Mental Retardation

Encyclopedia of Childhood and Adolescence

The term mental retardation is commonly used to refer to people with an intelligence quotient (IQ) below 70. An IQ of 80-130 is considered the normal range, and 100 is considered average. According to the definition in the American Psychiatric Association 's Diagnostic and Statistical Manual (DSM-IV), a mentally retarded person is significantly limited in at least two of the following areas: self-care, communication, home living, social/interpersonal skills, self-direction, use of community resources, functional academic skills, work, leisure, health, and safety. Mental retardation affects roughly 1% of the American population. According to the U.S. Department of Education, about 11% of school-aged children were enrolled in special education programs for students with mental retardation.

There are four categories of mental retardation: mild, moderate, severe, and profound. The roughly 80% of retarded persons who are classified as mildly retarded have an IQ between 50 or 55 and 70. Mild retardation, which may not be detected in early childhood, usually involves little sensorimotor impairment. Persons in this category can be educated up to a sixth-grade level. With adequate vocational guidance, they can live and work productively in the community as adults, either independently or with some degree of supervision.

About 10% of retarded persons are classified as moderately retarded, with IQs generally between 35 and 50. Although they usually do not progress beyond the second-grade level academically, as adults they can take care of themselves within supervised settings and perform unskilled or semiskilled work.

Persons with severe retardation, who account for 3-4% of the retarded population, have serious language and motor impairment. They usually do not speak in early childhood but can learn communication and basic self-care during the school years. Their language skills may be limited to the most basic functional words necessary to meet their daily needs. As adults, they live either with their families, in group homes, or, when necessary, in facilities that can provide skilled medical or nursing care.

Profound retardation, which accounts for 1-2% of the retarded population, is usually associated with a neurological condition. It is characterized by severe sensorimotor difficulties beginning in early childhood and serious long-term limitations on both communication and the ability to care for oneself. Some profoundly retarded individuals are never able to speak or to be toilet trained. Most need constant care throughout their lives.

In addition to the categories of mild, moderate, severe, and profound retardation, separate categories are sometimes used to designate those retarded persons who can benefit from some degree of academic training. Those designated "educable mentally retarded" (EMR) can handle academic work at a third- to sixth-grade level, and usually have IQs that fall between 50 and 75. The "trainable mentally retarded" (TMR) have IQs of between 30 and 50 and can progress as far as second-grade level work. It is important to note that IQ scores are not foolproof ways of detecting the abilities and potential of mentally retarded children. Some children with lower IQs ultimately prove to be more capable of leading independent, productive lives than others who score higher. Factors such as emotional support, medical attention, and vocational training can play as great a role as IQ in determining the future of a retarded child.

Rate of mental retardation among school-aged children by U.S. state, 1993

State	Children aged 6-17 with mental retardation per 1,000 population	State	Children aged 6-17 with mental retardation per 1,000 population
Alabama	31.4	Montana	7.1
Alaska	5.0	Nebraska	15.3
Arizona	7.7	Nevada	6.6
Arkansas	23.1	New Hampshire	4.0
California	4.5	New Jersey	3.2
Colorado	4.2	New Mexico	5.6
Connecticut	7.1	New York	5.7
Delaware	14.4	North Carolina	19.5
District of Columbia	13.5	North Dakota	8.9
Florida	14.8	Ohio	22.5
Georgia	20.0	Oklahoma	19.7
Hawaii	8.0	Oregon	7.6
Idaho	12.1	Pennsylvania	14.0
Illinois	10.4	Rhode Island	5.9
Indiana	17.8	South Carolina	21.9
Iowa	21.2	South Dakota	9.3
Kansas	10.9	Tennessee	14.3
Kentucky	25.5	Texas	6.4
Louisiana	12.9	Utah	6.9
Maine	6.2	Vermont	11.8
Maryland	6.5	Virginia	11.8
Massachusetts	13.8	Washington	8.3
Michigan	10.3	West Virginia	21.1
Minnesota	11.1	Wisconsin	4.6
Mississippi	12.9	Wyoming	5.7
Missouri	12.5

Causes of mental retardation

There are many different causes of mental retardation, both biological and environmental. In about 5% of cases, retardation is transmitted genetically, usually through chromosomal abnormalities, such as Down syndrome or fragile X syndrome. Down syndrome occurs when there is an extra chromosome in the 21st pair of chromosomes (known as trisomy 21). People with Down syndrome have 47 chromosomes instead of the normal 46. The disorder occurs in one out of every 600-700 births worldwide. Women over 35 are at greater risk of bearing a child with Down syndrome than younger women, and Down syndrome births are over 20 times more likely in women over 45 than in those under the age of 30. Children and adults with Down syndrome demonstrate both mental and motor retardation. Most are severely retarded, with IQs between 20 and 49, and prone to a number of physical problems, including poor vision, hearing and heart defects, and low resistance to respiratory infections. Individuals with Down syndrome (formerly called mongoloidism) also have distinctive physical features, including upward-slanting, almond-shaped eyes and a short, stocky build with a short neck and a smaller than average skull, which is usually flat in back.