Intellectual Disability

Summary and Description

What is Intellectual Disability?

Intellectual Disability (ID) is a term defined by a person who has below-average intelligence or mental ability and significant limitations in adaptive behavior (AAIDD, 2015). Children with ID are able to gain skills, but learn at a slower pace compared to their peers (WebMD, 2015). ID is a developmental disability with onset prior to age 18 (AAIDD, 2015).

What is intellectual functioning?

As previously stated, intellectual functioning is one of the two deficits a child with ID struggles with. Intellectual functioning is also referred to as intelligence, or IQ. Intelligence refers to a person's mental capacity (AAIDD, 2015). A person with ID may struggle in the areas of learning, reasoning, problem solving, and decision making (AAIDD, 2015; WebMD, 2015). An IQ test can be conducted to determine this portion of the diagnosis. A person with ID will have an IQ between 70-75 or below (The Arc, 2011).

What is adaptive behavior?

Adaptive behaviors are a large skill set needed for communicating in day-to-day life. Specifically, adaptive behaviors include conceptual skills, social skills, and practical skills (AAIDD, 2015). Someone with ID may show deficits with literacy, number concepts, social responsibility, self-esteem, activities of daily living, schedules, and more.

Diagnostic Criteria

As stated in the definition, ID has three main deficits. These deficits are in the areas of intellectual functions, adaptive behavior, and onset of intellectual and adapted benefits occur during the developmental period (DSM-V, 2013). To be diagnosed with ID, a person must meet the following criteria in all three areas:

(a) Diagnosing intellectual functions:

Intellectual functions are typically assessed with an intelligence quotient (IQ) test. IQ tests are standardized assessments that assess a person's intelligence. Regarding the intellectual functions criteria, the DSM-V (2013) indicates criteria for this area has been met if they score two standard deviations or more below the population average, including a margin of measurement error.

(b) Diagnosing adaptive functioning:

Adaptive functioning is composed of three different domains: conceptual, social, and practical (DSM-V, 2013). This criteria is met when at least one of these domains is impaired and needs continuous support (DSM-V, 2013). This area is typically assessed using clinical evaluation and individualized measures (DSM-V, 2013).

(c) Onset:

The onset of the above deficits occurs during the developmental period, which is before the age of 18 (DSM-V, 2013).

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Severity in children with ID ranges from mild to profound. According to the DSM-V (2013), the following are criteria for the different severity levels of ID.


  • IQ 50 to 70
  • Slower than typical in all developmental areas
  • No unusual physical characteristics
  • Able to learn practical life skills
  • Attains reading and math skills up to grade levels 3 to 6
  • Able to blend in socially
  • Functions in daily life

This includes about 85% of people with ID (Gluck, 2014). Those with mild ID had read but most likely have difficulty with reading comprehension (Gluck, 2014). A person with mild ID may achieve academic success.


  • IQ 35 to 49
  • Noticeable developmental delays (i.e. speech, motor skills)
  • May have physical signs of impairment (i.e. thick tongue)
  • Can communicate in basic, simple ways
  • Able to learn basic health and safety skills
  • Can complete self-care activities
  • Can travel alone to nearby, familiar places

This includes about 10% of people with ID (Gluck, 2014). Those with moderate ID may have adequate communication skills but will struggle to complex communication (Gluck, 2014). For example, a person with moderate ID may struggle with problem solving, decision making, and understanding facial expressions. People with moderate ID can care for themselves, but typically need support (Gluck, 2014).


  • IQ 20 to 34
  • Considerable delays in development
  • Understands speech, but little ability to communicate
  • Able to learn daily routines
  • May learn very simple self-care
  • Needs direct supervision in social situations

This includes 3-4% of people with ID (Gluck, 2014). Those with severe ID can only communicate on a basic level (Gluck, 2014). A majority of people with severe ID are not able to live alone. They are not able to perform self-care activities independently and need supervision (Gluck, 2014).


  • IQ less than 20
  • Significant developmental delays in all areas
  • Obvious physical and congenital abnormalities
  • Requires close supervision
  • Requires attendant to help in self-care activities
  • May respond to physical and social activities
  • Not capable of independent living

This includes 1-2% of people with ID (Gluck, 2014). Individuals with severe ID require 24 hour supervision. They depend on others throughout the entire day and have severely limited communication abilities (Gluck, 2014). Finally, individuals with severe ID typically have physical limitations (Gluck, 2014).


It is estimated about 1% to 3% of Americans have a developmental disability (The Arc, 2011). However, it is believed there are actually a larger number of Americans have ID but are incorrectly diagnosed with a different developmental disability (The Arc, 2011).

Etiology & Contributing Factors

There are many different causes of ID. Intellectual disability is typically caused by any condition that impairs brain development before birth, after birth, or during childhood (The Arc, 2011). Down syndrome, Fetal Alcohol Spectrum Disorder, and Fragile X syndrome are the most common causes of ID, but the cause of about one third of intellectual disability cases are unknown (The Arc, 2011). The list below describes other common causes of ID:

  • Genetic conditions such as inheritance, abnormalities of genes, phenylketonuria, and other genetic disorders (The Arc, 2011).
  • Problems during pregnancy such as drug and alcohol use as well as illness of the mother during pregnancy (The Arc, 2011).
  • Problems at birth such as low birth weight and oxygen deprivation (The Arc, 2011).
  • Problems after birth include childhood diseases such as whooping cough, chicken pox, and measles (The Arc, 2011).
  • Poverty and cultural deprivation because children growing up in poverty are more likely to be malnourished and often receive inadequate healthcare (The Arc, 2011).

What don't we know?

ID is so common there is so much we do know about it. However, there are a few unknowns about intellectual disability.

  • The cause of ID in one third of cases
  • How ID can relate to psychological disorders
  • ID as a risk factor for other health issues later in life

Key Characteristics

The common characteristics of intellectual disability varies significantly due to the various severities of ID. However the following is a list of key characteristics in people with ID (Gluck, 2014):

  • Developmental delays
  • May have physical signs of impairment
  • Reading comprehension difficulties
  • Difficulties communicating at complex levels
  • Difficulty with social cues
  • Typically need support in various activities of daily living

I thought this was MR?

ID was previously referred to as mental retardation (MR), however the term is no longer used.
What Is An Intellectual Disability?


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(2015). Intellectual disability. The Arc. Retrieved from

(2015). Intellectual disability. WebMD. Retrieved from

Gluck, S. (2014). Mild, moderate, severe intellectual disabilities. Healthy Place. Retrieved from

Morstad, D. (2012). How prevalent are intellectual and developmental disabilities in the United States? Bethesda Institute. Retrieved from