About IDDs

About Intellectual and Developmental Disabilities (IDDs)

What are IDDs? (source: US Department of Health and Human Services / National Institutes of Health)

IDDs are differences that are usually present at birth and that uniquely affect the trajectory of the individual’s physical, intellectual, and/or emotional development. Many of these conditions affect multiple body parts or systems.

Intellectual disability starts any time before a child turns 18 and is characterized by differences with both:

  • Intellectual functioning or intelligence, which include the ability to learn, reason, problem solve, and other skills; and
  • Adaptive behavior, which includes everyday social and life skills.

The term “developmental disabilities” is a broader category of often lifelong challenges that can be intellectual, physical, or both.

“IDD” is the term often used to describe situations in which intellectual disability and other disabilities are present.

It might be helpful to think about IDDs in terms of the body parts or systems they affect or how they occur. For example:

Nervous system

These disorders affect how the brain, spinal cord, and nervous system function, which can affect intelligence and learning. These conditions can also cause other issues, such as behavioral disorders, speech or language difficulties, seizures, and trouble with movement. Cerebral palsy, Down syndrome, Fragile X syndrome, and autism spectrum disorders (ASDs) are examples of IDDs related to problems with the nervous system.

Sensory system

These disorders affect the senses (sight, hearing, touch, taste, and smell) or how the brain processes or interprets information from the senses. Preterm infants and infants exposed to infections, such as cytomegalovirus, may have reduced function with their eyesight and/or hearing. In addition, being touched or held can be difficult for people with ASDs.


These disorders affect how the body uses food and other materials for energy and growth. For example, how the body breaks down food during digestion is a metabolic process. Problems with these processes can upset the balance of materials available for the body to function properly. Too much of one thing, or too little of another can disrupt overall body and brain functions. Phenylketonuria (PKU) and congenital hypothyroidism are examples of metabolic conditions that can lead to IDDs.


Individuals with degenerative disorders may seem or be typical at birth and may meet usual developmental milestones for a time, but then they experience disruptions in skills, abilities, and functions because of the condition. In some cases, the disorder may not be detected until the child is an adolescent or adult and starts to show symptoms or lose abilities. Some degenerative disorders result from other conditions, such as untreated problems of metabolism.

The exact definition of IDD, as well as the different types or categories of IDD, may vary depending on the source of the information.

For example, within the context of education and the Individuals with Disabilities Education Act (IDEA), a law that aims to ensure educational services to children with disabilities throughout the nation, the definition of IDD and the types of conditions that are considered IDD might be different from the definitions and categories used by the Social Security Administration (SSA) to provide services and support for those with disabilities. These definitions and categories might also be different from those used by healthcare providers and researchers.

For more information about disabilities included in IDEA visit https://www.parentcenterhub.org/disability-landing/ . For information about SSA and disabilities, visit https://www.ssa.gov/planners/disability/.