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About Intellectual
and Developmental Disabilities (I/DD)

What is an
intellectual disability?

An intellectual disability is a disability that occurs before age 18. People with this disability experience significant limitations in 2 main areas:

1. Intellectual functioning

2. Adaptive behavior.


These limitations are expressed in the person's conceptual, social, and practical everyday living skills. A number of people with intellectual disability are mildly affected, making it difficult to recognize without visual cues. Intellectual disability is diagnosed through the use of standardized tests of intelligence and adaptive behavior. Individuals with intellectual disabilities who are provided appropriate personalized supports over a sustained period generally have improved life outcomes (AAIDD, 2011). In fact, many adults with intellectual disabilities can live independent, productive lives in the community with support from family, friends, and agencies like The Arc.

How many people
have Intellectual 

An estimated 4.6 million Americans have an intellectual or developmental disability (Larson, 2000). Prevalence studies may not identify all people with intellectual disabilities. Many school-age children receive a diagnosis of "learning disability", "developmental delay", "behavior disorder", or "Autism", instead of "intellectual disability".

What is 

Intelligence refers to a general mental capability. It involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience.


Intelligence is represented by Intelligent Quotient (IQ) scores obtained from standardized tests given by trained professionals. Intellectual disability is generally thought to be present if an individual has an IQ test score of approximately 70 or below.

What is Adaptive Behavior?

Adaptive behavior is the collection of conceptual, social, and practical skills that have been learned by people in order to function in their everyday lives.

Significant limitations in adaptive behavior impact a person's daily life and affect his or her ability to respond to a particular situation or to the environment.


Standardized testing aims to measure the following skills:

  • Conceptual Skills: Receptive and expressive language, reading and writing, money concepts, self-direction

  • Social Skills: Interpersonal, responsibility, self-esteem, follows rules, obeys laws, is not gullible, avoids victimization

  • Practical Skills: Personal activities of daily living, such as eating, dressing, mobility, and toileting; instrumental activities of daily living, such as preparing meals, taking medication, using the telephone,using transportation, and doing housekeeping activities; occupational skills; maintaining a safe environment


A significant deficit in one area impacts individual functioning enough to constitute a general deficit in adaptive behavior (AAIDD, 2011).

How does
having a disability
affect someone's life?

The effects of intellectual disabilities vary considerably among people who have them, just as the range of abilities varies considerably among all people.


Children may take longer to learn to speak, walk and take care of their personal needs, such as dressing or eating. It may take students with intellectual disabilities longer to learn in school.


As adults, some will be able to lead independent lives in the community without paid supports, while others will need significant support throughout their lives. In fact, a small percentage of those with intellectual disabilities will have serious, lifelong limitations in functioning.


However, with early intervention, appropriate education and supports as an adult, every person with an intellectual disability can lead a satisfying, meaningful life in the community.

How can supports help?

Supports include the resources and individual strategies necessary to promote the development, education, interests, and well-being of a person.


Supports enhance individual functioning. Supports can come from family, friends, and community, or from a service system.


Job coaching is one example of a support often needed by a new employee with intellectual disabilities.


Supports can be provided in many settings, and a "setting" or location by itself is not a support.

What is the definition of Developmental Disabilities?

According to the Developmental Disabilities Act (Pub. L 106-402), the term "developmental disability" means a severe, chronic disability that:

1. is attributable to a mental or physical impairment or a combination of those impairments

2. occurs before the individual reaches age 22

3. is likely to continue indefinitely

4. results in substantial functional limitations in three or more of the following areas of major life activity: (i) self-care, (ii) receptive and expressive language, (iii) learning, (iv) mobility, (v) self-direction, (vi) capacity for independent living, and (vii) economic self-sufficiency

5. reflects the individual's need for a combination and sequence of special, interdisciplinary, or generic services, individualized supports, or other forms of assistance that are of lifelong or extended duration and are individually planned and coordinated.


Before the age of ten, an infant or child with developmental delays may be considered to have an intellectual or developmental disability if his or her disabilities are likely to meet the above criteria without intervention.

How does the DD definition compare with the AAIDD definition of intellectual disability?

The major differences are in the age of onset, the severity of limitations, and the fact that the developmental disability definition does not refer to an IQ requirement.


Many individuals with intellectual disability will also meet the definition of developmental disability. However, it is estimated that at least half of individuals with intellectual disability will not meet the functional limitation requirement in the DD definition.


The DD definition requires substantial functional limitations in three or more areas of major life activity, while the intellectual disability definition requires significant limitations in one area of adaptive behavior. 

Those with developmental disabilities include individuals with Cerebral Palsy, Epilepsy, developmental delay, Autism and Autism Spectrum Disorders, Fetal Alcohol Syndrome Disorder (FASD), or any of hundreds of specific syndromes that can result in impairment of general intellectual functioning or adaptive behavior similar to that of a person with intellectual disabilities.

Why do some people still use the term "mental retardation"?

The term "mental retardation" is an outdated term that may offer special protections in some states; however, with the passage of Rosa's Law in 2010, many states have replaced all terminology from "mental retardation" to "intellectual disability".


Although some still use the term "mental retardation" to be eligible for some services in a few states, in no case does having that label guarantee that supports will be available.


The Arc does not encourage the use nor promote the term "mental retardation". The general public, including families, individuals, funders, administrators, and public policymakers at local, state, and federal levels are becoming aware of how offensive this term is and The Arc is actively working to make sure the public at large now uses the preferred term of intellectual or developmental disability.


  • American Association on Intellectual & Developmental Disabilities. (2011). Intellectual Disability: Definition, Classification, and Systems of Supports, 11th Edition. Washington, DC: American Association on Intellectual & Developmental Disabilities.

  • Developmental Disabilities Assistance and Bill of Rights Act of 2000. PL106-402.

  • Larson, S.L. et al. (2000). Prevalence of Mental Retardation And/Or Developmental Disabilities: Analysis of the 1994/1995 NHIS-D. MR/DD Data Brief. Minneapolis, MN: Institute on Community Integration, University of Minnesota.


Revised March 1, 2011.​

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