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Child Psychiatrist /Adult Psychiatrist

What is Intellectual Disability?

Updated: Sep 8, 2023

Intellectual disability involves problems with general mental abilities that affect functioning in two areas:

  • Intellectual functioning (such as learning, problem solving, judgement).

  • Adaptive functioning (activities of daily life such as communication and independent living).

Additionally, the intellectual and adaptive deficit begin early in the developmental period.

Intellectual disability affects about 1% of the population, and of those about 85% have mild intellectual disability. Males are more likely than females to be diagnosed with intellectual disability.

Diagnosing Intellectual Disability

Intellectual disability is identified by problems in both intellectual and adaptive functioning.

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. 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.

a person and a child in a wheelchair

Three areas of adaptive functioning are considered:
  • Conceptual – language, reading, writing, math, reasoning, knowledge, memory.

  • Social – empathy, social judgment, communication skills, the ability to follow rules and the ability to make and keep friendships.

  • 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.

Intellectual disability is identified as mild (most people with intellectual disability are in this category), moderate or severe. The symptoms of intellectual disability begin during childhood. Delays in language or motor skills may be seen by age two. However, mild levels of intellectual disability may not be identified until school age when a child has difficulty with academics.


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.

a child that is using a laptop


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 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 supports and services can help, such as:

  • 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.

Related and Co-occurring 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.

  • Autism spectrum disorder

  • Attention-deficit hyperactivity disorder

  • Impulse control disorder

  • Depression

  • Anxiety disorders.

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).


  1. The term intellectual disability used in DSM-5 replaces “mental retardation” used previously.

  2. The majority of people, 68%, have IQ scores between 85 and 115.

  3. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition. Arlington, VA APA Publishing. 2013.


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