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Intellectual Disability (Intellectual Developmental Disorder)

Reviewed by Psychology Today Staff

Intellectual disability is a disorder marked by below-average intellectual function and a lack of skills necessary for independent daily living. The condition begins in the developmental period.

The general mental abilities that are examined to diagnose intellectual disability include reasoning, problem-solving, planning, abstract thinking, judgment, learning from instruction and experience, and practical understanding. These abilities are measured using individually administered tests of intelligence by a trained clinician. Additionally, people with intellectual disabilities may struggle with the skills needed to function in daily life, such as communication, social participation, and independent living without ongoing support.

While previous versions of the DSM defined severity of intellectual disability by IQ score, severity is now defined by the ability to meet the demands of daily life, as compared with peers. Severity of intellectual disability is categorized as mild, moderate, severe, or profound. Education, job training, support from family, and individual characteristics such as motivation and personality can all contribute to the ability of individuals with an intellectual disability to adapt to the demands of everyday life.

Other behavioral traits associated with intellectual disability—but not deemed criteria for a diagnosis—include aggression, dependency, impulsivity, gullibility, passivity, self-injury, stubbornness, low self-esteem, low frustration tolerance, and high risk of suicide. It is common for people with intellectual disabilities to have co-occurring mental, neurodevelopmental, medical, and physical conditions. For example, other mental disorders and epilepsy are three to four times higher in people with intellectual disabilities than in the general population. If a genetic condition has caused the intellectual disability, the person may have other impairments as well.

Intellectual disability affects about 1 percent of the population, and the prevalence of severe intellectual disability is approximately six per 1,000 people, according to the DSM-5.

  • Failure to meet intellectual developmental markers
  • Difficulties learning academic skills
  • Lack of curiosity
  • Immaturity in social interactions compared with peers
  • Difficulty regulating emotions and behavior
  • Support needed in daily living tasks compared with peers
  • Spoken language is limited

Deviations in normal adaptive behaviors depend on the severity of the condition. Mild intellectual disability may be associated with academic difficulties and a somewhat concrete approach to solving problems. Severe intellectual disability is associated with limited communication and the need for support with all activities of daily living.

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Causes of intellectual disability are numerous, and specific causes may be unknown in many cases.

The failure to adapt normally and grow intellectually may become apparent early in life or, in the case of mild intellectual disability, may not become recognizable until school age or later. An assessment of age-appropriate adaptive behaviors can be made by using developmental screening tests. The failure to achieve developmental milestones is suggestive of intellectual deficits.

A family may suspect intellectual disability if motor skills, language skills, and other cognitive skills do not seem to be developing in a child or are developing far more slowly than those among the child's peers.

The degree of impairment from intellectual disability ranges widely, from mild to profound. Less emphasis is now placed on the degree of intellectual disability and more on the amount of intervention and care required for daily life.

Causes of intellectual disability can be roughly broken down into several categories:

  • Trauma (prenatal and postnatal), such as oxygen deprivation before, during, or after birth
  • Infection (congenital and postnatal)
  • Brain malformations
  • Chromosomal abnormalities
  • Genetic abnormalities and inherited metabolic disorders
  • Seizure disorders
  • Nutritional deficits such as severe malnutrition
  • Environmental influences (alcohol, other drugs, toxins such as lead or mercury, teratogens)
  • Severe and chronic social deprivation
Can a specific condition cause intellectual disability in some but not all individuals?

Yes, this is often the case. Autism is one such condition. Another is a group of disorders known as cerebral palsy, in which motor skills, balance, and posture are compromised.  CP does not by definition involve cognitive impairment.  One-half of individuals with CP have no intellectual impairment whatsoever. However, among the 50 percent who do exhibit intellectual disability,  some 20 percent may have intellectual disabilities that are moderate to severe. Often, severe physical impairment is accompanied by severe intellectual disability. However, some individuals are seriously impaired physically but they do not suffer any extreme intellectual disability, and there are individuals who are mildly impaired physically but significantly impaired intellectually.

Spina bifida is another such condition. 80 percent of children with spina bifida have normal intelligence. This congenital defect is the exposure of part of the spinal cord and its meninges through an opening in the spine. It can cause lower limb paralysis and, in some cases,  intellectual disability.

Can injury, illness, or illness cause intellectual disability?

Yes. Head injuries, malnourishment, drowning, and environmental toxins like lead can affect cognitive function. In addition, infections and illnesses including measles, meningitis, and whooping cough can likewise affect intellectual ability.


There is no cure for intellectual disability, but proper support and services can greatly improve an individual's quality of life. To develop an appropriate treatment plan, an assessment of age-appropriate adaptive behaviors should be made using developmental screening tests. The objectives of these tests are to determine which developmental milestones have been missed. The primary goals of treatment are to develop the person's potential to the fullest and to allow them to participate in as many aspects of their community as possible. Special education and training may begin as early as infancy; in fact, early intervention is a critical part of treatment.

It is necessary for a specialist to evaluate the individual for coexisting disorders that may require treatment. Behavioral approaches are important in understanding and working with individuals with intellectual disabilities.

Can intellectual disability disorder be treated?

Early diagnosis and ongoing interventions can improve adaptive functioning throughout one's childhood and into adulthood. Because intellectual developmental disorder is not a disease, it cannot be "cured," nor should an individual with cognitive impairment be considered mentally ill. Many people with intellectual disability can and do function independently and can master activities of daily living, such as food shopping, fully maintaining personal grooming and hygiene, etc. Interventions for those with moderate to severe cognitive impairment often focus on these skills. 

What is the best way to speak to someone with an intellectual disability?

It is often helpful to speak slowly (but not necessarily at a higher volume), to someone with an intellectual disability. Ask whether a person needs help with a specific task before offering to do it for them. Do not treat them as you would a young child; if you are interacting with a teen or an adult, try to engage with them as you would with a peer. 

Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition    
National Institutes of Health
National Library of Medicine
The American Association on Intellectual and Developmental Disabilities 
Centers for Disease Control
The Special Olympics
Last updated: 04/19/2022