Intellectual Disability: Everything you need to know to understand it

In this article we address in depth what is the intellectual disability: symptoms and diagnostic criteria, types of intellectual disability and its characteristics, causes and evolution. Also, discover useful tips that can help you relate better to people with intellectual disability.

Intellectual disability
Intellectual disability

Intellectual disability has been taboo for many years, surrounded by stigma and exclusion. Concepts, definitions, and contexts are changing. We now focus more on the person and their goals and needs more than on their limitations. If you have doubts and want to know more about intellectual disability, if you don’t want to stay confined to the myths spread on intellectual disability, I invite you to continue reading.

What is intellectual disability?- Definition

Intellectual disability involves a number of significant skill limitations. In other words, people with these disabilities have intellectual limitations and in adaptive behavior. Therefore the interaction with the environment that is not adapted for them is difficult to handle. The American Association for Intellectual and Developmental Disabilities (AAIDD), defines intellectual disability as follows:

Intellectual disability is characterized by significant limitations in intellectual functioning and adaptive behavior manifested in adaptive conceptual, social, and practical skills. This disability originates before the person turns 18″

Schalock established that if personalized appropriate supports are maintained over a long period of time, the general functioning or performance of the person with an intellectual disability will improve.

The current approach has led to change the term mental retardation for the term intellectual disability. This new term not only adapts to clinical terminology but it is also less offensive for people with intellectual disability. Other terms that are no longer used are mental deficiency, cognitive disability, psychic disability, mental retardation, abnormal or subnormal. Intellectual disability is not a mental illness. 

Is intellectual disability the same as a developmental disability? Developmental disability is a broader term containing intellectual disability, cerebral palsy, autism spectrum disorders and other conditions that are largely related to intellectual disability (or characteristics similar to intellectual disability).

Symptoms and diagnostic criteria for intellectual disability

When we talk about intellectual disability, we must always keep in mind the individual differences that this might pose for each person. Nor should we forget that the environment can play an important part in the adaptation or adjustment of the person. 

The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) which includes, within neurodevelopmental disorders the intellectual disability characterizes it by:

  • A. Deficits in intellectual functions, such as reasoning, problem-solving, planning, abstract thinking, judgment, academic learning and learning from experience, and practical understanding confirmed by both clinical assessment and individualized, standardized intelligence testing.
  • B. Deficits in adaptive functioning that result in failure to meet developmental and sociocultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more activities of daily life, such as communication, social participation, and independent living, and across multiple environments, such as home, school, work, and recreation.
    • Difficulties at the conceptual or academic level: that is, they have difficulty performing tasks involving memory, attention, language, reading, writing, mathematical reasoning, acquiring practical skills, problem-solving skills, etc.
    • Difficulties in the social domain: Little awareness of their thoughts, feelings, and experiences with others, empathy, skills for interpersonal communication, the ability to make friends, etc.
    • Difficulties in the practical domain: Related to the degree of learning and how the person is self-sufficient in different situations in life, such as personal care, work responsibility, money management, leisure, task organization or appropriate behavior control.

    The last of the diagnostic criteria for intellectual disability is that the onset of intellectual and adaptive deficiencies. They must always happen during the developmental period, but age and characteristic traits will depend on the cause, type, and severity of intellectual disability. While it is true that child development is a complex process based on the biological, psychological and social evolution of the person. It is important to take into account the series of milestones or fundamental evolutionary moments that imply progress and mastery of simple skills that will facilitate other complex ones.

Other that DSM-5 criteria we have to take into account other characteristics and symptoms that can help us recognize an intellectual disability.

  • In cases of more severe intellectual disability, milestones of motor development, language development or social development are reached later than in the general population. These can be identified in the first years of life, however, when the intellectual disability is low it may not be identified until the child is at school when academic difficulties become evident.
  • As for cognitive development, children with intellectual disabilities go through the same evolutionary stages as children without intellectual disabilities and in the same order, but their pace of development is slower. People with intellectual disabilities have primary periods of thought when reasoning, however, others show skills related to more advanced levels of thinking.
  • When intellectual disability is associated with a genetic syndrome there may be physical characteristics genetically associated.
  • When intellectual disability is acquired, for example, after an illness, it can be started abruptly: after meningitis, encephalitis or after a blow (trauma) to the skull during development.
  • People with intellectual disabilities have between 3 and 4 times more mental, neurological, medical and physical disorders than the general population.
  • The mental health problems in people with intellectual disabilities are the same as in people without disabilities, however, the prevalence is greater. The most common are mood disorders, schizophrenia, anxiety symptoms and disorders and sleep disturbances.
  • Regarding physical health problems, people with intellectual disabilities are at greater risk and prevalence for diseases such as obesity, epilepsy, diabetes, HIV and STDs, dementia, and others. That is why it is necessary to create health programs aimed at meeting their needs.
  • Another of the intellectual functions that are usually most affected in people with intellectual disabilities is language and communication. In general, people with intellectual disabilities have adequate language that equals their younger peers. The greater the severity of intellectual disability, the greater the severity of language problems.
  • Behavior alterations are another of the problems generally associated with intellectual disability and can be caused by several factors: such as the discomfort caused by the difficulty in communicating or expressing their personal desires and needs. Emotional problems related to exclusion and social discrimination or simply as a way of expressing or communicating what they can not otherwise transmit (frustration, worry, nervousness …).
  • These problematic behaviors, because of their intensity, frequency or duration, will negatively affect the personal development and communitiy participation of the person with intellectual disability. Generally, these behaviors serve a purpose to the person who performs it.

Types of intellectual disability and characteristics: Mild, Moderate, Severe and Profound 

When diagnosing an intellectual disability, the severity of the disability must be specified at four levels: mild, moderate, severe and profound.

Traditionally, this classification was based on IQ scores obtained through intelligence tests or tests. In such a way that the person with mild intellectual disability would have obtained IQ scores of between 50-55 and 70 points; With moderate intellectual disability between 35-40 and 50-55 points; With severe intellectual disability between 20-25 and 35-40 points and with profound intellectual disability scores lower than 20-25 points (remember that the average score of the general population is between 90 and 110).

The most current classification of intellectual disability is based on adaptive behavior and could be summarized as follows:

1.Mild Intellectual Disability

A person with a mild intellectual disability the conceptual domains affected, such as abstract thinking, cognitive flexibility, short-term memory and the functional use of academic skills such as reading or handling money. As for the social domain and in comparison with their peers, immaturity appears in social interactions and increases the risk of being manipulated. Finally, they need support to complete complex tasks of daily living.

2. Moderate Intellectual Disability

The person with a moderate intellectual disability needs continuous assistance to complete daily basic activities of the day-to-day, and others may need to take some responsibility for that person (example: sign an informed consent). Regarding the social domain, oral language (which is the main tool we have to communicate socially) is much less complex than that of people without disabilities. They may not adequately interpret certain social cues and need communicative support to establish successful interpersonal relationships. Finally, with a longer teaching period and additional support they can develop various skills and abilities.

3. Severe Intellectual Disability

When intellectual disability is severe, conceptual and cognitive skills are much more limited. The person has little understanding of language and numerical concepts such as time or money. Caregivers should provide extensive support to perform daily activities. Since oral language is very limited in both vocabulary and grammar, speech is formed only by simple words or phrases that could be improved by alternative means, such as non-verbal language. Communication and social focus on the here and now. The person requires constant support and supervision for all daily living activities (cooking, personal hygiene, choice of wardrobe, etc.).

4. Profound Intellectual Disability

 The person could use some objects (brushes, etc.) and acquire some visual-spatial skills such as pointing. However, the motor and sensory problems that are usually associated can prevent the functional use of objects. Social skill is also very limited in terms of the compression of both verbal and gestural communication. The person can understand very simple instructions and express basic desires or emotions through simple and non-verbal communication. The person is dependent on all aspects, although, if there are no major motor or sensory impairments, they could participate in some basic activities.

Main syndromes associated with intellectual disability

1-Down’s Syndrome

Down Syndrome is caused by a chromosomal abnormality in pair 21, that is, the person has an extra chromosome in that pair, therefore, they have 47 chromosomes instead of the 46 that typically appear in the human karyotype. This syndrome is the most frequent cause of intellectual disability of genetic origin.

Three different types are now known: simple or pure trisomy 21 (more than 90% of cases), chromosomal translocation (whose characteristics are similar to the previous type) and mosaicism, a less frequent type that affects only 1% of the cases. The latter is also known because it can show all, some and even none of the typical features associated with Down Syndrome, depending on the percentage of cells carrying extra information.

At the physical level, people with Down syndrome usually have a flat and straight face (brachycephaly), muscular hypotonia, short stature; neck, extremities, fingers, and ears generally short; nose and eyes inclined upwards and a small mouth with a large tongue.

At the behavioral level, there is slowness to process, structure, interpret and elaborate the information, generating a mild to moderate intellectual disability. They also often have difficulty maintaining attention, retaining information in memory and spatiotemporal orientation. Language production is usually poor, however, despite this, they usually have a good capacity of social adaptation.

Intellectual Disability- Down Syndrome
Intellectual Disability- Down Syndrome

2- X-Fragile Syndrome

X-fragile syndrome is a disorder of hereditary origin that happens more in men than in women and is the result of a molecular anomaly on the X chromosome. It is the second genetic cause of intellectual disability

People with this syndrome have four main features: elongated face with a broad forehead and prominent chin, large and detached ears, great joint mobility and macroorchidism (excessive testicular development). Behaviorally, they usually present mild or moderate intellectual disability, language alterations such as speech delay or absence of language. Hyperactivity, attention deficit, extreme shyness or frequent stereotypes such as flapping or biting their hands. In women, cognitive impairment is usually lower.

3-Williams syndrome

Williams Syndrome is a genetic disorder characterized by the loss of genes in one of the chromosomes. Physically it is associated with a determined form of the face: elongated and thin, large lips, clear eyes with a star pattern on the iris and flattened nose. In a large number of cases, the cardiovascular system is affected. People with this syndrome have intellectual disability, usually between mild and moderate and psychomotor difficulties. They usually have a rich vocabulary, good skills for social interaction, as well as for music and a good memory.

4- Angelman’s syndrome

Angelman Syndrome is a genetic disorder due to alterations in chromosome 15. It is associated with delays at the neurological level. Physically they have low pigmentation in their feet, their hair, and in their eyes. They have a large mouth with elongated teeth and forward jaw. They also have a smaller head size and some characteristic spots on the iris. The intellectual disability in this syndrome is usually severe or profound, with severe conditions in speech and language, delayed psychomotor development, alterations of movement, balance and hand stereotypies. Another characteristic feature is the presence of a smile that remains without cause.

5- Prader-Willi syndrome

Prader-Willi syndrome is a congenital disorder derived from the absence of normal parent gene activity on chromosome 15 and other chromosomal abnormalities. During early childhood, they may present problems with feeding and low muscle tone. It is also common they have low stature, small hands and feet, hypogonadism, compulsive food intake and obesity. In this syndrome, intellectual disability is not key to diagnose since 32% of people with this syndrome have a normal IQ. Although in general, they usually present speech disorders and cognitive limitations in processing information and short-term memory.

6- Cri Du Chat Syndrome (Syndrome 5p)

The syndrome is due to an alteration in chromosome 5. Physically they have a small size head, a rounded face, separated eyes, a wide nasal bridge, malformación in the ears and a small jaw. They also have small hands and deformities in the feet and the palate in addition to eyesight problems such as strabismus. At a more behavioral level, the distinguishing feature is the baby’s crying, it is similar to the meow of a cat and has no communicative utility. There is a significant delay in motor development, they usually present severe intellectual disability and a very limited attention span.

Causes of intellectual disability

The causes of intellectual disability are multiple: from genetic diseases to alterations caused by the environmental. Currently, the cause of intellectual disability is considered to be an interaction between four risk factors: biomedical, social, behavioral and educational. They interact throughout a person’s life and are passed on between generations.

Some examples of risk factors are:

Causes before birth (prenatal)

  • At a biomedical level: chromosomal disorders, disorders associated with a single gene, syndromes, maternal diseases or parents age.
  • At a social level: maternal malnutrition, domestic violence, lack of access to healthcare or poverty.
  • At a behavioral level: drug use, alcohol, tobacco and parental immaturity.
  • At an educational level: cognitive impairment of the parents or lack of preparation for parenthood.

Causes during birth (perinatal)

  • At a biomedical level: prematurity in childbirth, birth injuries or neonatal disorders.
  • At a social level: lack of parental care.
  • At a behavioral level: rejection by the parents or abandonment of the child.
  • At an educational level: lack of medical care after medical discharge.

Causes after birth (postnatal)

  • At a biomedical level: trauma or cranioencephalic lesions, malnutrition, meningoencephalitis, epileptic disorders or degenerative disorders.
  • At a social level: poor interaction between child and caregiver, lack of adequate stimulation, family poverty, chronic illness in the family.
  • At a behavioral level: mistreatment and abandonment, domestic violence, inadequate safety measures, social deprivation (isolation) or problematic behaviors of the child.
  • At an educational level: deficits in upbringing, late diagnosis, inadequate early care services, inadequate special education services or deficient family support.

Despite current knowledge, medical advances and efforts to promote detection as early as possible, the fact is that in many cases the specific causes of intellectual disability are unknown.

How does an intellectual disability develop?

The evolution or development of an intellectual disability has as many possibilities as people with this existing condition. One of the most important points in which research in this field is focused is the early diagnosis, that way professionals carry out an intervention with the child as soon as possible in all the affected areas.

Another important point that I have stressed throughout the article is the importance of providing the appropriate therapy and support to the person with intellectual disability.

This implies that therapies must be individualized and an evaluation will have to be carried out prior in order to meet the person’s needs. Keep in mind that not all people with disabilities are the same, nor do they need the same therapies nor even require the same intensity of those therapies in all areas and activities of their life.

Hence the current focus is on planning support systems based on the person with intellectual disability, individually, identifying their goals and their desired life experiences and emphasizing the strengths, not the limitations they may have. The main goal should be to improve their quality of life. 

Here you have a video where Loretta Claiborne explains what it’s like to live with an intellectual disability.

Advice for parents of children with intellectual disabilities

Every person, whether or not he has an intellectual disability, is different. Each family is also different and deal with situations differently. However, parents play a primary and fundamental role in our life. Here are some helpful tips:

1. If you see behaviors or reactions that are not expected in your child or if you have doubts about whether something is happening to your child, ask for professional help as soon as possible. This is a very important point since you dispel your doubts and also allow for the intervention to happen as soon as possible.

2. Once the diagnosis of intellectual disability is confirmed, it is possible that negative feelings such as guilt, anger, sadness appear. This is perfectly understandable and normal, you will have to give yourself time to process them and then take steps towards acceptance and recognition. You can do this with help from other parents who have gone through the same or with a professional.

3. A very important challenge will be to differentiate the things that we can change from those that we can’t. This allows us to spend our resources on things we can change and not waste time on other irrelevant aspects.

4. Research information about your child’s disability and everything you can do throughout his development. Keep informed about recent studies with scientific evidence and therapies as well as through families that experience the same disabilities.

5. Find out about therapies that might be useful for your child and your family.

6. Value professional opinions as much as other nonprofessional.

7. Think about your child best interests based on their possibilities and not on their limitations. Think about their needs, what can make them happy and progress by offering the appropriate therapies that they need. This will allow them to achieve their highest level of autonomy and the highest quality of life possible.

8. Share and enjoy with your child, considering that although he will go through the same developmental stages at a slower pace than other children, you can still enjoy every minute of it. Don’t underestimate him or always treat him like a child.

9. And remember, even if we are different, we are all people with the same rights and the same opportunities as the others.

Intellectual Disability
Intellectual Disability


Verdugo M.Á., Guillén, V.M. y Vicente, E. (2014). Discapacidad intelectual. En L. Expeleta y J. Toro (Eds.) Psicopatología del desarrollo (pp.169-190). Madrid, España: Pirámide

Guía para padres de FEAPS: No estás solo. 3ª edición.

This article is orginally in Spanish written by Rosa Calderón Vicente, translated by Alejandra Salazar. 

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