intellectual Disability’ is currently gaining favour as the preferred term by health professionals, but it is not consistently used. This review will use ‘Intellectual Disability’ for the purposes of consistency and will use UK based terminology and experience for examples.  This reflects changes in international coding and classifications, which are described below.

    The term ‘mental age’ is no longer acceptable, though comparisons of individual skills with typically developing individuals of different chronological ages still occurs as part of understanding degree of delay.


    The International Classification of Diseases version 10 published by the World Health Organisation in 1993, is still in use. Currently the terminology used is ‘mental retardation’ which reflects an IQ of 69 or below with social competence difficulties (daily living skills) which are seen within the developmental period. This would place an individual’s IQ in approximately the bottom 2% of the population i.e. outside of the ‘normal’ range (World Health Organization [WHO], 1992).

    ICD-11 (developed, awaiting implementation)

    ICD-11 (World Health Organization website, 2017) is awaiting implementation at the time of writing. It is available on line and it is planned that the terminology will change to a ‘Disorder of Intellectual Development’ to parallel the changes that have occurred between DSM IV (American Psychiatric Association [APA], 2000) and DSM 5 (American Psychiatric Association [APA], 2013).

    DSM 5

    The Diagnostic Statistical Manual version 5 (APA, 2013) is produced by the American Psychiatric Association and was revised in 2013. The current terminology used is Intellectual Disability or Intellectual Developmental Disorder (IDD) which equates to:

    1. Deficits in intellectual functioning. (These should be confirmed by clinical assessment and standardised intelligence testing. i.e. an IQ of 70 or below),
    2. Deficits in adaptive functioning (that without ongoing support would limit functioning in daily life) and
    3. An onset during the developmental period (i.e. childhood).

    Note– DSM 5 includes an IQ of 70 within Intellectual Disability, whereas ICD10 does not.

  • What we know already


    Intellectual Disability affects approximately 2% of the population. Rates vary depending upon the source of information e.g. Office for national statistics data is based upon parental/self –report. Health or education data sets use the available evidence (there is often over or under reporting and the language used differs, so accuracy is dubious). Epidemiological research may use cognitive tests and developmental questionnaires to validate diagnoses. Rates appear to be rising in children following improvements in neonatal/paediatric healthcare and improved survival rates following premature birth or life-limiting physical illnesses.


    Intellectual Disabilities may be caused by a variety of reasons. They may have no identified cause, they may be due to a genetic condition that might be new or known within a family, or due to a physical health reason e.g. infection, head trauma, a degenerative neurological condition, a bleed or lack of oxygen to the brain (cerebral palsy) or poisoning by substances such as lead to name a few. Severe neglect or lack of educational opportunities may also affect intellectual development.

    Levels of severity (ICD-10)


    This is a description given to IQ scores slightly above the intellectual disability range. It is not actually an intellectual disability and is best described as a learning difficulty (World Health Organization, 2004).


    Individuals with mild intellectual disabilities have an IQ within the range of 50 to 69. Speech will eventually be in full sentences. Academically they will be significantly behind e.g. approximately 5 years behind by age 16 and at least 2 years behind by the start of school. Support needs will vary for each individual, though a degree of independence will be possible by adulthood with the ability to enter into employment. By adulthood, developmentally the individual will be functioning with skills equivalent to a 9 to 11-year-old (World Health Organization, 2004).


    Individuals with a moderate intellectual disability have an IQ within the range of 35 to 49. Delays are more marked than in an individual with a mild intellectual disability. Most will develop self-care skills and be able to communicate adequately. Support is generally required life-long. By adult hood, developmentally an individual will function with skills equivalent to a 6 to 9-year-old (World Health Organization, 2004).


    Severe intellectual disabilities have an IQ range of 20 to 34. Delays are significant and continuous support needs should be expected. By adult hood an individual may be functioning with skills equivalent to a 3 to 6-year-old (World Health Organization, 2004).


    Individuals with profound intellectual disabilities are likely to be pre-verbal and have multiple other disabilities and require lifelong support for most or all activities of daily living.  The IQ range is under 20, however formal testing is often not possible (World Health Organization, 2004).

    Why is it important to identify an intellectual disability?

    The Individual

    It is important to an individual, because it may help with their own understanding of their strengths and difficulties as well as with identity development.

    Family/carers/support networks

    It is important to a family or carers, because it may help with their understanding of an individual’s strengths, difficulties and needs. Reasonable adjustments can be made e.g. communication, parenting strategies, maintaining safety etc.


    Education is adapted to ability. Knowledge of an intellectual disability may lead to the creation of individualised Education Health Care Plans (EHCPs). Educational provision can be modified to reflect ability and EHCPS help to coordinate multiagency input. Young people may need access to specific educational provisions e.g. special schools or support bases.

    Social Care

    Many social care teams distinguish between individuals with or without a disability. This can alter services offered.


    Health provision e.g. Child and adolescent or adult mental health services often distinguish between whether they are commissioned for individuals with an ID. Some will cover mild ID but not moderate to profound ID. So, having an ID identified, may affect service offered and the expertise of the staff available to the young person e.g. LD nurses or Child and Adolescent Intellectual Disability psychiatrists. These clinicians have additional expertise in the recognition of mental health difficulties within ID and the modification of therapeutic input to meet identified need.

    It is really important that all health provision and services are of the highest quality for children, young people and adults with an ID. Their lives matter like others and they often need extra care and support because they may not be able to express themselves like others or indicate pain or other symptoms of ill-health.

    Third Sector

    Various charitable or similar services have ID within their remit.

    Criminal Justice/ Police

    Identification of an ID may alter police and court response including making reasonable adjustments, use of appropriate adults, modified court processes etc.


    Benefits may be related to the identification of diagnoses or specific needs e.g. Disability Living Allowance or Personal Independence Payment


    Identification of an intellectual disability can affect the use of legislation e.g. equality legislation and reasonable adjustments or capacity legislation.

    This legislation offers legal protection to children, young people and adults with an ID for reasonable adjustments and the need to avoid diagnostic overshadowing etc.

    Population Data/Registers

    Addition to local population registers, where they exist, allows services to be correctly commissioned and funded to meet the expected needs of the ID population.

    Research and use of evidence base

    Knowledge of the presence of an ID may affect the evidence base for treatments that could be offered. ID may also affect whether an individual might be able to be included in scientific research.

  • Identification

    The Identification of an Intellectual Disability, an A to H Framework (IDID A2H©)

    The following alphabetical framework is designed as an aide memoire for the identification of an Intellectual Disability (in children, young people or adults). It will help identify available information sources, gaps in knowledge about an individual’s ability and guide decision making regarding a likely diagnosis and generating a needs-based plan.  Making a ‘diagnosis’ of an Intellectual Disability is the remit of psychologists and medically trained professionals. The framework can be used by other professional groups to ‘formulate’ and gather the appropriate information to establish a likelihood of an Intellectual Disability and guide diagnostic processes or access to appropriate services. It is a longer version of the IDID A2H: short version© and the IDID A2H: Parent version©. The Professional IDID A2H Evidence Collection Framework© is an editable version that can be used as a tool for gathering evidence and planning.

    It has been created by the author and is free to use clinically. It can be reproduced for non-commercial uses. Any publication in hard copy or electronically requires permission from the author, acknowledgement of the copyright and authorship. No reasonable request will be refused. Contact


    Evidence from education is helpful for understanding an individual’s level of functioning. This may be current or predicted attainment or historical information. If in education, it is useful to consider current and past attainment. This may be compared against established expectations for an age group e.g. national curriculum levels. Information may be gained from school or college reports, direct communication with a teacher or results of tests e.g. examinations or specific learning ability tests.

    If the person is out of education, it is important to consider how much education was received, whether it was in an appropriate setting and what attainments were made.

    Diagnoses may have already been made e.g. of a learning difficulty. Care should be taken to establish whether these are specific e.g. dyslexia or more generalised and what level of difficulty has been assigned.  A general rule of thumb is that a generalised moderate to profound and multiple learning difficulty, equates to a mild to profound intellectual disability.

    Attendance and engagement in education is also an important factor. Not attending, not engaging, having a specific learning difficulty or behavioural/emotional difficulties that interfere with learning and attainment, or not receiving appropriate education to meet an individual’s needs, are likely to contribute to lower attainment irrespective of level of intelligence. It is therefore important to understand the context.

    Behaviours of daily living

    These are also known as adaptive behaviours or activities of daily living. They reflect a range of skills that are learnt and develop over time. These behaviours include managing personal hygiene, dressing skills, eating, toileting, ambulating, engaging in leisure activities, skills in domestic living to name a few. These behaviours may be assessed in different environments e.g. home and school.

    To make a diagnosis of an intellectual disability, evidence of significant delay in adaptive behaviours is required. There are various tests that can contribute towards understanding the degree of delay in adaptive behaviours e.g. ABAS (Pearson website, 2015), ABS (Assessment Psychology Online website, 2001) and Vineland (Pearson website, 2005). Training in the administration of these tests and their interpretation may be required. Direct observation is also important.

    Cognitive assessments

    Formal assessments of cognition are important in making an accurate diagnosis of an intellectual disability. Formal assessment will calculate a full-scale IQ score and will also produce sub- section scores that describe different areas of skill or difficulty. The IQ score classifies an individual as having an intellectual disability and also gives a reference range of IQ scores that an individual might lie within, with a high degree of certainty.

    A variety of IQ tests are available covering different ages or levels of verbal ability. E.g. WIPPSI (Pearson website, 2012), WISC (Pearson website, 2014), WAIS (Pearson website, 2010), or WNV (Pearson website, 2006). These tests are carried out by appropriately trained psychologists. In the UK, Educational Psychologists may also use the BAS (Fountas & Pinnell Literacy website, n.d.).

    There are some limitations to cognitive assessments. IQ is supposed to be stable over time however it can worsen if an individual is losing skills or ability e.g. if they have a neurodegenerative condition or if they are choosing or not able to engage with the formal testing e.g. if they are refusing to engage, or deliberately underperforming, if their primary language is not English or if they are not able to communicate or focus.

    Short screening IQ tests are available e.g. Ammons and Ammons (Frank & George, 1971) or the KBIT (Pearson Clinical website, 2004); however, they do not produce the level of information that is important to understand an individual’s strengths and difficulties. They may be useful for briefly establishing the likelihood of an intellectual disability e.g. by non-psychologists.

    Development (other)

    Development occurs in a range of areas and at different rates. Physical development includes growth, fine motor, and gross motor and sensory development. Socio- emotional development includes social skills, the development of attachments, play and behaviours. Other areas of development include sleep, speech and language and communication. Investigation and assessment may point towards specific delays or more generalised delays. The degree of these should be considered and needs based plans generated.

    There are formal tests to assess these areas e.g. CELF for speech and language (Pearson website, 2013), development scales e.g. Denver and Schedule of Growing skills (GL Assessment website, n.d.) and assessments of behaviours e.g. Adaptive Behaviour Assessment System (Pearson website, 2018) and Adaptive Behaviour Scale (Pearson website, 2005). Some of these can only be carried out by appropriately qualified professionals.

    The presence of differing patterns of developmental delay may indicate other conditions e.g. speech, language or communication disorders or other neurodevelopmental disorders.

    Environmental Influence

    It is important to consider the influences around an individual. This may include understanding the different environments that they live and operate within. The most obvious is the home environment (including the general socio-economic setting) and whether it is supportive of development and learning. In children and young people, school is often the other major influence on development and learning. The therapeutic influence on development of significant time spent in differing settings should also be considered. E.g. if an individual is being neglected and are in an inappropriate educational setting, the likelihood of an individual meeting their academic and developmental potential is less than if they were in an educational setting that did meet their needs.

    Factors (other)

    Other factors may be present either as a cause of the intellectual disability e.g. genetic conditions or in addition to the intellectual disability. These may either be masked by the intellectual disability or mask the intellectual disability itself. This is called diagnostic overshadowing. Other neurodevelopmental conditions e.g. ADHD or ASD are likely candidates for diagnostic overshadowing.

    Mental Health and Behavioural disorders as well as physical health problems are more common in individuals with intellectual disabilities and presentations may be atypical. E.g. tooth ache may present as behaviour that challenges others or a mood disorder may present with more biological symptoms e.g. sleep, appetite or energy level disturbance, rather than clear psychological symptoms. Neurological disorders are common e.g. epilepsy.

    Attachment difficulties may also present following early childhood adversity and/or differences in parental responses to a child with a disability. The incidence of abuse and need for safeguarding may also be higher in young people with ID than the general population.

    The above factors should be considered holistically, alongside an understanding of the young person’s IQ and adaptive behaviours, so that their unique strengths and difficulties and environmental challenges are understood.

    General impression

    The 3 main outcomes for use of the IDID A2H© are:

    • Not an Intellectual Disability i.e. IQ and/or adaptive functioning are within normal range
    • Possible Intellectual Disability i.e. further investigation is required over time
    • Confirmed Intellectual Disability i.e. IQ is 70 or below and there are significant difficulties with adaptive functioning

    It is important to be clear about what terminology is being used and the meaning of this and to communicate this understanding to the individual and involved parties (if appropriate).

    The diagnosis may be supplemented by a formulation to understand the factors involved e.g. how is the individual presenting? What has caused or contributed to the diagnosis and what is protective or supportive.

    How to meet an individual’s needs

    In partnership with the person and their parent/carer, generate a person centred multiagency, multidisciplinary care plan to meet the needs identified in A to G. Plans should include recommendations for the interventions or adaptations required to meet the identified needs (irrespective of the diagnostic outcome). It is important to communicate the findings and plan to others in the individual’s networks e.g. family/carers, education, social care and health.

  • Areas of uncertainty

    Confusing Terminology


    In the UK, education systems describe levels of ‘learning difficulty’. These are not exactly the same as an intellectual disability. They can be specific e.g. Dyslexia or more generalised. Moderate to severe generalised learning difficulties and below, are roughly equivalent to an Intellectual Disability. These may be diagnosed by Educational Psychologists and influence the education provision that a child will receive. This understandably can lead to confusion for parents and professionals about a person’s diagnosis and needs.


    To add to this, many children and young people with delays are initially seen by paediatricians and associated services e.g. child development centres with multidisciplinary inputs e.g. Occupational Therapy, Physiotherapy, speech and language therapy, portage, education psychologists and health visitors. The term ‘Global Developmental Delay’ is often used, but should only be used before the age of 5 and does not specify a level of delay. Many of these children could be re-diagnosed with an intellectual disability at aged 5, however this assessment is not always done and confusion about diagnosis and needs can result.

    Mental Health

    In the UK, the predominant terms used by Mental Health services are ‘Learning Disabilities’ and more recently ‘Intellectual Disabilities’. These terms are used interchangeably. The Royal College of Psychiatrists have renamed the Learning Disability faculty to the faculty of Intellectual Disabilities (Royal College of Psychiatrists, 2018) and there are changes occurring in how psychiatrists are describing themselves e.g. ID psychiatrist, though often they still work within services that use the Learning Disability term. Assessments are usually made by clinical psychologists and involve formal cognitive testing and information gathering about adaptive behavioural function (daily living skills), academic progress and speech, language and communication.

    Social Care

    The terminology generally used is disabilities, which encompasses more than intellectual disabilities. Social care does not make diagnoses and have to rely on health and education to identify the appropriate diagnoses to allow access to service.


    In the UK, most reports, legislation or guidance either from the government, associated bodies or advocacy groups use ‘Learning Disability’. Many of the more recent initiatives combine ASD (Autism Spectrum Disorder) with Learning Disabilities. Approximately half of children and young people with ASD have an intellectual disability and a quarter of those with intellectual disabilities have ASD (National Institute for Health and Care Excellence, 2017). Many of the needs are similar; however, they are not always exactly the same. This has caused some confusion when separating out recommendations, and when multiagency input is required e.g. between health, social care and education.  Some data sets used for public health analysis, not only merge ASD and intellectual disabilities but also children and adults, making a lifespan commentary on results and recommendations.


    To add, further confusion, terminology does not remain constant between countries, for instance in the US the term Learning Disability reflects a wider group including those with specific learning difficulties e.g. dyslexia, Asperger syndrome and ADHD. This can cause issues when looking at research findings, when attending educational events or when reading text books on the topic.


    When reading the evidence base, care is needed when extrapolating from either adult or child and adolescent research. These often exclude intellectual disabilities and are therefore not always interpretable for individuals with intellectual disabilities.  Children and young people are not small adults with intellectual disabilities and adults with intellectual disabilities are not big children and adolescents. Research may also exclude more severe levels of intellectual disabilities.

  • What’s in the pipeline?
  • Useful organisations and resources

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