After the discovery and establishment of psychometric and factorial methodology in the study of intelligence at the beginning of the last century by Alfred Binet and Simon (1905) and later by Terman (1916) and Weschler in the 1930s, the IQ became the central factor in the assessment of intellectual capacity.

However, the most recent proposal of the American Association on Mental Retardation (AAMR) of 1992 seems to overcome some of the drawbacks that the first formula had associated.

Intellectual Disability as a Neurodevelopmental Disorder

Neurodevelopmental Disorder (or Neurodevelopmental Disorders, according to the DSM-V) is understood as any illness related to an alteration during the process of maturation of the nervous system that affects inadequate functioning at the level of behaviour, thought, movement, learning, perceptive-sensory capacity and other complex mental functions.

The set of manifestations that can occur as a consequence are of a very considerable variety, since attention must be paid both to the location of the dysfunction, to the influential environmental factors and also to the moment of development when such alteration takes place.

Neuroscience is the discipline that deals with the study and research of NDT, 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 along two dimensions: time (developmental-decline) and phenomenological (cognitive-emocinal).

Its characteristics

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

Thus, there are no known associated biological markers (neuroimaging tests or analyses) from which a TND can be diagnosed unequivocally. 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 , a fact that on certain occasions may make an exact diagnosis of the case difficult, since all the labels present 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 relations in a case of autism and language disorder).

Types of Neurodevelopmental Disorders

Generically, TNDs can be classified into three main categories according to the criteria:

Whether or not a specific cause is identified

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

Genetic alterations linked to a structural alteration

It is easier to delimit, since the phenotypical deviations are clearly identifiable (delection, duplication, translocation, chromosomal dysomia or trisomia, etc), as in the case of Williams Syndrome .

TND linked to a known environmental cause

It is usually considered its influence in interaction with genetic factors, for example fetal intoxication by maternal alcohol consumption 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 human beings.

Thus, the distinction between the classificatory levels of Intellectual Disability according to the individual’s IQ was taken as the only determining reference. Let us see a more detailed description of each of these categories:

Mild Mental Retardation

It comprises an IC between 55 and 70 and presents a proportion of 85% of the total cases. As it is the least significant level in severity, it is difficult to distinguish in the first years of life. In this case, social and communication skills or atomic ability are rather preserved, although they require some type of supervision and follow-up. No major difficulties are observed in the development of a satisfactory life.

Moderate Mental Retardation

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

Severe Mental Retardation

Severe Mental Retardation is associated with an IQ between 25 and 40 and occurs in 3-4% of total cases. Their linguistic ability is very limited but they 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 is present in 1-2% of the population with MR. At this level clear and severe motor, sensory and cognitive difficulties are observed . They require constant and permanent supervision and a high degree of structuring of the environment in which they interact.

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 places emphasis on giving the definition of Mental Retardation a somewhat more positive and optimistic connotation in terms of mainly valuing the capacities and potentialities of the individual with intellectual dysfunction, as well as the supports he or she needs to achieve these goals.

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

Mental Retardation Assessment Dimensions

In particular, the large dimensions proposed by the AAMR on which to assess at a functional level the skills available to the child and to which he or she can reach with a global multidisciplinary intervention :

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

Unlike the previous ones, this proposal places emphasis on the social context and on determining what resources are required to guarantee the greatest number of learnings, autonomy and well-being of the child in his or her day-to-day life, instead of taking the deficits and difficulties presented by the child as a central factor.

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

This new conception presupposes the following postulates: the consideration of cultural and linguistic, communicative and behavioral diversity; the need for the existence of individualized support at the community level; the co-existence of potentialities in other adaptive areas or personal abilities; the assumption of the improvement of the person’s functioning by providing him/her with appropriate support over 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 on a personal and social level, enabling greater development by emphasising his qualities rather than his difficulties.

Bibliographic references:

  • Artigas-Pallarés, J. and Narbona, J. (2011): Disorders of Neurodevelopment. 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) El cambio de paradigma en la concepción del Retardo Mental: La nueva definición de la AAMR. Siglo Cero.