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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 disability 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 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 disability 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 disability than in the general population. If a genetic condition has caused the intellectual disability, a person may also have the characteristic physical features of that condition (as in Down syndrome).

Intellectual disability affects about 1 percent of the population, and prevalence for 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.

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 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


There is no cure for intellectual disability, but proper supports 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 person for coexisting disorders that may require treatment. Behavioral approaches are important in understanding and working with individuals with intellectual disability.

Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition    
National Institutes of Health
National Library of Medicine
Last updated: 02/07/2019