Diseases & Conditions


Mental Retardation/Intellectual Disability

Mental retardation/intellectual disability is significantly subaverage intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily living.

The long-used term “mental retardation” has acquired an undesirable social stigma. Because of this stigma, doctors and health care practitioners have begun replacing it with the term "intellectual disability." Because this change is recent, the term "mental retardation/intellectual disability" (MR/ID) is used to mark the transition in terminology.

MR/ID is not a specific medical disorder like pneumonia or strep throat, and it is not a mental health disorder. People with MR/ID have significantly below average intellectual functioning that limits their ability to cope with two or more activities of normal daily living (adaptive skills). These activities include the ability to communicate; live at home; take care of oneself, including making decisions; participate in leisure, social, school, and work activities; and be aware of personal health and safety.

People with MR/ID have varying degrees of impairment. While recognizing each person's individuality, doctors find it helpful to classify a person's level of functioning. Intellectual functioning levels can be based on the results of developmental quotient (DQ) tests and intelligence quotient (IQ) tests or on the level of support needed. Support is categorized as intermittent, limited, extensive, or pervasive. Intermittent means occasional support; limited means support such as a day program in a sheltered workshop; extensive means daily, ongoing support; and pervasive means a high level of support for all activities of daily living, possibly including full-time nursing care.

Based only on IQ test scores, about 3% of the total population are considered to have MR/ID. However, if classification is based on the need for support, only about 1% of people are classified as having significant mental (cognitive) limitation.

Levels of Mental Retardation/Intellectual Disability


Intelligence Quotient (IQ) Range

Ability at Preschool Age (Birth to 6 Years)

Ability at School Age (6 to 20 Years)

Ability at Adult Age (21 Years and Older)

Mild 52-69 Can develop social and communication skills; motor coordination is slightly impaired; often not diagnosed until later age Can learn up to about the 6th-grade level by late teens; can be expected to learn appropriate social skills Can usually achieve enough social and vocational skills for self-support; may need guidance and assistance during times of unusual social or economic stress

Moderate 36-51 Can talk or learn to communicate; social awareness is poor; motor coordination is fair; can profit from training in self-help Can learn some social and occupational skills; can progress to elementary school level in schoolwork; may learn to travel alone in familiar places May achieve self-support by performing unskilled or semiskilled work under sheltered conditions; needs supervision and guidance when under mild social or economic stress

Severe 20-35 Can say a few words; able to learn some self-help skills; has limited speech skills; motor coordination is poor Can talk or learn to communicate; can learn simple health habits; benefits from habit training May contribute partially to self-care under complete supervision; can develop some useful self-protection skills in controlled environment

Profound 19 or below Extreme cognitive limitation; little motor coordination; may need nursing care Some motor coordination; limited communication skills May achieve very limited self-care; usually needs nursing care


A wide variety of medical and environmental conditions can cause MR/ID. Some are genetic; some are present before or at the time of conception; and others occur during pregnancy, during birth, or after birth. The common factor is that something interferes with the growth and development of the brain. However, doctors can identify a specific cause in only about one third of people with mild MR/ID and in two thirds of people with moderate to profound MR/ID.

Some Causes of Mental Retardation/Intellectual Disability

Before or At Conception Inherited disorders (such as phenylketonuria, Tay-Sachs disease, neurofibromatosis, hypothyroidism, and fragile X syndrome) Chromosome abnormalities (such as Down syndrome) During Pregnancy Severe maternal malnutrition Infections with HIV, cytomegalovirus, herpes simplex, toxoplasmosis, rubella virus Toxins (such as alcohol, lead, and methylmercury) Drugs (such as phenytoin, valproate, isotretinoin, and cancer chemotherapy) Abnormal brain development (such as porencephalic cyst, grey matter heterotopia, and encephalocele) Preeclampsia and multiple births During Birth Insufficient oxygen (hypoxia) Extreme prematurity After Birth Brain infections (such as meningitis and encephalitis) Severe head injury Malnutrition of the child Severe emotional neglect or abuse Toxins (such as lead and mercury) Brain tumors and their treatments


Some children with MR/ID have abnormalities apparent at birth or shortly thereafter. These abnormalities may be physical as well as neurologic and may include unusual facial features, a head that is too large or too small, deformities of the hands or feet, and various other abnormalities. Sometimes children have an outwardly normal appearance but have other signs of serious illness, such as seizures, lethargy, vomiting, abnormal urine odor, and failure to feed and grow normally. During their first year, many children with more severe MR/ID have delayed development of motor skills, and are slow to roll, sit, and stand.

However, most children with MR/ID do not develop symptoms that are noticeable until the preschool period. Symptoms become apparent at a younger age in those more severely affected. Usually, the first problem parents notice is a delay in language development. Children with MR/ID are slower to use words, put words together, and speak in complete sentences. Their social development is sometimes slow, because of cognitive impairment and language deficiencies. Children with MR/ID may be slow to learn to dress and feed themselves. Some parents may not consider the possibility of cognitive impairment until the child is in school or preschool and is unable to keep up with age-appropriate expectations.

Children with MR/ID are somewhat more likely than other children to have behavioral problems, such as explosive outbursts, temper tantrums, and physically aggressive behavior. These behaviors are often related to specific frustrating situations compounded by an impaired ability to communicate and control impulses. Older children may be gullible and easily taken advantage of or led into minor misbehavior.

About 10 to 40% of people with MR/ID also have a mental health disorder (dual diagnosis). In particular, anxiety and depression are common, especially in children who are aware that they are different from their peers or who are maligned and mistreated because of their disability.


Many children are evaluated by teams of professionals, including a pediatric neurologist or developmental pediatrician, a psychologist, speech pathologist, occupational or physical therapist, special educator, social worker, or nurse.

Doctors evaluate a child suspected of having MR/ID by testing intellectual functioning and looking for a cause. Even though the cause of the child's MR/ID may be irreversible, identifying a disorder that caused the disability may allow doctors to predict the child's future course, prevent further loss of skills, plan any interventions that can increase the child's level of functioning, and counsel parents on the risk of having another child with that disorder.

Newborns with physical abnormalities or other symptoms suggestive of a condition associated with MR/ID often need laboratory tests to help detect metabolic and genetic disorders. Imaging tests, such as computed tomography (CT) or magnetic resonance imaging (MRI), may be performed to look for structural problems within the brain. An electroencephalogram (EEG) records the brain's electrical activity and is used to evaluate a child for possible seizures. A chromosome analysis, urine and blood tests, and x-rays of bones can also help rule out suspected causes of MR/ID.

Some children who are delayed in learning language and mastering social skills have conditions other than MR/ID. Because hearing problems interfere with language and social development, a hearing evaluation is typically performed. Emotional problems and learning disorders also can be mistaken for MR/ID. Children who have been severely deprived of normal love and attention (see Child Neglect and Abuse: Physical Neglect ) for long periods of time may seem to have MR/ID. A child with delays in sitting or walking (gross motor skills) or in manipulating objects (fine motor skills) may have a neurologic disorder not associated with MR/ID.

Because mild developmental problems are not always noticed by parents, doctors routinely perform developmental screening tests during well-child visits. Doctors use simple questionnaires, such as the Ages and Stages Questionnaires or Child Development Inventories, to quickly evaluate the child's cognitive, verbal, and motor skills. Parents can help the doctor determine the child's level of functioning by completing a Parents' Evaluation of Developmental Status (PEDS) test. Children who perform significantly below their age level on these screening tests are referred for formal testing.

Formal testing has three components: interviews with parents, observations of the child, and norm-referenced tests. Some tests, such as the Stanford-Binet Intelligence Test and the Wechsler Intelligence Scale for Children-IV (WISC-IV), measure intellectual ability. Other tests, such as the Vineland Adaptive Behavior Scales, assess areas such as communication, daily living skills, social abilities, and motor skills. Generally, these formal tests accurately compare a child's intellectual and social abilities with those of others in the same age group. However, children of different cultural backgrounds, non–English-speaking families, and very low socioeconomic status are more likely to perform poorly on these tests. For these reasons, a diagnosis of MR/ID requires that the doctor integrate the test data with information obtained from parents and direct observations of the child. A diagnosis of MR/ID is appropriate only when both intellectual and adaptive skills are significantly below average.

Prevention and Prognosis

Prevention applies to environmental, genetic, and infectious disorders as well as to accidental injuries. Fetal alcohol syndrome is a highly common and totally preventable cause of MR/ID. The March of Dimes and other groups concerned about the prevention of MR/ID focus much of their efforts on alerting women to the seriously damaging effects of drinking alcohol during pregnancy. Doctors may recommend genetic testing for people who have a family member or other child with a known inherited disorder, particularly ones related to MR/ID, such as phenylketonuria, Tay-Sachs disease, or fragile X syndrome. Identification of a gene for an inherited disorder allows genetic counselors to help parents evaluate the risk of having an affected child. Women who plan to get pregnant should receive necessary vaccinations, particularly against rubella. Women who are at risk for infectious disorders that may be harmful to a fetus, such as rubella and human immunodeficiency virus (HIV), should be tested before getting pregnant.

Proper prenatal care lowers the risk of having a child with MR/ID. Folic acid, a vitamin supplement, taken before conception and early in pregnancy can help prevent certain kinds of brain abnormalities. Advances in the practices of labor and delivery and in the care of premature infants have helped to reduce the rate of MR/ID related to prematurity.

Certain tests, such as ultrasound, amniocentesis, chorionic villus sampling, and various blood tests, can be performed during pregnancy to identify conditions that often result in MR/ID. Amniocentesis or chorionic villus sampling is often used for women at high risk of having a baby with Down syndrome, especially those aged 35 and older, and for women with family histories of metabolic disorders. Maternal serum alpha-fetoprotein is a helpful screen for neural tube defects, Down syndrome, and other abnormalities. A few conditions, such as hydrocephalus and severe Rh incompatibility (see Pregnancy Complications: Rh Incompatibility ), may be treated during pregnancy. Most conditions, however, cannot be treated, and early recognition can serve only to prepare the parents and allow them to consider the option of abortion.

Because MR/ID sometimes coexists with serious physical problems, the life expectancy of children with MR/ID may be shortened, depending on the specific condition. In general, the more severe the cognitive disability and the more physical problems the child has, the shorter the life expectancy. However, a child with mild MR/ID has a relatively normal life expectancy, and health care is improving long-term health outcomes for people with all types of developmental disabilities. Many people with mild to moderate MR/ID can support themselves, can live independently, and can be successful at jobs that require basic intellectual skills.


A child with MR/ID is best cared for by a multidisciplinary team consisting of the primary care doctor; social workers; speech, occupational, and physical therapists; neurologists or developmental pediatricians; psychologists; nutritionists; educators; and others. Together with the family, these people develop a comprehensive, individualized program for the child, which is begun as soon as the diagnosis of MR/ID is suspected. The parents and siblings of the child also need emotional support and sometimes counseling. The whole family should be an integral part of the program.

The full array of a child's strengths and weaknesses must be considered in determining what kind of support is needed. Factors such as physical disabilities, personality problems, mental illness, and interpersonal skills are all taken into consideration. Affected children with coexisting mental health disorders such as depression may be given appropriate drugs in dosages similar to those given to children without MR/ID. However, giving a child drugs without also instituting behavioral therapy and environmental changes is usually not helpful.

All children with MR/ID benefit from special education. The federal Individuals with Disabilities Education Act (IDEA) requires public schools to provide free and appropriate education to children and adolescents with MR/ID or other developmental disorders. Education must be provided in the least restrictive, most inclusive setting possible–where the children have every opportunity to interact with non-disabled peers and have equal access to community resources.

A child with MR/ID usually does best living at home. However, some families cannot provide care at home, especially for children with severe, complex disabilities. This decision is difficult and requires extensive discussion between the family and their entire support team. Having a child with severe disabilities at home requires dedicated care that some parents may not be able to provide. The family may need psychologic support. A social worker can organize services to assist the family. Help can be provided by day care centers, housekeepers, child caregivers, and respite care facilities. Most adults with MR/ID live in community-based residences that provide services appropriate to the person's needs, as well as work and recreational opportunities.

Last full review/revision October 2006 by Stephen Brian Sulkes, MD

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Introduction: Mental Health Disorders: Merck Manual Home Edition


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In This Topic



Children's Health Issues


Mental Health Disorders


Introduction · Asperger's Disorder and Pervasive Developmental Disorder Not Otherwise Specified · Autism · Childhood Disintegrative Disorder · Childhood Schizophrenia · Conduct Disorder · Depression · Manic-Depressive Illness · Oppositional Defiant Disorder · Rett's Disorder · Separation Anxiety Disorder · Somatoform Disorders · Suicidal Behavior

Source: The Merck Manual Home Edition