The term "mental retardation" conjures up different meanings for different people. A useful definition is provided by the American Association on Mental Retardation: "Mental retardation is a particular state of functioning that begins in childhood and is characterized by limitation in both intelligence and adaptive skills." Mental retardation reflects the "fit" between the capabilities of individuals and the structure and expectations of their environment. It is characterized by significantly subaverage intellectual functioning, existing concurrently with related limitations in two or more of the following skill areas: communication, home living, community use, health and safety, leisure, self-care, social skills, self-direction, functional academics, and work. It is evident that deficits in intelligence and adaptive skills will be related to the complexity of the society in which the individual lives.

Categorizations of the severity of mental retardation have been established based on IQ scores. The four levels of severity are mild retardation (IQ range 50-70), moderate retardation (IQ range 35-50), severe retardation (IQ range 20-35), and profound (IQ range less than 20). Rather than use a classification based on the severity level, a classification based on the type and intensity of support needed also is now in practice: intermittent, limited, extensive, or pervasive. Persons with mild retardation usually are capable of living with some degree of independence in the community and can usually work successfully at simple jobs. The great majority—85 percent—of cases of mental retardation fall into this category. The remaining 15 percent of cases are at the moderate, severe, and profound levels, with only approximately 1 percent to 2 percent at the profound level. These last three levels are sometimes grouped together as severe. Profoundly affected individuals require constant care and supervision.

Several causes of mental retardation are becoming known, although in many cases it may not be possible to ascribe mental retardation to a specific cause. Just because a disorder is congenital (present at birth) does not necessarily imply that the disorder is genetic. Agents that are capable of affecting the developing fetus such as alcohol, mercury, infections, maternal phenylketonuria, and many other substances may lead to mental retardation. Many single-gene disorders and chromosomal abnormalities produce mental retardation as part of their syndromes, or disorders characterized by multiple effects. A large-scale study of severely mentally retarded patients institutionalized in Wisconsin, summarized by Sarah Bundey in 1997, indicated that 11.8 percent of the cases were caused by chromosomal abnormality, 6.5 percent by single-gene defects, 16.3 percent by multiple congenital anomaly syndromes, 14.7 percent by central nervous system malformations such as hydrocephalus, 32.1 percent by central nervous system dysfunction due to perinatal or unidentified prenatal causes including cerebral palsy, 8.5 percent by infectious disease, 3.9 percent by postnatal brain damage, and 1.2 percent by infantile psychosis; 4.3 percent were unclassified. It was noted that the number of patients with Down syndrome was low because they were admitted less frequently. Other surveys have shown that Down syndrome accounts for about one-third of mentally retarded patients.

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