mental retardation

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mental retardation

mental retardation, below average level of intellectual functioning, usually defined by an IQ of below 70 to 75, combined with limitations in the skills necessary for daily living. Daily living skills include such things as communication, the ability to care for oneself, and the ability to work. The definition of mental retardation has evolved over the years. Prior categorizations of mental retardation, defined solely by IQ, have largely been abandoned in favor of an approach that looks at how much support the retarded person needs in various areas of his or her life at any given time. Such support can range from intermittent help in such things as finding housing or a job, to pervasive, daily, lifelong help in all areas.


There are several hundred possible causes of mental retardation. They include genetic conditions (e.g., Down syndrome, fragile X syndrome); prenatal problems (e.g., fetal alcohol syndrome, rubella, malnutrition); problems apparent at birth (e.g., low birth weight and prematurity); and problems that occur after birth (e.g., injuries and childhood diseases like measles that can lead to meningitis and encephalitis). The most commonly identified causes of mental retardation are Down syndrome, fragile X syndrome, and fetal alcohol syndrome. In many cases the cause is never known.


Most mentally retarded children are capable of learning new things, both in and out of a formal school setting, but they may learn at a slower pace than other children. Schools are responsible for providing an appropriate education for retarded children. Many teachers and parents feel that the practice of mainstreaming, or inclusion, which places such children in standard classrooms for at least part of the day, helps them to feel more a part of society and helps others to better understand their special needs and capabilities.


Many cases of mental retardation are now prevented by improved health care. Vaccines against rubella and measles prevent an estimated 3,000 cases of mental retardation in the United States yearly. Vaccination against Haemophilus influenzae b (Hib), a cause of childhood meningitis, is expected to prevent 3,000 more. Prevention of Rh disease (see Rh factor), screening and treatment for phenylketonuria, and emphasis on prenatal care and the dangers of poor nutrition or alcohol consumption during pregnancy have also resulted in a decrease in cases of mental retardation in the United States. Mental retardation rates in poor nations, however, are rising.


The treatment of mentally retarded people has always reflected the changes in society. They have been officially referred to as idiots and as the feebleminded. The introduction of the IQ test was followed by a classification system that used such terms as moron (IQ of 51–70), imbecile (26–50), and idiot (0–25); later these terms were softened and classifications redefined somewhat to mild (IQ of 55–70), moderate (40–54), severe (25–39), and profound (0–24) retardation. The term mentally retarded itself, although still commonly used, has been replaced in some settings by the term developmentally disabled.

Mentally retarded people have been subjected to unnecessary institutionalization and, as a result of the eugenics movement, involuntary sterilization. The deinstitutionalization movement of the 1970s reflected a concern for the civil rights of mentally retarded. Very few of the mentally retarded are now institutionalized; most now live independently, with their families, or in group homes. The emphasis on education and self-sufficiency seen in the late 20th cent. mirrors a similar movement in the 1840s.


See M. Adams, Mental Retardation and Its Social Dimensions (1971); A. Clarke et al., ed., Mental Retardation: The Changing Outlook (1985); E. Zigler, Understanding Mental Retardation (1986); American Association on Mental Retardation, Mental Retardation: Definition, Classification, and Systems of Support (1992).

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Mental retardation

A developmental disability characterized by significantly subaverage general intellectual functioning, with concurrent deficits in adaptive behavior. The causes are many and include both genetic and environmental factors as well as interactions between the two. In most cases the diagnosis is not formally made until children have entered into school settings. In the preschool years, the diagnosis is more likely to be established by evidence of delayed maturation in the areas of sensory-motor, adaptive, cognitive, social, and verbal behaviors. By definition, evidence of mental retardation must exist prior to adulthood, where vocational limitation may be evident, but the need for supervision or support may persist beyond the usual age of social emancipation.

From the aspect of etiology, mental retardation can be classified by prenatal, perinatal, or postnatal onset. Prenatal causes include genetic disorders, syndromal disorders, and developmental disorders of brain formation. Upward of 700 genetic causes have been suggested as associated with the development of mental retardation. Many environmental influences on the developing fetus, for example, infection, and other unknown errors of development may account for mental retardation.

Perinatal causes include complications at birth, extreme prematurity, infections, and other neonatal disorders. Postnatal causes include trauma, infections, demyelinating and degenerative disorders, consequences of seizure disorders, toxic-metabolic disorders, malnutrition, and environmental deprivation. Often no specific cause can be identified for the mental retardation of a particular individual.

Individuals with mental retardation are typically subclassified in terms of the manifest severity of cognitive disability as reflected by the ratio of mental age to chronological age, or intelligence quotient (IQ). Subaverage intellectual functioning is defined as an IQ score of at least two standard deviations below the mean, or approximately 70 to 75 or below. Mild, moderate, severe, and profound degrees of mental retardation refer to two, three, four, or five standard deviations below the normal IQ for the general population.

Limitations in adaptive behavior must also be demonstrable in order to satisfy diagnostic criteria for mental retardation. This criterion is important because certain artistic or other gifts may not be revealed by formal IQ testing, and different levels of learning difficulty may be accentuated by the demands of specific environments. Outside such environments, an individual may navigate a normal course in life.

A specific genetic or other cause of mental retardation may also predispose to other medical or neurologic conditions. In these circumstances, the comorbid medical conditions may increase the likelihood of emotional or behavioral problems, or contribute to the challenges with which a given child must contend. Thus, the identification of cause can be important in planning for the medical, educational, and treatment needs of a particular individual.

Considerable progress has been made in both prevention and treatment. Diet is a method of treatment following early detection of phenylketonuria; warnings regarding alcohol consumption during pregnancy, lead exposure in infancy, and disease immunization and therapy are measures for prevention of retardation. Advances in prenatal, obstetrical, and neonatal care and genetic counseling have had the effect of reducing the incidence or the severity of various conditions. Energetic training and the application of psychosocial techniques have resulted in improved social performance and adaptive behavior in many persons with mental retardation.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

mental retardation

[′men·təl ‚rē‚tär′dā·shən]
An abnormal slowness of mental function and behavior patterns relative to age and development.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.