What is intellectual disability?

What is intellectual disability? 

Intellectual and developmental disability (IDD), formerly known as mental retardation, is a cognitive impairment caused by trauma to the brain, environmental and/or genetic factors making it develop differently than a typical human brain. 

It comes under the umbrella of neurodevelopmental disorders or disorders that present themselves in the developing years up to 18-22 [1] years of age*. It is a lifelong condition, not an illness.

IDD limits a person’s ability to learn at an expected speed, and do activities of daily life as it affects mental and physical development, including their ability to communicate and learn things, including social skills. 

There are two key medical criteria to diagnose IDD

  • IQ of below 70
  • Significant limitations in two or more adaptive areas (skills needed to live, work, and play in groups) as revealed in standardised tests such as Vineland adaptive behaviour scale

An IDD does not always make one mentally dull, like is being discovered, but skills needed for activities of daily life, including self-care and communication, can be significantly impeded, like in many cases diagnosed under the autism spectrum disorder.

An intellectual disability is different from a developmental disability. For instance, cerebral palsy is a developmental disability caused by lack of oxygen supply to the brain at the time of birth. It may or may not manifest with an intellectual challenge. Co-occurrence of an intellectual disability and developmental disability is common but not compulsory.  

An intellectual disability is also not the same as a learning difference (disability), such as dyslexia, which refers to the differences in the way our brain receives and processes information. 

A person can also develop symptoms associated with IDD because of accidents, diseases, infections or strokes. In such cases, the condition is not a neurodevelopmental disability, but a neurological disability since the onset is not in developing years.

Signs and Symptoms

There are signs and symptoms that should be observed before testing a child’s IQ and other adaptive skills. The biggest giveaway is delayed achievement of growth milestones as compared to other children. It alters the developmental trajectory in the following areas of life:

  • Physical development

Delay in sitting up, crawling, walking, grasping

Delay in speech onset, clarity, identifying objects

Issues with making eye contact, or keeping focus

  • Intelligence or cognitive development

Ability to problem solve

Store information in memory

Make evaluations, judgements

  • Adaptive functioning (home, school, community)

Impaired social communication

Difficulty reading emotions

Conception of language, reading, writing, time and money management


A neurodevelopmental disability shows up in the developing years, and detecting one successfully for correct interventions requires a battery of tests and observational exams to be conducted. 

  • Intellectual functioning or IQ test (done between 4 and 8 years of age)
  • Adaptive functioning test 
  • Detailed family and social history 
  • Social and behavioural observation of the child through interviews with primary caregivers, and teachers
  • Detailed physical examination
  • Genetic, chromosomal and neurological testing, brain scans like MRI etc
  • Testing for co-occurring conditions, including mental health tests


There are identified and unidentified causes of intellectual and developmental disability, but for many children and adults the root cause of their condition remains unknown. 

The known causes include: 

1) brain injury before, at or after birth, leading to intellectual disability, like with cerebral palsy, or Shaken Baby Syndrome.

2) Genetic conditions as presented in the Autism Spectrum Disorder, Prader-Willi Syndrome, Down Syndrome, phenylketonuria (PKU), Williams Syndrome, Apert Syndrome.

3) Complications caused by mother’s continued consumption of alcohol through the pregnancy leading to fetal alcohol syndrome

4) Diseases including whooping cough, the measles, or meningitis in the early years

5) Disease caused by viral or bacterial infections and/or stroke of the head or the heart

6) Toxicity, like exposure to lead or mercury

7) Extreme malnutrition because of poverty

Early diagnosis goes a long way in improving the quality of life outcomes of a person with IDD.
85% cases of intellectual disability are mild to moderately severe [2]. Life outcomes improve with early intervention through therapy and integrated or inclusive schooling, helping persons with IDDs (PwIDDs) lead meaningful lives.

Grades of Severity 

Not all IDDs are the same, and neither are the persons who live with the condition. The physical health symptoms, IQ level and accompanying adaptive and behavioural deficiencies together act as a measure of severity of the condition. It defines what the person will be able to learn, and to what degree. 

For instance, a teen with mild Down Syndrome can lead an independent life while another with a more severe manifestation may need to be in an assisted living setup. 

Degree of severity IQ score Prevalence
(Overall 3% population)
Mild 55-69 85%
Moderate 40-54 10%
Severe  25-39 3-4%
Profound <25 1-2%
Normative range 85-110

Since the severity also changes from person to person — IQ score being just one measure of IDD — timely intervention and inclusive schooling can play a big role in strengthening a person’s capability to learn and function.

Let’s look at what different degrees of IDD severity can look like.


  1. Slower than typical in all developmental areas
  2. No unusual physical characteristics
  3. Able to learn all practical skills of daily living and safety
  4. Delays in acquiring reading and writing skills but capable of achieving academic success
  5. Able to blend in socially
  6. Capable of living independently


  1. Noticeable developmental delays in speech, motor skills. May face some difficulty communicating, slow in understanding and using language
  2. May have physical signs of impairment 
  3. Able to learn all practical skills of daily living, health and safety
  4. Can acquire basic proficiency in reading, writing and math
  5. Able to participate independently in social activities. May face difficulty with social cues and judgment
  6. Generally unable to live alone, but can use public transport to visit familiar places


  1. Considerable delays in reaching all developmental milestones
  2. Understand language, but limited ability to communicate
  3. Able to learn daily routines
  4. Can learn simple self-care, but need daily supervision and support
  5. Require assistance in social situations
  6. Cannot live independently


  1. Significant delays in all developmental areas
  2. Pronounced and apparent physical and cognitive impairment, possible immobility
  3. Inability to comply with instructions
  4. Require help for all self-care activities
  5. May respond to social activities around
  6. Need lifelong care and support.