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The evolution of the concept of Intellectual Disability

The evolution of the concept of Intellectual Disability

February 27, 2024

After the discovery and establishment of the psychometric and factorial methodology in the study of intelligence at the beginning of the last century by Alfred Binet and Simon (1905) and later, Terman (1916) and Weschler in the decade of the 30s, the Intellectual Coefficient passed to be the central factor in the evaluation of intellectual capacity.

But nevertheless, the most recent proposal of the American Association on Mental Retardation (AAMR) of 1992 seems to save some of the disadvantages that the first formula had associated.

  • Related article: "Types of intelligence tests"

Intellectual Disability as a Neurodevelopmental Disorder

As a Neurodevelopmental Disorder (or Neurological Development Disorders, according to the DSM-V) is understood any disease related to an alteration during the process of maturation of the nervous system that affects inappropriate functioning at the level of behavior, thinking, movement, learning, perceptive-sensitive capacity and other complex mental functions.

The set of manifestations that can occur as a consequence are of a very considerable variety, since both the location of the dysfunction, influential environmental factors and the moment of development in which said alteration occurs must be addressed.

Neuroscience is the discipline that is responsible for the study and investigation of ODD, as well as other neurodegenerative disorders, static injury disorders and psychiatric disorders. In certain cases, The same pathology can be considered within more than one of these categories , which differ from each other in two dimensions: time (development-decline) and the phenomenological (cognitive-emotional).


Among the characteristics attributed to TND, is the difficulty of distinguishing whether the origin of the external manifestation of the underlying symptomatology is derived from a TND or a type of normative functioning, such as the case of distractibility ( which may be due to an affectation of the structures that regulate attention capacity or it may be a marked personality trait, simply).

A) Yes, no biological markers are known (associated neuroimaging or analysis tests) from which an ODD can be unequivocally diagnosed. The subjectivity of the evaluator therefore plays a significant role in the diagnosis made of the case.

Secondly, TNDs have a very high comorbidity with other pathologies , fact that in certain occasions can make difficult an exact diagnosis of the case in that all the present labels must be detected. On the other hand, the delimitation between the symptomatology attributable to one disorder and another is also complex, since many of them share common criteria (for example, the difficulty in social relationships in a case of autism and language disorder).

  • Related article: "Intellectual and Development Disability"

Types of Neurodevelopment Disorders

In a generic way, TND can be classified into three main categories according to the criteria:

Whether a specific cause is identified or not

In this case the genetic influence is a significant causative factor . The most widely used classification manuals (DSM and CIE) include communication, learning, hyperactivity, and autism spectrum disorders. In the case of behavioral disorders, schizophrenic disorders and Tourette's disorder, the difference in age of onset must be taken into account for each of them, so depending on the case they may also be included in this first category.

Genetic alterations linked to a structural alteration

Of simpler delimitation, since the phenotypic deviations are clearly identifiable (deletion, duplication, translocation, disomías or chromosomal trisomías, etc), as in the case of Williams Syndrome .

TND linked to a known environmental cause

Its influence on interaction with genetic factors is usually considered, for example, fetal intoxication for the maternal consumption of alcohol or pathologies derived from the action of valproic acid.

The traditional conceptualization of Intellectual Disability

As indicated at the beginning of these lines, the last century was marked by the rise of psychometric scales on the assessment and quantification of the level of intelligence in the human being.

Thus, the only decisive reference was the distinction between the classification levels of Intellectual Disability according to the Intellectual Coefficient (CI) of the individual. Let's see a more detailed description of each of these categories:

Mild Mental Retardation

Understand an IC located between 55 and 70 and it presents a proportion of 85% of the total of cases. Being the least significant level in severity is difficult to distinguish in the first years of life. In this case, the social and communication skills or the capacity of atonomy are rather well preserved, although they require some type of supervision and follow-up. There are no great difficulties to achieve the development of a satisfactory life.

Moderate Mental Retardation

A second level of greater severity with a prevalence of 10% is Moderate Mental Retardation, to which an IQ is attributed between 40 and 55. In this case the level of social and communicative development is lower and they must be supervised during working and adult life, although they can adapt to community life in most cases.

Severe Mental Retardation

Severe Mental Retardation is associated with an IC between 25 and 40 and occurs in 3-4% of total cases. His linguistic capacity is very limited but are able to acquire elementary self-care habits . They need a considerable level of support and help for their adaptation to community life.

Deep Mental Retardation

Deep Mental Retardation is characterized by an IQ of less than 25 and they present it between 1 and 2% of the population with MRI. At this level they are observed clear and serious motor, sensory and cognitive difficulties . They require constant and permanent supervision and a high structuring of the environment in which they interact.

  • Related article: "Types of intellectual disability (and characteristics)"

The descriptive dimensions of intellectual functioning

The most recent proposal of the American Association on Mental Retardation (AAMR) implies a drastic change in the conception of intellectual disability and puts the emphasis on endowing the definition of Mental Retardation a more positive and optimistic connotation in terms of assessing mainly the capabilities and potential of the individual with intellectual dysfunction, as well as the support you need to achieve those goals.

Thus, the proposed definition of the AAMR on Mental Retardation explains it as a series of substantial limitations in intellectual functioning, which is significantly lower than the average and which manifests before the age of 18.

Dimensions of evaluation of Mental Retardation

In particular, the large dimensions proposed by the AAMR on which to assess functionally the skills available to the child and to which it can arrive with a global multidisciplinary intervention :

  • Intellectual skills.
  • Adaptive behavior at a conceptual, social and practical level.
  • Participation, interactions and social roles.
  • Physical, mental health, etiology of possible alterations.
  • Social context, relating to the environment, culture and opportunities for access to this type of stimulation.

Unlike the previous ones, there is an emphasis in this proposal on the social context and on determining what resources are required to guarantee the greatest number of learning, autonomy and well-being of the child on a day-to-day basis, instead of taking as a central factor the deficits and difficulties that the child presents.

This has several advantages both in terms of the decrease in negative labeling that is usually associated with individuals with this type of deficit, since the definition gives a major role to the potentialities and abilities to be developed by the child. In addition, this new definition s It is more oriented to determine the type of intervention that will be necessary for the specific case to obtain the highest possible level of development (environmental, social, personal and intellectual adaptations).

In this new conception the following postulates are presupposed: the consideration of cultural and linguistic diversity, communicative and behavioral; the need for the existence of individualized support at the community level; the co-existence of potentialities in other adaptive areas or personal capacities; the assumption of the improvement of the functioning of the person by providing the appropriate supports for a continuous period of time.

In short, it seems that the most recent definition of Mental Retardation aims to provide a more practical, positive and adaptive perspective that will facilitate a greater integration of the individual, both personally and socially, allowing a greater development emphasizing its qualities rather than its difficulties.

Bibliographic references:

  • Artigas-Pallarés, J. and Narbona, J. (2011): Neurodevelopmental Disorders. Barcelona: Viguera Editores.
  • American Psychiatric Association (APA, 2013). DSM-V. (American Psychiatric Association, Diagnostic and statistical manual of mental disorders, Washington, DC).
  • Verdugo A. (1994) The paradigm shift in the conception of Mental Retardation: The new definition of the AAMR. Century Zero.
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