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What is Intellectual Disability?

Intellectual disability1 refers to neurodevelopmental conditions that affect functioning in two areas:

  • Cognitive functioning, such as learning, problem solving and judgement.
  • Adaptive functioning, activities of daily life such as communication skills and social participation.

Additionally, the intellectual and adaptive deficit begin early in the developmental period, typically before age 18 years for diagnosis.

Intellectual disability affects about 1% of the population, and of those about 85% have mild intellectual disability. In high-income countries, 2–3% of children have an intellectual disability.

Diagnosing Intellectual Disability

Intellectual disability is identified by significant limitations in both intellectual functioning and adaptive behavior.

Intellectual functioning is measured with individually administered and psychometrically valid, comprehensive, culturally appropriate, psychometrically sound tests of intelligence. While a specific full-scale IQ test score is no longer required for diagnosis, standardized testing is used as part of diagnosing the condition. A full-scale IQ score of around 70 to 75 indicates a significant limitation in intellectual functioning.2 However, the IQ score must be interpreted in the context of the person’s difficulties in general mental abilities. Moreover, scores on subtests can vary considerably so that the full-scale IQ score may not accurately reflect overall intellectual functioning. Therefore, clinical judgment is needed in interpreting the results of IQ tests.

What is intellectual functioning? Intellectual functioning incorporates the characteristics of intelligence, the abilities assessed by standardized intelligence tests, and the consensus view that intellectual functioning is influenced by other human functioning dimensions and by systems of supports.

Traditionally, cognitive or intellectual functioning has been measured through the intelligence quotient (IQ) tests, with an IQ of less than 70 recommended for a clinical diagnosis of Intellectual Disability. Currently, clinical diagnosis also requires a score of two or more standard deviations below the population norm (approximately less than the 2nd/3rd percentile) on a standardized measure of adaptive skills such as the vineland adaptive behavior scales.

What is adaptive behavior? Adaptive behavior is the collection of conceptual, social, and practical skills that have been learned and are performed by people in their everyday lives, which include the following:

Three areas of adaptive functioning are considered:3

  1. Conceptual – language, reading, writing, math, reasoning, knowledge, memory.
  2. Social – empathy, social judgment, communication skills, the ability to follow rules and the ability to make and keep friendships.
  3. Practical – independence in areas such as personal care, job responsibilities, managing money, recreation, and organizing school and work tasks.

Adaptive functioning is assessed through standardized measures with the individual and interviews with others, such as family members, teachers and caregivers.

Age of onset is the third element for a diagnosis of ID. This third criterion is essential because it establishes the age-related parameters for determining when ID originates or is first manifested. The age of onset criterion “before the individual attains age 22” found in the 12th edition of the AAIDD Manual is based on recent research showing that important brain development continues into our 20s.

The clinical symptoms and signs of intellectual disability are first recognized during infancy and childhood. Intellectual disability is identified as mild (most people with intellectual disability are in this category), moderate, or severe or profound.


There are many different causes of intellectual disability. It can be associated with a genetic syndrome, such as Down syndrome or Fragile X syndrome. It may develop following an illness such as meningitis, whooping cough or measles; may result from head trauma during childhood; or may result from exposure to toxins such as lead or mercury. Other factors that may contribute to intellectual disability include brain malformation, maternal disease and environmental influences (alcohol, drugs or other toxins). A variety of labor- and delivery-related events, infection during pregnancy and problems at birth, such as not getting enough oxygen, can also contribute.


Intellectual disability is a life-long condition. However, early and ongoing intervention may improve functioning and enable the person to thrive throughout their lifetime. Underlying medical or genetic conditions and co-occurring conditions frequently add to the complex lives of people with intellectual disability.

Once a diagnosis of intellectual disability is made, help for individuals with intellectual disability is focused on looking at the individual’s strengths and needs, and the supports he or she needs to function at home, in school/work and in the community.

Services for people with intellectual disabilities and their families can provide support to allow full inclusion in the community. Many different types of treatments and services can help, such as:

  • Treatment of medical complications.
  • General preventive medical care.
  • Treatment of co-morbid medical and mental health conditions.
  • Treatment of challenging behaviors.
  • Rehabilitation services.
  • Early intervention (infants and toddlers).
  • Special education.
  • Family support (for example, respite care support groups for families).
  • Transition services from childhood to adulthood.
  • Vocational programs.
  • Day programs for adults.
  • Housing and residential options.
  • Case management.

Under federal law (Individuals with Disabilities Education Act, IDEA, 1990), early intervention services work to identify and help infants and toddlers with disabilities. Federal law also requires that special education and related services are available free to every eligible child with a disability, including intellectual disability.

In addition, supports can come from family, friends, co-workers, community members, school, a physician team, or from a service system. Job coaching is one example of a support that can be provided by a service system. With proper support, people with intellectual disabilities are capable of successful, productive roles in society.

A diagnosis often determines eligibility for services and protection of rights, such as special education services and home and community services. The American Association of Intellectual and Developmental Disabilities (AAIDD) stresses that the main reason for evaluating individuals with intellectual disabilities is to be able to identify and put in place the supports and services that will help them thrive in the community throughout their lives.

Tips for Parents

  • Ask for help, learn about your child’s disability.
  • Connect with other parents of children with disabilities.
  • Be patient; learning may come slower for your child.
  • Encourage independence and responsibility.
  • Educate yourself on the educational services your child deserves.
  • Learn the laws that are written to help your child live their best life.
  • Look for opportunities in your community for social, recreational and sports activities (such as Best Buddies or Special Olympics).

Related Conditions

Some mental health, neurodevelopmental, medical and physical conditions frequently co-occur in individuals with intellectual disability, including autism spectrum disorder, cerebral palsy, epilepsy, attention-deficit hyperactivity disorder, impulse control disorder, and depression and anxiety disorders. Identifying and diagnosing co-occurring conditions can be challenging, for example recognizing depression in an individual with limited verbal ability. Family caregivers are very important in identifying subtle changes. An accurate diagnosis and treatment are important for a healthy and fulfilling life for any individual.

Physician Review

Rubí E. Luna, M.D.
UCLA Semel Institute for Neuroscience and Human
Child and Adolescent Psychiatry Fellow, PGY-5

March 2024

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