Mental Retardation, disorder in which a person’s overall intellectual functioning is well below average, with an intelligence quotient (IQ) around 70 or less. Individuals with mental retardation also have a significantly impaired ability to cope with common life demands and lack some daily living skills expected of people in their age group and culture. The impairment may interfere with learning, communication, self-care, independent living, social interaction, play, work, and safety. Mental retardation appears in childhood, before age 18.
About 1 percent of the general population has mental retardation, although some estimates range as high as 3 percent. Mental retardation is slightly more common in males than in females. It occurs in people of all racial, ethnic, education, and economic backgrounds.
II DEGREES OF SEVERITY
Mental health clinicians have defined four degrees of severity of mental retardation based on IQ score. These are mild retardation (IQ range 50-55 to about 70), moderate (IQ range 35-40 to 50-55), severe (IQ range 20-25 to 35-40), and profound (IQ level below 20-25). People of average intelligence score from about 90 to 110 on IQ tests. See also Intelligence; Psychological Testing: Intelligence Tests.
Mildly affected individuals comprise about 85 percent of people with retardation. They often cannot be distinguished from normal children until they attend school. Although they learn more slowly, people with mild retardation usually can develop academic skills equivalent to the sixth-grade level. As adults, they can work and live in the community if helped when they experience unusual social or economic stress. Some may marry and have children.
About 10 percent of people with mental retardation are moderately retarded. They can progress to about the second-grade level in academic skills. By adolescence, they usually have good self-care skills—such as eating, dressing, and going to the bathroom—and can perform simple tasks. As adults, most can work at unskilled or semiskilled jobs with supervision.
Severe retardation affects 3 to 4 percent of mentally retarded individuals. Severely retarded individuals may learn to talk during childhood and develop basic self-care skills. In adulthood they can perform simple tasks with close supervision. They often live in group homes or with their families.
About 1 to 2 percent of retarded people have profound mental retardation and require constant care. Profoundly retarded individuals can understand some language, but they have little ability to talk. They often have a neurological condition that accounts for their retardation.
Scientists can identify a specific cause in 60 to 70 percent of mental retardation cases. Causes include genetic conditions, disorders that occur as a fetus develops during pregnancy, and problems during or after birth. Some cases of mental retardation have multiple causes.
A Genetic Causes
Genetic causes include single-gene defects such as Fragile X syndrome and chromosomal disorders such as Down syndrome. Scientists in 1992 identified Fragile X syndrome as the most common inherited cause of mental retardation, responsible for up to 10 percent of cases. People with this condition inherit a defective gene that results in a weak spot on the X chromosome, a sex chromosome. The weak part of the chromosome is susceptible to breaking. Fragile X syndrome is more likely to cause retardation in males then females.
Chromosomal disorders, which occur in about 7 out of every 1000 infants, involve an abnormal number of chromosomes or changes in the structure of a chromosome. Down syndrome occurs when people inherit all or part of an extra copy of a pair of chromosomes known together as chromosome 21. Although regarded as genetic disorders, chromosomal disorders are not necessarily inherited. Both parents may have normal genes, with the defect resulting from a random error when chromosomes reproduce.
Other genetic causes of mental retardation are inborn errors of metabolism. They involve inheritance of a defective gene unable to produce enzymes or proteins needed for critical cell functions. Scientists have identified more than 300 gene disorders involving inborn errors of metabolism. Many can result in mental retardation, including phenylketonuria (PKU), Tay-Sachs disease, galactosemia, homocystinuria, maple syrup urine disease, and biotinidase deficiency.
Another common cause of mental retardation, congenital hypothyroidism, occurs in about 1 in every 4000 births. Infants with this disorder are unable to produce enough thyroxine, a hormone secreted by the thyroid gland. Mental retardation and stunted growth result unless they receive thyroid replacement therapy.
B External Causes
A variety of problems during a woman’s pregnancy can cause mental retardation in her child. These problems include malnutrition; a mother’s use of alcohol or drugs; environmental toxins such as lead and mercury; viral infections, including rubella (see German Measles) and cytomegalovirus; and untreated diseases such as diabetes mellitus. Fetal alcohol syndrome results from excessive consumption of alcohol during pregnancy and is the most common preventable cause of mental retardation in the United States. It occurs in 1 to 3 out of every 1000 births. Malnutrition during pregnancy is a common cause of mental retardation in developing countries, where many women do not consume adequate amounts of protein and other necessary nutrients.
Some cases of mental retardation result from problems during birth, including premature birth, very low birth weight, and stresses to the fetus such as deprivation of oxygen. Infectious diseases during childhood, which are easily preventable through immunization, also can cause mental retardation when they result in complications. For example, measles, chicken pox, and whooping cough may lead to encephalitis and meningitis, which can damage the brain.
Physical trauma to the brain can also cause mental retardation. Brain damage may result from accidental blows to the head, near drowning, severe child abuse, and childhood exposure to such toxins as lead and mercury. Experts believe that poverty and a lack of stimulation during infancy and early childhood can be factors in mental retardation. Children raised in poor environments are more likely to experience malnutrition, lack of routine medical care, and environmental health hazards.
Newborn screening programs can prevent some cases of mental retardation by identifying inherited conditions that may lead to retardation. All states in the United States require a blood test for congenital hypothyroidism and phenylketonuria (PKU). Infants with PKU cannot metabolize the amino acid phenylalanine, found in foods that contain protein. Once identified, these infants can be given a low-phenylalanine diet that prevents retardation. Some states require newborn screening for additional inherited diseases.
Adult screening tests can identify carriers of other conditions before couples conceive a child. Individuals and couples with a family history of mental retardation can seek genetic counseling to evaluate their own risks and need for screening. Specialized laboratory tests, including amniocentesis, can detect Down syndrome and other genetic disorders in the early stages of pregnancy.
Proper prenatal care, avoidance of alcohol and drugs during pregnancy, and routine immunization against measles and other childhood diseases can prevent some forms of retardation. With pregnant women at risk of bearing a baby with a very low birth weight, magnesium sulfate treatments can reduce the risk of mental retardation in the infant by 70 percent.
New ways of preventing mental retardation may emerge as the Human Genome Project identifies more disease-causing genes. This research may lead to new screening tests and gene therapies that can remove defective genes that cause mental retardation and replace them with normal genes.
V TREATMENT AND CARE
Treatment and care of mentally retarded people has changed greatly in modern times. Until the 1800s, families kept children with retardation at home, hidden from public view. Later, state governments built large institutions to house the retarded, and physicians advised parents to institutionalize retarded children. Few retarded children had the opportunity for education and training. Experts now recognize that mental retardation is not always a lifelong disorder. Some individuals diagnosed with mild mental retardation as children may gradually develop new skills through early intervention and educational services. As adults, they may function in everyday life at a level that no longer warrants a diagnosis of retardation.
All but the most profoundly retarded people usually can best develop their full potential by living in the community. Most people with mental retardation have the capacity to learn, advance intellectually, develop job and social skills, and become full participants in society. They may marry, have families, and be indistinguishable from other people. In order to achieve their potential, mentally retarded children need special education and training, which ideally begins in infancy and continues until they establish an adult role.
Early intervention programs can provide specialized teaching and other services for infants, toddlers, and preschool children. Such programs try to optimize development of the individual’s strengths. Whenever possible, children with mental retardation attend the same school they would attend if they did not have mental retardation. But they receive instruction modified for their specific needs. Federal law in the United States requires that every school district provide services to mentally retarded children in regular schools. See Education of Students with Mental Retardation.
B Living Arrangements
The number of mentally retarded people living in large, state-sponsored institutions has declined since the 1960s and many of these institutions have closed. Most mentally retarded people live in one of a variety of community settings: group homes, supervised homes, adult foster care, personal care homes, board-and-care homes, and other settings.
Group homes provide care, supervision, and training for a small number of unrelated individuals. In supervised apartments or homes, individuals live alone or with roommates. Trained staff live in a separate unit in the same location. In adult foster care, a mentally retarded person lives with a family other than his or her own family. The foster family provides meals, a comfortable home environment, and assistance with daily living skills. Staff in a personal care home can provide help with dressing, bathing, and other personal needs. Board-and-care homes provide sleeping rooms and meals. Some social-service agencies provide assistance for people with retardation to live in the same kind of rented or owned apartments or houses as other people in the community. Profoundly retarded people may live in nursing homes that provide daily nursing care.
C Employment Opportunities
Many people with mental retardation are capable of working in a variety of full- or part-time jobs. Studies have shown that most employers are satisfied with the performance, work attendance, and loyalty of people with mental retardation. Employees with retardation do not have more accidents on the job than other workers, nor do they raise employee health insurance or benefit costs. Because people with retardation may take longer to master job tasks, supervisors may spend additional time with them during the first few days or weeks of employment. Job coaching programs established in some communities provide a specialist who helps in the initial training of a mentally retarded individual. Employers who hire individuals with mental retardation may be eligible for government tax credits and other incentives. People with more severe mental retardation may work in other settings, including special facilities known as sheltered workshops.
Despite their ability to work, most people with mental retardation do not have jobs. Surveys indicate that only about 36 percent of mentally retarded adults have full-time or part-time jobs. Reasons for this low employment rate may include a lack of training in vocational and social skills, lack of encouragement from others, and a scarcity of community programs that aid people with mental retardation in finding and maintaining employment. In addition, employers may hesitate to hire people with mental retardation because of uncertainty over how to provide accommodations for their disability.