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Intellectual Disability (intellectual developmental disorder)

According to the DSM-V1, an intellectual disability (intellectual developmental disorder) is a neurodevelopmental disorder that begins in childhood and is characterized by intellectual functioning deficits and adaptive behaviour deficits in conceptual, social, and practical areas of living.

"It is important to remember that intellectual functioning and adaptive behavior are two separate and distinct constructs that complement each other and significant deficits in each are necessary but alone insufficient to meet criteria for a diagnosis of intellectual disability." (Tassé, M.J, 2016).

M.J, Tassé (2016) explains that intellectual disability has long been categorized as a developmental condition with an onset prior to the end of the developmental period, and historically set at 18 years of age. There have been changes and revisions about the chronological age cut-off, which defined the developmental period. It is now up to the judgment of the clinician to define the developmental cut-off period.

What is meant by intellectual functioning deficits?

Significant limitations in intellectual functioning such as “reasoning, problem-solving, planning, abstract thinking, judgment, academic learning, and learning from experience”—confirmed by clinical evaluation and individualized standard IQ testing (APA, 2013, p. 33).

What is meant by adaptive functioning?

It is a collection of conceptual, social, and practical skills that are learned and performed by people in their everyday lives.

  • Conceptual skills—language and literacy; money, time, and number concepts; and self-direction.

  • Social skills—interpersonal skills, social responsibility, self-esteem, gullibility, naïveté (i.e., wariness), social problem solving, and the ability to follow rules/obey laws and to avoid being victimized.

  • Practical skills—activities of daily living (personal care), occupational skills, healthcare, travel/transportation, schedules/routines, safety, use of money, use of the telephone. (AAIDD, 2013)


Cleaned up and updated!


Paradigm Shift


"Although the AAIDD abandoned some 30 years ago (see: Luckasson et al., 1992) using severity levels to define the condition of intellectual disability, DSM-5 (American Psychiatric Association, 2013) has continued to maintain a system of severity levels for intellectual disability.


A major paradigm shift did, however, occur in this most recent revision of the DSM when APA chose to abandon the use of IQ scores as the determinant of the severity levels of intellectual disability (i.e., mild, moderate, severe and profound). Instead, DSM-5 has proposed using the individual’s adaptive functioning level across conceptual, social and practical skills to guide clinical judgment in determining the severity level of intellectual disability. This clearly signalled the APA’s desire to discourage the over-reliance on IQ scores and recognition of the greater emphasis that should be placed on the more comprehensive and predictive construct of adaptive behavior." (Tassé, M.J, 2016).

Severity of Intellectual Disability

The severity of intellectual disability varies from one individual to another. For the purpose of determining the severity of intellectual disability, an assessment must be done by a qualified clinical professional such as a psychiatrist or a psychologist.

Intellectual functioning is measured with individually administered and psychometrically valid, comprehensive, culturally appropriate, psychometrically sound tests of intelligence.


The level of severity does not only depend on IQ results but MUST take into account the level of adaptive functioning of the individual as well. According to the DSM-V, the level of adaptive functioning will determine the level of supports required.

For reference purposes only, the following table offers an overview of the classification of  Intellectual Disability Severity.

Information taken from: 9 Clinical Characteristics of Intellectual Disabilities, Mental Disorders and Disabilities Among Low-Income Children. Committee to Evaluate the Supplemental Security Income Disability Program for Children with Mental Disorders; Board on the Health of Select Populations; Board on Children, Youth, and Families; Institute of Medicine; Division of Behavioral and Social Sciences and Education; The National Academies of Sciences, Engineering, and Medicine; Boat TF, Wu JT, editors.Washington (DC): National Academies Press (US); 2015 Oct 28.


Moderate Intellectual Disability

“Children with moderate Intellectual Disability often show signs of their intellectual and adaptive impairments as infants or toddlers (Jacobson & Mulick, 1996).  Their motor skills usually develop in a typical fashion, but parents often notice delays in learning to speak and interacting with others. These children often seem less interested in their surroundings compared to their age mates. They are often first identified as having Intellectual Disability as toddlers or preschoolers when they show little or no language development. Instead, they rely mostly on gestures and single-word utterances. By the time they begin school, these children usually speak in short, simple phrases and show self-care skills similar to typically developing toddlers. However, they display problems mastering basic reading, writing, and mathematics. By adolescence, these children are able to communicate effectively with others, have basic self-care skills, and have simple reading and writing abilities. They may continue to have trouble with reading a newspaper, performing arithmetic, or handling money.”  (Weiss, R., 2014, p. 93)

Severe Intellectual Disability

“Children with severe Intellectual Disability are usually first identified in infancy (Jacobson & Mulick, 1996). They almost always show early delays in basic developmental milestones, such as sitting up and walking. They also usually show one or more biological anomalies that are indicative of a genetic or medical disorder. They require ample supervision from parents and caregivers. By the time they begin school, they may be able to move on their own and perform some basic self-care skills, such as feeding, dressing, and using the toilet. They may communicate using single words and gestures. As adults, their speech continues to be limited and difficult to understand, although their ability to understand others is often better developed. They are usually unable to read or write, but they may be able to perform simple daily living tasks under close supervision.” (Weiss, R., 2014, p. 93)


How common is intellectual disability?

  It’s best to consider people living with an intellectual disability and define their level of disability in terms of the support they need.  “This approach sees the effect of the disability as something that will vary and can be increased or reduced by external factors. It does not view intellectual disability as an unchangeable characteristic of the individual. The effect of the disability or the capacity of the person is a function not only of the disability but also a function of their environment and the support they receive.”    (Intellectual Disability Rights Service, 2016)

According to Special Olympics International (2017), approximately 1-3 percent of the global population has an intellectual disability -- as many as 200 million people. In the United States, approximately 6.5 million people have an intellectual disability.

It is estimated that 2% of Canadians have an intellectual disability.2  In Quebec, it is estimated that 1% of people over the age of 15 have an intellectual disability.3

Intellectual disability is significantly more common in low-income countries – 16.41 in every 1,000 people. Disabilities overall are more common in low-income countries.

The United Nations Development Program estimates that 80 % of all people with disabilities live in low-income countries. While people with disabilities represent approximately one in 10 people worldwide, they are one in every five of the world’s poorest people.



Disability Creation Process Model

The Disability Creation Process model enables us to identify and explain the causes and consequences of disease, trauma, and disruptions to the development of a person. It demonstrates that social participation is not only the result of one’s identity, choices, impairments to organs, and abilities and inabilities but also of characteristics of a person’s living environment. (Fougeyrollas, P. & Robin, J.P., 2013)

The level of social participation is explained as a result of the interaction between the person and his/her environment. It is by either acting on personal factors or on environmental factors that we can modify someone’s level of social participation.

Risk factors are an element belonging to an individual or within the environment that is likely to cause disease, trauma or any other disruption to a person’s integrity or development.

Personal factors, which are internal, correspond to a person’s characteristics, such as age, sex, socio-cultural identity, organic systems, capabilities.

Environmental factors constitute either facilitators or obstacles regarding an individual’s life habits. Environmental factors enable social participation or, on the contrary, worsen a disabling situation.

Facilitator refers to an environmental factor that contributes to the accomplishment of life habits (when interacting with personal factors).

An obstacle is an environmental factor or situation that hinders the accomplishment of life habits (when interacting with personal factors).” (Fougeyrollas, P. & al., 1999)

Disability Creation Process Model

For more information about the Disability Creation Process Model follow this link


Disability is not a definite status but an evolving notion.

It is relative, varying over time, gender, age, context and environment.

It is a situation that can be modified by reducing impairments or developing aptitudes

(acting on personal factors) as well as by adapting the environment (acting on environmental factors).

When acting on such factors, it is, therefore, possible to transform a disabling situation into a situation

of social participation.” (Agence Française de Développement, 2017)



  1. The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) is the 2013 update to the American Psychiatric Association's (APA) classification and diagnostic tool. In the United States, the DSM serves as a universal authority for psychiatric diagnoses.                           

  2. Canadian Association for Community Living - www.cacl.ca/about-us.

  3. Institute de la statistique du Québec - http://www.stat.gouv.qc.ca/statistiques/sante/services/incapacites  /limitation-maladies-chroniques-utilisation.pdf




Bergeron, G., Ducharme, M., Fortin, M., & Vézina, M.M. (2013).  Guideline for Advisors of Post-Secondary Students with Disabilities. AQICESH


Buntinx, W.H.W & Schalock R.L. (2010). Models of Disability, Quality of Life, and Individualized Supports: Implications for Professional Practice in Intellectual Disability. Journal of Policy and Practice in Intellectual Disabilities, 7(4), 283-294.


Comité National d’expertise sur l’évaluation des apprentissages pour les élèves ayant une DIP (2012-2016). Encadrements, données de literature et argumentaire ayant mené à l’application de la démarche de planification et d’évaluation en DIP.


Fougeyrollas, P. & Robin J.P. (2013). The Interactive Person-Environment Disability Prevention Process: A conceptual Framework and Methodology for Intervention and Social Participation Outcomes Measurement in the Field of Rehabilitation and Inclusive Urban and Local Inclusive Development. A Proposal for the Expected Revision of ICF. International Network on the Disability Creation Process

Tassé, M.J. (2016).  Defining intellectual disability: Finally we all agree... almost — Defining and determining intellectual disability. Spotlight on Disability Newsletter, American Psychological Association. https://www.apa.org/pi/disability/resources/publications/newsletter/2016/09/intellectual-disability, 2020


Weis, Robert, (2014). Intellectual Disability and Developmental Disorders in Children. In Robert Weis, Editor (Ed.),  Introduction to Abnormal Child and Adolescent Psychology, (pp. 88-126). Thousand Oaks, California, Sage Publications Inc.




American Association on Intellectual and Developmental Disorders. (2017, February). Definition of Intellectual Disability. Retrieved from http://aaidd.org/intellectual-disability/definition#.WMvJEmMg3sE

Canadian Association for Community Living. (2017, February). Definitions and Terminology. Retrieved from http://www.cacl.ca/about-us

Institut de la statistique Québec (2017, February). Enquête québécoise sur les limitations d’activités, les maladies chroniques et le vieillissement 2010-2011 : Utilisation des services de santé et des services sociaux des personnes avec incapacité – Volume 2. Retrieved from http://www.stat.gouv.qc.ca/statistiques/sante/services/incapacites/limitation-maladies-chroniques-utilisation.pdf