Mental retardation

Mental retardation is described as below-average general intellectual function with associated deficits in adaptive behavior that occurs before age 18.

Causes, incidence, and risk factors 

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

Failure to adapt normally and grow intellectually may become apparent early in life or, in the case of mild retardation, 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 at a far slower rate than the child’s peers.

The degree of impairment from mental retardation has a wide range from profoundly impaired 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.

Risk factors are related to the causes. Mental retardation affects about 1 to 3% of the population.

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

  • unexplained (This category is the largest and a catchall for undiagnosed incidences of mental retardation.)  
  • trauma (prenatal and postnatal)       o intracranial hemorrhage before or after birth (for example, periventricular hemorrhage)       o lack of oxygen to the brain before, during, or after birth       o severe Head injury  
  • infectious (congenital and postnatal)       o congenital rubella       o Meningitis       o congenital CMV       o encephalitis       o Congenital toxoplasmosis       o listeriosis       o HIV infection  
  • chromosomal abnormalities       o errors of chromosome numbers (Down’s syndrome)       o defects in the chromosome or chromosomal inheritance (fragile X syndrome, Angelman syndrome, Prader-Willi syndrome)       o chromosomal translocations (a gene is located in an unusual spot on a chromosome, or location on a different chromosome than usual) and deletions (cri du chat syndrome)  
  • genetic abnormalities and inherited metabolic disorders       o galactosemia       o Tay-Sachs disease       o phenylketonuria       o Hunter syndrome       o Hurler syndrome       o Sanfilippo syndrome       o metachromatic leukodystrophy       o adrenoleukodystrophy       o Lesch-Nyhan’s syndrome       o Rett syndrome       o tuberous sclerosis  
  • metabolic       o Reye’s syndrome       o congenital hypothyroid       o very high bilirubin levels in babies       o hypoglycemia (poorly regulated Diabetes mellitus)  
  • toxic       o intrauterine exposure to alcohol, cocaine, amphetamines, and other drugs       o methylmercury poisoning       o lead poisoning  
  • nutritional       o malnutrition  
  • environmental       o poverty       o low socioeconomic status       o deprivation syndrome


  • failure to meet intellectual developmental markers  
  • persistence of infantile behavior  
  • lack of curiosity  
  • decreased learning ability  
  • inability to meet educational demands of school

Note: 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.

Signs and tests 

  • development significantly less than that of peers  
  • an intelligence quotient (IQ) less than two standard deviations below the mean (This is frequently classified as a score below 70 on a standardized IQ test where 100 is the mean.)  
  • adaptive behavior score  
  • abnormal Denver developmental screening test


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. This includes 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 and treat those disorders. Behavioral approaches are important in understanding and working with mentally retarded individuals.

Expectations (prognosis) 

The outcome is related to the aggressiveness of treatment, personal motivation, opportunity, and associated conditions. Many people lead productive lives while functioning independently; others require a structured environment to be most successful.


Complications vary. They may include:

  • social isolation  
  • inability to care for self  
  • inability to interact with others appropriately

Calling your health care provider 
Call your health care provider if you have any concerns about your child’s development, if you notice a lack of normal development with motor or language skills in a child, or if there are associated disorders that need treatment.


Genetic: Prenatal screening for genetic defects and genetic counseling for families at risk for known heritable disorders can decrease the incidence of genetically caused mental retardation.

Social: Government programs to insure adequate nutrition are available to the underprivileged in the first and most critical years of life, which can reduce retardation associated with malnutrition. Social programs to reduce poverty and provide good education can impact the mild “retardation” associated with impoverished and lower socioeconomic status. Early intervention in abuse and deprivation will also help.

Toxic: Environmental programs to reduce lead and mercury exposure and other toxins will reduce toxin-associated retardation. However, the benefits may take years to become apparent. Increased awareness by the public of the effects of alcohol and drugs during pregnancy can reduce the incidence of retardation.

Infectious: The prevention of congenital rubella syndrome is probably one of the best examples of a successful program to prevent one form of mental retardation. Constant vigilance, such as in the relationship of cats, Toxoplasmosis, and pregnancy, helps to reduce retardation that results from this infection.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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