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What are Intellectual and Developmental Disabilities?
Intellectual disability is the most common developmental disability.
There are many kinds of intellectual disabilities—and many causes.
What Are Intellectual and Developmental Disabilities?

Intellectual and developmental disabilities (IDD) refer to “a group of developmental conditions characterized by significant impairment of cognitive functions, which are associated with limitations of learning, adaptive behavior and skills.”1

Developmental disabilities (DD) are defined by the Centers for Disease Control and Prevention as “a group of conditions due to an impairment in physical, learning, language, or behavior areas. These conditions begin during the developmental period, may impact day-to-day functioning, and usually last throughout a person’s lifetime”.2

Intellectual disability is the most common developmental disability.

Intellectual disability (ID) is defined by the Centers for Disease Control and Prevention as “a term used when there are limits to a person’s ability to learn at an expected level and function in daily life.”3

According to the American Association of Intellectual and Developmental Disabilities (AAIDD), an individual has ID if he or she demonstrates, before the age of 22:

  • Limited intellectual functioning, as indicated by an IQ around 70
  • Significant limitations in two or more adaptive areas (skills that are needed to live, work, and play in the community, such as communication or self-care)4
What is an Intellectual Disability?
Intellectual Disability
What is intellectual disability? Learn about what qualifies as an intellectual disability.

Note: the definition of the developmental period for ID changed from "before age 18" to "before age 22" in the 12th Edition of the American Association on Intellectual and Developmental Disabilities (AAIDD) Manual.1 This video was made before the 12th Edition of the AAIDD Manual was released, and therefore uses the "before age 18" developmental period criteria.

How does IDD happen?
Disability results from the interaction between individuals with a health condition, such as cerebral palsy, Down syndrome and depression, with personal and environmental factors including negative attitudes, inaccessible transportation and public buildings, and limited social support.5

There are a number of medical conditions associated with a diagnosis of IDD. For some people, the cause is unknown.

The most common causes of health conditions associated with IDD are:
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Genetic conditions

Sometimes IDD is caused by genes inherited from parents. Examples of genetic conditions are Down syndrome, Fragile X syndrome, and phenylketonuria (PKU).
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Complications during pregnancy

IDD can result when the baby does not develop inside the mother typically. For example, there may be a difference in the way the baby’s cells divide. A woman who drinks alcohol or gets an infection like rubella during pregnancy may also have a baby with IDD.
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Problems during birth

If there are complications during labor and birth, such as a baby not getting enough oxygen, he or she may have IDD.
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Diseases or toxic exposure

Diseases like whooping cough, the measles, or meningitis can cause IDD. They can also be caused by extreme malnutrition, not getting appropriate medical care, or by being exposed to poisons like lead or mercury.
How Common are Intellectual and Developmental Disabilities?
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In 2019 there were an estimated 107.62 million individuals with intellectual disabilities globally, forming about 2% of the global population.6
That equates to approximately 6.5 million people in the United States with an intellectual disability.  
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16.6% of the population in the United States has any developmental disability. That equates to approximately 55 million people.
This number includes attention deficit/hyperactivity disorder (ADHD), learning disability, Autism Spectrum Disorder (ASD), intellectual disability, and other developmental delay. 7
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Intellectual disability often occurs alongside other developmental disabilities, like ASD. Significantly higher numbers of Black children with ASD are also diagnosed with ID, compared with Hispanic or White children.8
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Language Referring to People with IDD

The language we use is important. Read more about the language to use when referring to people with IDD.
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Easy Read & Accessibility

Easy Read is a format of presenting information that is accessible to all people. Creating and using easy read language is one additional way that we can reach as wide an audience as possible.
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Intellectual and Developmental Disabilities

Learn about health conditions associated with IDD.
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