Intellectual Disability: Definition and Types

Intellectual Disability: Definition and Types
Sara Clemente

Written and verified by psychologist and journalist Sara Clemente.

Last update: 21 December, 2022

A person with an intellectual disability has fewer or less developed cognitive resources than what is normal for their age. It makes learning much more difficult, and it takes greater effort to communicate in certain contexts with certain messages. This type of disability is usually identified before the age of 18 and affects 1% of the world population.

But it is important to note that intellectual disability is not a mental illness, but rather a developmental disorder. All people with this type of disability are essentially like us: they have their own dreams, interests, tastes and preferences. We must not stigmatize the disorder. What better way to avoid stigmas than to learn more about it!

Intellectual functioning and adaptive behavior

There are different degrees of intellectual disability, and each comes with its own difficulties. It creates problems with reasoning, planning, problem solving, abstract thinking and learning, all due to slow and incomplete cognitive skill acquisition.

Their adaptive capacities may also be very limited, with conceptual matters as well as social and practical matters. Thus, their ability to express themselves linguistically, or their reading and writing abilities, are poorly developed, as well as their sense of responsibility and self-esteem.

In daily activities, such as grooming, personal care and food preparation, the level of autonomy depends on the severity of the disability.

Impact on health and social interactions

An intellectual disability may be accompanied by certain alterations in the individual’s physical and mental health. These, in turn, can affect other aspects of well-being.

Some syndromes that may accompany intellectual disability are: Rett syndrome, Dravet syndrome, Prader-Willi syndrome, Down syndrome, Asperger’s syndrome, and fragile X syndrome (Martin-Bell). Diseases such as obesity, diabetes, HIV, STD’s, and dementia are also more likely.

Additionally, the individual’s communication, interaction and social participation may be affected. Their intellectual and adaptive limitations make it impossible for them to normally participate in community life. It affects all their environments: home, school, work and leisure time.

an intellectual disability in a child.

The role of IQ

IQ by itself is not enough to diagnose an intellectual disability. In addition to quantitatively defining intelligence with IQ, it is necessary to make a deeper assessment of the person’s intellectual functioning.

IQ is the relationship between a person’s mental age and chronological age. The first age corresponds to intellectual development: how they perform intellectually compared to the average level. Chronological age is biological.

We consider an IQ less than 70 to indicate an intellectual disability. We diagnose the other extreme, giftedness, when the index is above 130. IQ also classifies the degree of intellectual disability.

Types of intellectual disabilities

According to the DSM 5, there are mild, moderate, severe and profound classifications.

Mild (IQ of 50-55 to 70)

85% of people with intellectual disabilities have a mild disability.

  • Conceptual domain: mildly affected abstract thinking, functional abilities, cognitive flexibility and short-term memory.
  • Social domain: immature social interactions, which puts them at risk for manipulation.
  • Practical domain: requiring supervision, guidance and assistance when carrying out daily life tasks. This help is very important particularly in stressful situations.
  • They often seem no different than children without this disability until they get older.

Moderate (IQ of 35-40 to 50-55)

10% are moderate disabilities.

  • Conceptual domain: requiring continuous assistance to complete daily activities. Sometimes it is even necessary for other people to take over some of their responsibilities. With moderate supervision, they are able to learn skills related to their own personal care. They can perform unskilled or semi-skilled jobs, but always with supervision.
  • Social domain: when communicating verbally, their language is less rich and complex than people without disabilities. This means that they cannot interpret certain social subtleties correctly and that they have problems making new relationships.
  • Practical domain: with support and continuous instruction they can develop certain skills and abilities.
Down's syndrome.

Severe (IQ of 20-25 to 35-40)

3-4% of people with an intellectual disability have a severe disability.

  • Conceptual domain: very limited, especially with numerical concepts. They need constant support in many areas.
  • Social domain: their oral language is very elementary, their sentences are grammatically simple and they have a limited vocabulary. They communicate very simply and are limited to the present.
  • Practical domain: requiring constant supervision for all everyday tasks.

Profound (IQ of 20-25)

Although they are a minority (1% -2%), most are related to an identified neurological disease.

  • Conceptual domain: clearly impacted. They only think about the physical world and non-symbolic processes. With instruction, they can acquire certain skills like pointing. Associated motor and sensory difficulties often prevent the functional use of objects.
  • Social domain: precarious understanding of verbal and gestural communication. They express themselves in a very basic, simple and mostly non-verbal way.
  • Practical domain: they are totally dependent in all areas. Only if they have no motor or sensory impairments can they participate in certain basic activities.

We need public support if we want to create an easier and more accessible environment for people with intellectual disabilities. Otherwise, these environmental limitations will only add to the limitations they already have.

In any case, we should never forget that the person always comes before the disability. A person with an intellectual disability has feelings, dreams, and something to contribute, just like us.


This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.