Mental Retardation


Mental retardation (MR) is a condition diagnosed before age 18, usually in infancy or prior to birth, that includes below-average general intellectual function, and a lack of the skills necessary for daily living. When onset occurs at age 18 or after, it is called dementia, which can coexist with an MR diagnosis. Intelligence level as determined by individual standard assessment is below 70, and the ability to adapt to the demands of normal life is impaired. This is important because it distinguishes a diagnosis of MR from individuals with low IQ scores who are able to adapt to the demands of everyday life. Education, job training, support from family, and individual characteristics such as motivation and personality can all contribute to the ability of individuals with MR to adapt.

Other behavioral traits associated with MR (but not deemed criteria for an MR diagnosis) include aggression, dependency, impulsivity, passivity, self-injury, stubbornness, low self-esteem, and low frustration tolerance. Some may also exhibit mood disorders such as psychotic disorders and attention difficulties, though others are pleasant, otherwise healthy individuals. Sometimes physical traits, like shortness in stature and malformation of facial elements, can set individuals with MR apart, while others may have a normal appearance.

Mental retardation affects about 1 percent to 3 percent of the population.


  • Failure to meet intellectual developmental markers
  • Persistence of infantile behavior
  • Lack of curiosity
  • Decreased learning ability
  • Inability to meet educational demands of school

Deviations in normal adaptive behaviors depend on the severity of the condition. Mild retardation may be associated with a lack of curiosity and quiet behavior. Severe mental retardation is associated with infantile behavior throughout life.


Causes of mental retardation are numerous, but a specific reason for mental retardation is determined in only 25 percent of cases.

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

A family may suspect mental retardation if motor skills, language skills, and self-help skills do not seem to be developing in a child or are developing far more slowly than among the child's peers.

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

Causes of mental retardation can be roughly broken down into several categories:

  • unexplained (the largest category)
  • trauma (prenatal and postnatal) such as oxygen deprivation before, during or after birth
  • infection (congenital and postnatal)
  • Chromosomal abnormalities
  • Genetic abnormalities and inherited metabolic disorders
  • metabolic disorders
  • toxins such as lead or mercury poisoning
  • nutritional deficits such as severe malnutrition
  • environment


In order 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 goal of treatment is to develop the person's potential to the fullest. Special education and training may begin as early as infancy. Attention is given to social skills to help the person function as normally as possible.

It is important for a specialist to evaluate the person for coexisting affective disorders that may require treatment. Behavioral approaches are important in understanding and working with mentally retarded individuals.


  • National Institutes of Health
  • National Library of Medicine
Last reviewed 12/27/2015