Clare Nicholson: Misalignment between the DSM-5 and the Mental Capacity Act

Clare Nicholson issues a call for clarity in defining severe intellectual disability

While I was completing my research into healthcare interactions that involve people with severe intellectual disabilities (ID), I realised that there seems to be some misalignment between the definition of severe ID provided in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), the American Psychiatric Association’s current classification and diagnostic tool, and a guiding principle of the Mental Capacity Act (MCA), the legislation aimed at safeguarding those who may lack the capacity to make and express decisions for themselves. [1,2]

Even though the DSM-5 is not routinely used in healthcare settings in Britain, this apparent misalignment is still noteworthy for British healthcare practitioners and researchers. Having taught both psychology and speech and language therapy to undergraduate students, I know that DSM-5 is still referred to in higher education on both academic and clinical courses, albeit often alongside the International Statistical Classification of Diseases and Related Health Problems (ICD-10). [3] The DSM-5 is a document that many health professionals and researchers are likely to be familiar with and use for reference. While the definition of severe intellectual disabilities in ICD-10 and the awaited ICD-11 is of interest, the striking misalignment noted here is between the DSM-5 and the MCA.

The MCA’s principle of assuming capacity

The MCA is regularly used in health and social care settings to ensure that people are as involved as possible with decisions that affect them. Although sections of the MCA are devoted to including people who may lack the capacity to consent, this is not the act’s sole focus. Indeed, the act’s first guiding principle is to assume capacity. The Social Care Institute for Excellence summarises this principle by stating “Every adult has the right to make his or her own decisions and must be assumed to have capacity to do so unless it is proved otherwise. This means that you cannot assume that someone cannot make a decision for themselves just because they have a particular medical condition or disability.” [4] Capacity is judged on a case by case basis and mental capacity assessments should be completed to assess the person’s capacity to make said decision. [5]

The DSM-5’s specifier of lacking capacity

The DSM-5 defines ID as “a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains.” A table is provided to specify various levels of severity of ID (pp. 34 – 36). [1] These are defined by adaptive behaviours and not IQ. When it comes to the deficits that people with severe ID experience in the practical domain, it states that “The individual cannot make responsible decisions regarding well-being of self or others” (p.36).

The problem

Recent conversations about the definition and classification of ID have focused on moving away from both the term “mental retardation” (in the USA) and the use of IQ tests as a sole diagnostic tool. [7] While these are important changes, the discussion lacks a closer examination of the language used to describe specific impairments in particular domains. We should strive to use consistent, clear language and to treat people with respect. If people with severe ID are capable of making responsible decisions that affect their well-being (as I would suggest), then they should be involved in them. And it is up to us to ensure that there is no space for confusion within the terminology used across various professional publications.

Clare Nicholson is a Psychology postgraduate. I lecture speech and language therapy students at City University and am completing my PhD, examining interactions involving people with severe intellectual disabilities, at Anglia Ruskin University.

Competing interests: None declared.

Peer reviewed.


  1. American Psychiatric Association. DSM 5. American Psychiatric Association; 2013 May 27.
  2. The Mental Capacity Act 2005 [Internet]. 2015 [cited 3 March 2015]. Available from:
  3. World Health Organization. The ICD‐10 classification of mental and behavioural disorders: clinical descriptions and diagnostic guidelines. Geneva, 1992
  4. SCIE: Mental Capacity Act resource [Internet]. 2015 [cited 3 March 2015]. Available from:
  5. The Department for Constitutional Affairs. The Mental Capacity Act 2005 Code of Practice. United Kingdom: Crown Copyright; 2007.
  6. Willner P, Jenkins R, Rees P, Griffiths VJ, John E. Knowledge of mental capacity issues in community teams for adults with learning disabilities. Journal of Applied Research in Intellectual Disabilities. 2011 Mar 1;24(2):159-71.
  7. Tassé MJ, Luckasson R, Nygren M. AAIDD proposed recommendations for ICD-11 and the condition previously known as mental retardation. Intellectual and developmental disabilities. 2013 Apr;51(2):127-31.