Mental Retardation
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Mental Retardation

Mental retardation is often poorly understood because its effects vary greatly among those who have it. About 85% of people with mental retardation are mildly affected. They are only a little slower than average in learning new information and skills. As children, their mental retardation is not readily apparent and may not be identified until they enter school. As adults, many are able to lead somewhat independent lives.

The remaining 15% of people with mental retardation—those with IQs under 50—have serious limitations in their ability to function. However, with early intervention and appropriate support, they can also lead satisfying lives.

What Is Mental Retardation?

Mental retardation begins in childhood and is characterized by limitations in both intelligence and adaptive skills. The following three criteria must be met for a diagnosis of mental retardation:

  • Intellectual functioning (IQ) below 70
  • Significant limitations exist in two or more adaptive skill areas. These include:
    • Communication
    • Community use
    • Functional academics (reading, writing, basic math)
    • Health and safety
    • Home living
    • Leisure
    • Self-care
    • Self-direction
    • Social skills
    • Work
  • The condition begins before age 18

What Causes Mental Retardation?

Any condition that impairs development of the brain before birth, during birth, or during childhood can cause mental retardation. The three major causes are Down syndrome, fetal alcohol syndrome, and fragile X. Several hundred causes have been discovered, but in about one-third of those affected, the cause remains unknown.

The main causes can be categorized as follows:

Genetic Conditions

Genetic abnormalities may be inherited from parents, or may be caused by environmental factors.

More than 500 genetic diseases are associated with mental retardation. For example:

  • PKU (phenylketonuria)—Children born with this rare genetic disorder cannot metabolize phenylalanine (PHE), which is an amino acid found in food. Without proper treatment, PKU can lead to mental retardation.
  • Down syndrome—In a normal fertilized egg, chromosomes exist in pairs, but in the case of Down syndrome, there are three of chromosome 21.
  • Fragile X syndrome—This is caused by mutations of the FMR1 gene, the leading cause of inherited mental retardation.

Problems During Pregnancy

Use of alcohol or drugs by a pregnant mother can cause mental retardation. Smoking can increase the risk as well. Other risks during pregnancy include:

  • Malnutrition
  • Certain environmental toxins
  • Illnesses of a mother during pregnancy that can be passed on to her infant, such as:
  • X-rays or radiation exposure—Typical diagnostic x-rays are considered acceptable in pregnancy if they are necessary for the mother’s well-being.
  • Maternal substance abuse such as:
    • Alcohol
    • Drugs
  • Prescription medications such as:
    • Isotretinoin (Accutane)
    • Phenytoin (Dilantin)
  • Maternal diseases such as:

Problems at Birth

Prematurity and low birth weight may sometimes lead to mental retardation. These conditions may be associated with bleeding in or around the brain. However, other birth conditions or physical stress in the newborn stage may injure an infant's brain.

Problems After Birth

Other conditions that can damage a child's brain and possibly lead to mental retardation include:

In addition, poisoning from lead , mercury , carbon monoxide, and other environmental toxins can cause permanent damage to a child's brain and nervous system.

Poverty and Cultural Deprivation

Children in poor families may become mentally retarded because of:

  • Malnutrition
  • Disease-producing conditions
  • Inadequate medical care
  • Environmental health hazards
  • Lack of stimulation to help the brain grow

How Is Mental Retardation Diagnosed?

The American Association on Mental Retardation has outlined a process for diagnosing and classifying a person with mental retardation. It consists of three steps:

  • Standardized intelligence tests and adaptive skills tests given by a qualified person.
  • Description of the person's strengths and weaknesses across four dimensions. This may be done with testing, interviews, and observation. The four dimensions are:
    • Intellectual and adaptive behavior skills
    • Psychological/emotional considerations
    • Physical/health/medical considerations
    • Environmental considerations
  • Determination, by an interdisciplinary team, of the level of support needed across the four dimensions.

How Is Mental Retardation Treated?

The best assistance for people with mental retardation begins with diagnosis and help early in life. Treatment includes:

  • Family counseling
  • Training in daily living skills
  • Education
  • Job training
  • Housing services

With enough education and support, many people with mental retardation can learn to take care of their basic needs and to live in the community.

Can Mental Retardation Be Prevented?

Newborn screening followed by proper treatment can prevent mental retardation resulting from certain conditions. Examples include:

  • Phenylketonuria (PKU)
  • Congenital hypothyroidism

Vaccines can prevent certain infectious diseases that may lead to mental retardation such as:

Other interventions that can reduce the risk of mental retardation include:

  • Removing lead from the environment
  • Using child safety seats and bicycle helmets
  • Early intervention programs with high-risk infants and children
  • Early and comprehensive prenatal care
  • Abstaining from alcohol, tobacco, and drugs during pregnancy


American Association on Intellectual and Developmental Disabilities

Disabilities Home Page
The Centers for Disease Control and Prevention


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The AAIDD definition. American Association on Mental Retardation website. Available at: Accessed June 17, 2008.

The Arc website. Available at:

Children who are mentally retarded. The American Academy of Child and Adolescent Psychiatry website. Available at: Updated July 2004. Accessed June 17, 2008.

Intellectual disability. EBSCO Health Library website. Available at: Updated May 2008. Accessed June 17, 2008.

Last reviewed May 2008 by Ryan Estévez, MD, PhD, MPH

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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