mental retardation

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Mental Retardation



Mental retardation is a developmental disability that first appears in children under the age of 18. It is defined as an intellectual functioning level (as measured by standard tests for intelligence quotient) that is well below average and significant limitations in daily living skills (adaptive functioning).


Mental retardation occurs in 2.5-3% of the general population. About 6-7.5 million mentally retarded individuals live in the United States alone. Mental retardation begins in childhood or adolescence before the age of 18. In most cases, it persists throughout adulthood. A diagnosis of mental retardation is made if an individual has an intellectual functioning level well below average and significant limitations in two or more adaptive skill areas. Intellectual functioning level is defined by standardized tests that measure the ability to reason in terms of mental age (intelligence quotient or IQ). Mental retardation is defined as IQ score below 70-75. Adaptive skills are the skills needed for daily life. Such skills include the ability to produce and understand language (communication); home-living skills; use of community resources; health, safety, leisure, self-care, and social skills; self-direction; functional academic skills (reading, writing, and arithmetic); and work skills.
In general, mentally retarded children reach developmental milestones such as walking and talking much later than the general population. Symptoms of mental retardation may appear at birth or later in childhood. Time of onset depends on the suspected cause of the disability. Some cases of mild mental retardation are not diagnosed before the child enters preschool. These children typically have difficulties with social, communication, and functional academic skills. Children who have a neurological disorder or illness such as encephalitis or meningitis may suddenly show signs of cognitive impairment and adaptive difficulties.
Mental retardation varies in severity. The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) is the diagnostic standard for mental healthcare professionals in the United States. The DSM-IV classifies four different degrees of mental retardation: mild, moderate, severe, and profound. These categories are based on the functioning level of the individual.

Mild mental retardation

Approximately 85% of the mentally retarded population is in the mildly retarded category. Their IQ score ranges from 50-75, and they can often acquire academic skills up to the 6th grade level. They can become fairly self-sufficient and in some cases live independently, with community and social support.

Moderate mental retardation

About 10% of the mentally retarded population is considered moderately retarded. Moderately retarded individuals have IQ scores ranging from 35-55. They can carry out work and self-care tasks with moderate supervision. They typically acquire communication skills in childhood and are able to live and function successfully within the community in a supervised environment such as a group home.

Severe mental retardation

About 3-4% of the mentally retarded population is severely retarded. Severely retarded individuals have IQ scores of 20-40. They may master very basic self-care skills and some communication skills. Many severely retarded individuals are able to live in a group home.

Profound mental retardation

Only 1-2% of the mentally retarded population is classified as profoundly retarded. Profoundly retarded individuals have IQ scores under 20-25. They may be able to develop basic self-care and communication skills with appropriate support and training. Their retardation is often caused by an accompanying neurological disorder. The profoundly retarded need a high level of structure and supervision.
The American Association on Mental Retardation (AAMR) has developed another widely accepted diagnostic classification system for mental retardation. The AAMR classification system focuses on the capabilities of the retarded individual rather than on the limitations. The categories describe the level of support required. They are: intermittent support, limited support, extensive support, and pervasive support. To some extent, the AAMR classification mirrors the DSM-IV classification. Intermittent support, for example, is support needed only occasionally, perhaps during times of stress or crisis. It is the type of support typically required for most mildly retarded individuals. At the other end of the spectrum, pervasive support, or life-long, daily support for most adaptive areas, would be required for profoundly retarded individuals.

Causes and symptoms

Low IQ scores and limitations in adaptive skills are the hallmarks of mental retardation. Aggression, self-injury, and mood disorders are sometimes associated with the disability. The severity of the symptoms and the age at which they first appear depend on the cause. Children who are mentally retarded reach developmental milestones significantly later than expected, if at all. If retardation is caused by chromosomal or other genetic disorders, it is often apparent from infancy. If retardation is caused by childhood illnesses or injuries, learning and adaptive skills that were once easy may suddenly become difficult or impossible to master.
In about 35% of cases, the cause of mental retardation cannot be found. Biological and environmental factors that can cause mental retardation include:


About 5% of mental retardation is caused by hereditary factors. Mental retardation may be caused by an inherited abnormality of the genes, such as fragile X syndrome. Fragile X, a defect in the chromosome that determines sex, is the most common inherited cause of mental retardation. Single gene defects such as phenylketonuria (PKU) and other inborn errors of metabolism may also cause mental retardation if they are not found and treated early. An accident or mutation in genetic development may also cause retardation. Examples of such accidents are development of an extra chromosome 18 (trisomy 18) and Down syndrome. Down syndrome, also called mongolism or trisomy 21, is caused by an abnormality in the development of chromosome 21. It is the most common genetic cause of mental retardation.

Prenatal illnesses and issues

Fetal alcohol syndrome affects one in 600 children in the United States. It is caused by excessive alcohol intake in the first twelve weeks (trimester) of pregnancy. Some studies have shown that even moderate alcohol use during pregnancy may cause learning disabilities in children. Drug abuse and cigarette smoking during pregnancy have also been linked to mental retardation.
Maternal infections and illnesses such as glandular disorders, rubella, toxoplasmosis, and cytomegalovirus infection may cause mental retardation. When the mother has high blood pressure (hypertension) or blood poisoning (toxemia), the flow of oxygen to the fetus may be reduced, causing brain damage and mental retardation.
Birth defects that cause physical deformities of the head, brain, and central nervous system frequently cause mental retardation. Neural tube defect, for example, is a birth defect in which the neural tube that forms the spinal cord does not close completely. This defect may cause children to develop an accumulation of cerebrospinal fluid on the brain (hydrocephalus). Hydrocephalus can cause learning impairment by putting pressure on the brain.

Childhood illnesses and injuries

Hyperthyroidism, whooping cough, chickenpox, measles, and Hib disease (a bacterial infection) may cause mental retardation if they are not treated adequately. An infection of the membrane covering the brain (meningitis) or an inflammation of the brain itself (encephalitis) cause swelling that in turn may cause brain damage and mental retardation. Traumatic brain injury caused by a blow or a violent shake to the head may also cause brain damage and mental retardation in children.

Environmental factors

Ignored or neglected infants who are not provided the mental and physical stimulation required for normal development may suffer irreversible learning impairments. Children who live in poverty and suffer from malnutrition, unhealthy living conditions, and improper or inadequate medical care are at a higher risk. Exposure to lead can also cause mental retardation. Many children have developed lead poisoning by eating the flaking lead-based paint often found in older buildings.


If mental retardation is suspected, a comprehensive physical examination and medical history should be done immediately to discover any organic cause of symptoms. Conditions such as hyperthyroidism and PKU are treatable. If these conditions are discovered early, the progression of retardation can be stopped and, in some cases, partially reversed. If a neurological cause such as brain injury is suspected, the child may be referred to a neurologist or neuropsychologist for testing.
A complete medical, family, social, and educational history is compiled from existing medical and school records (if applicable) and from interviews with parents. Children are given intelligence tests to measure their learning abilities and intellectual functioning. Such tests include the Stanford-Binet Intelligence Scale, the Wechsler Intelligence Scales, the Wechsler Preschool and Primary Scale of Intelligence, and the Kaufmann Assessment Battery for Children. For infants, the Bayley Scales of Infant Development may be used to assess motor, language, and problem-solving skills. Interviews with parents or other caregivers are used to assess the child's daily living, muscle control, communication, and social skills. The Woodcock-Johnson Scales of Independent Behavior and the Vineland Adaptive Behavior Scale (VABS) are frequently used to test these skills.


Federal legislation entitles mentally retarded children to free testing and appropriate, individualized education and skills training within the school system from ages 3-21. For children under the age of three, many states have established early intervention programs that assess, recommend, and begin treatment programs. Many day schools are available to help train retarded children in basic skills such as bathing and feeding themselves. Extracurricular activities and social programs are also important in helping retarded children and adolescents gain self-esteem.
Training in independent living and job skills is often begun in early adulthood. The level of training depends on the degree of retardation. Mildly retarded individuals can often acquire the skills needed to live independently and hold an outside job. Moderate to profoundly retarded individuals usually require supervised community living.
Family therapy can help relatives of the mentally retarded develop coping skills. It can also help parents deal with feelings of guilt or anger. A supportive, warm home environment is essential to help the mentally retarded reach their full potential.


Individuals with mild to moderate mental retardation are frequently able to achieve some self-sufficiency and to lead happy and fulfilling lives. To reach these goals, they need appropriate and consistent educational, community, social, family, and vocational supports. The outlook is less promising for those with severe to profound retardation. Studies have shown that these individuals have a shortened life expectancy. The diseases that are usually associated with severe retardation may cause the shorter life span. People with Down syndrome will develop the brain changes that characterize Alzheimer's disease in later life and may develop the clinical symptoms of this disease as well.


Immunization against diseases such as measles and Hib prevents many of the illnesses that can cause mental retardation. In addition, all children should undergo routine developmental screening as part of their pediatric care. Screening is particularly critical for those children who may be neglected or undernourished or may live in disease-producing conditions. Newborn screening and immediate treatment for PKU and hyperthyroidism can usually catch these disorders early enough to prevent retardation.
Good prenatal care can also help prevent retardation. Pregnant women should be educated about the risks of drinking and the need to maintain good nutrition during pregnancy. Tests such as amniocentesis and ultrasonography can determine whether a fetus is developing normally in the womb.



American Association on Mental Retardation (AAMR). 444 North Capitol St., NW, Suite 846, Washington, D.C. 20001-1512. (800) 424-3688.
The Arc. 900 Varnum Street NE, Washington, D.C. 20017. (202) 636-2950.


Americans With Disabilities Act (ADA) Page.

Key terms

Amniocentesis — A test usually done between 16 and 20 weeks of pregnancy to detect any abnormalities in the development of the fetus. A small amount of the fluid surrounding the fetus (amniotic fluid) is drawn out through a needle inserted into the mother's womb. Laboratory analysis of this fluid can detect various genetic defects, such as Down syndrome, or neural tube defects.
Developmental delay — The failure to meet certain developmental milestones, such as sitting, walking, and talking, at the average age. Developmental delay may indicate a problem in development of the central nervous system.
Down syndrome — A disorder caused by an abnormality at the 21st chromosome. One symptom of Down syndrome is mental retardation.
Extensive support — Ongoing daily support required to assist an individual in a specific adaptive area, such as daily help with preparing meals.
Hib disease — An infection caused by Haemophilus influenza type b (Hib). This disease mainly affects children under the age of five. In that age group, it is the leading cause of bacterial meningitis, pneumonia, joint and bone infections, and throat inflammations.
Inborn error of metabolism — A rare enzyme deficiency; children with inborn errors of metabolism do not have certain enzymes that the body requires to maintain organ functions. Inborn errors of metabolism can cause brain damage and mental retardation if left untreated. Phenylketonuria is an inborn error of metabolism.
Limited support — A predetermined period of assistance required to deal with a specific event, such as training for a new job.
Phenylketonuria (PKU) — An inborn error in metabolism that prevents the body from using phenylalanine, an amino acid necessary for normal growth and development.
Trisomy — An abnormality in chromosomal development. Chromosomes are the structures within a cell that carry its genetic information. They are organized in pairs. Humans have 23 pairs of chromosomes. In a trisomy syndrome, an extra chromosome is present so that the individual has three of a particular chromosome instead of the normal pair. An extra chromosome 18 (trisomy 18) causes mental retardation.
Ultrasonography — A process that uses the reflection of high-frequency sound waves to make an image of structures deep within the body. Ultrasonography is routinely used to detect fetal abnormalities.


1. pertaining to the mind.
2. pertaining to the chin.
mental disorder any clinically significant behavioral or psychological syndrome characterized by distressing symptoms, significant impairment of functioning, or significantly increased risk of death, pain, or other disability. Mental disorders are assumed to result from some behavioral, psychological, or biological dysfunction in the individual. The concept does not include deviant behavior, disturbances that are essentially conflicts between the individual and society, or expected and culturally sanctioned responses to particular events.
mental retardation less than average general intellectual functioning that brings with it some degree of impaired adaptation in learning, social adjustment, or maturation, or in all three areas; it is now classified as a developmental disability.

Mental retardation is a relative term. Its meaning depends on what society demands of the individual in learning, skills, and social responsibility. Many people who are considered developmentally challenged in the complex modern world would get along normally in a simpler society.

Diagnosis: There is no absolute measurement for retardation. At one time the different types were classified only according to the apparent severity of the retardation. Since the most practical standard was intelligence, the degree of retardation was based on the score of the patient on intelligence tests such as the intelligence quotient (IQ). The average person is considered to have an IQ of between 90 and 110, and those who score below 70 are considered mentally retarded.

In the past, the different groupings were classified in terms such as feebleminded, idiot, imbecile, and moron. Today, most health care providers use the following classifications: for IQ's from 50 to 70, mild; 35 to 50, moderate; 20 to 35, severe; under 20, profound. Whatever classifications are used, it is agreed that IQ measurements are only one part of the factors to be considered in determining mental retardation. Others, such as the patient's adaptability to surroundings, the services and training available, and the amount of control shown over his or her emotions, are also very important.

About 85 per cent of patients considered mentally retarded are in the least severe, or mild, group. Those in this group do not usually have obvious physical defects and thus are not always easy to identify as mentally retarded while they are still infants. Sometimes such a child's mental defects do not show up until the time of entering school, when the child has difficulty learning and keeping up with others in the same age group. Many persons who are in the mild category, as adults can find employment or a place in society suitable to their abilities, so that they are no longer identified as mentally retarded.

Cause: The cause of mental retardation is often unidentifiable; known ones are classified as either genetic or acquired. Genetic conditions include chromosomal abnormalities such as down syndrome and klinefelter's syndrome and errors of metabolism such as phenylketonuria, hypothyroidism, and tay-sachs disease. Acquired conditions may be prenatal, perinatal, or postnatal. Prenatal conditions include rubella and other viral infections, toxins, placental insufficiency, and blood type incompatibility. Perinatal causes are anoxia, birth injury, and prematurity. Postnatal causes may include infections, poisons, poor nutrition, trauma, and sociocultural factors such as deprivation.

Many conditions that can cause severe retardation can be diagnosed during pregnancy, and in some cases proper treatment can lessen or even prevent retardation. Proper care for the mother during pregnancy and for the baby in the first months of life is also important.


delay; hindrance; delayed development.
mental retardation subnormal general intellectual development, associated with impairment of either learning and social adjustment, maturation, or both; see also mental retardation.
psychomotor retardation a generalized slowing of physical and emotional reaction, such as that seen in major depression and in catatonic schizophrenia.

men·tal re·tar·da·tion

subaverage general intellectual functioning that originates during the developmental period and is associated with impairment in adaptive behavior. The American Association on Mental Deficiency lists eight medical classifications and five psychological classifications; the latter five replace the three former classifications of moron, imbecile, and idiot. Mental retardation classification requires assignment of an index for performance relative to a person's peers on two interrelated criteria: measured intelligence (IQ) and overall socioadaptive behavior (a judgmental rating of the person's relative level of performance in school, at work, at home, and in the community). In general an IQ of 70 or less indicates mental retardation (mild = 50/55-70; moderate = 35/40-50/55; severe = 20/25-35/40; profound = below 20/25); an IQ of 70-85 signifies borderline intellectual functioning.

mental retardation

n. Often Offensive
Intellectual disability.

mental retardation

a disorder characterized by subaverage general intellectual function with deficits or impairments in the ability to learn and to adapt socially. The cause may be genetic, biological, psychosocial, or traumatic.

mental retardation

Neurology Below-average general intellectual function with associated deficits in adaptive behavior that occurs before age 18; intellectual activity that is significantly below average for a population, and associated with impaired social function. Cf Psychological testing.

men·tal re·tar·da·tion

(men'tăl rē'tahr-dā'shŭn)
Subaverage general intellectual functioning that originates during the developmental period and is associated with impairment in adaptive behavior. Mental retardation classification requires assignment of an index for performance relative to a person's peers on two interrelated criteria: measured intelligence (IQ) and overall socioadaptive behavior. In general, an IQ of 70 or lower indicates mental retardation. Some clinicians and laypeople suggest this term may be offensive in some contexts.

mental retardation

Intellectual ability so much below average as to preclude the performance of most forms of work or other social functions. Mentally retarded people usually require supervision and guidance if they are to avoid distress or danger. There are degrees of mental deficiency. So far as INTELLIGENCE QUOTIENTS (IQs) can be measured in the mentally retarded, the mildly defective have IQs from 70 down to about 55; the moderately defective have IQs from 54 to 40; and the severely defective have IQs below 40.

men·tal re·tar·da·tion

(men'tăl rē'tahr-dā'shŭn)
Subaverage general intellectual functioning starting with developmental period, associated with impairment in adaptive behavior.

mental retardation,

n a disorder characterized by certain limitations in a person's mental functioning and in skills such as communicating, taking care of him or herself, and social skills. It has many causes, including genetic conditions such as Down syndrome, fragile X syndrome, and phenylketonuria (PKU) as well as problems during pregnancy or at birth, or due to health problems such as diphtheria, measles, or meningitis. It can also be caused by extreme malnutrition, not getting enough medical care, or being exposed to poisons such as lead or mercury.
References in periodicals archive ?
Strikingly, slightly more than 10% of these patients have some form of mental retardation.
Yet, through the ages, the understanding and treatment of persons with mental retardation have moved like a pendulum between extremes.
To top it all, even the fragile X mental retardation protein didn't have any effect on bantam.
Sample questions included the following: Have you ever read about mental retardation in a book, newspaper, or magazine?
The majority (89%) of students with mild mental retardation were identified as needing some type of vocational instruction.
Completion of the career education curriculum in grade school was significantly related to the future employment levels of all students with mental retardation and of females with severe learning disabilities.
Likewise, studies examining mental retardation have also illustrated a significantly negative social attitude.
Psychiatric aspects of diagnosis and treatment of mental retardation.
A key role of The Arc," explains Abeson, "is to define the future for people with mental retardation and their families.
Advocates point out that many people with mental retardation are taught to trust and obey authority figures: parents, teachers, and the police.
Enrollees who responded yes to the question on mental retardation were counted in that group regardless of other reported mental conditions.
Hostility against those with mental illness and mental retardation is particularly acute.

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