Intellectual disability is a condition in which people have substantial limitations in their mental abilities. People with intellectual disabilities score significantly below average on tests designed to measure intelligence. As a result of their intellectual limits, they experience difficulties in daily activities, such as learning, working, and caring for themselves. They also have difficulty with such social skills as understanding other people’s behavior or communicating thoughts and feelings.
People who show signs of intellectual disability before they reach 18 years of age are considered mentally handicapped. The most common symptom is a delay in achieving milestones of development. People with intellectual disabilities were often called mentally retarded. Many children with severe disability fail to sit up or walk at the usual age for these accomplishments. Children with less severe disabilities may be slow in learning to talk. Mild disability may escape detection until a child starts school and has trouble learning.
Physicians and social workers once advised parents of children with intellectual disability to place them in residential institutions. Experts now believe that living in a community is a better arrangement for all but the most serious cases. People with intellectual disability and their families need varying levels of support services to help them live successfully in a community.
Diagnosis
of intellectual disability first requires determining that a person has a serious intellectual impairment. The best available means for measuring intelligence is a test called an IQ test, which is still imperfect and inexact. IQ tests assign an approximate numerical value to intelligence. People of average ability score from about 90 to 110. People with intellectual disability score below about 75. For a discussion of IQ tests and their limitations, see the article Intelligence quotient.
No IQ test score alone is a sufficient basis for a diagnosis of intellectual disability. The score must be confirmed by expert evaluation of an individual’s ability to perform daily activities and to function in everyday situations. There are four broad categories of intellectual disability based on approximate IQ: (1) mild, (2) moderate, (3) severe, and (4) profound.
Mild disability.
People with mild intellectual disability form the largest group. These people have IQ’s from about the low 50’s to the low 70’s. Many people with mild intellectual disability can master reading and other schoolwork up to about the sixth-grade level. Most attend some special education classes and some regular classes. Many adults with mild intellectual disability who demonstrate good social skills can live with minimal supervision and work at jobs suited to their abilities.
Moderate disability.
People with moderate intellectual disability have IQ’s from about 40 to the low 50’s. Educational goals for children with moderate intellectual disability focus on teaching them to care for themselves and other practical skills. Some can learn a little reading and writing. In some cases, adults can work at suitable jobs at home or in special workshops.
Severe disability.
People with severe intellectual disability have IQ’s from about the mid-20’s to the high 30’s. Training for people with severe disability emphasizes learning to care for themselves and developing language and social skills. Individuals with severe intellectual disability require close supervision throughout life.
Profound disability.
People with profound intellectual disability have IQ’s below about 25. They remain at about the mental age of a baby or toddler. Most of them have only limited abilities to talk or care for themselves. People with profound disability need constant care.
Causes.
Intellectual disability can result from any factor that hinders healthy brain development. In many cases, doctors cannot identify any single cause for the condition. Factors involved in intellectual disability fall into the broad categories of (1) genetic and (2) environmental.
Genetic factors
involve faulty chemical instructions in genes, the hereditary material in every cell. In human cells, genes are carried on 46 microscopic threads called chromosomes that are arranged in 23 pairs. Some errors in the copying and division of genes or chromosomes can cause intellectual disability. In some cases, such errors are inherited. Others occur spontaneously for unknown reasons.
One common genetic cause of retardation is a disorder called Down syndrome. People with Down syndrome have an entire extra chromosome, for a total of 47. Another common genetic factor is called fragile X syndrome. This condition involves an abnormality in the X chromosome, one of the chromosomes that determines a person’s sex.
Many less common genetic factors involve genes that control a particular chemical pathway in the body. One such condition is phenylketonuria << `FEHN` uhl `KEE` tuh NYUR ee uh >> often abbreviated PKU. People with PKU cannot process one of the amino acids that make up proteins. This amino acid builds up and leads to brain damage if their diet is not controlled.
Environmental factors
include a wide variety of influences that can affect brain development before, during, or after birth. During pregnancy, a woman’s general health and nutrition greatly affect her unborn baby. Consumption of alcohol during pregnancy can lead to a condition in the baby called fetal alcohol syndrome. This syndrome causes many serious health problems, often including mental retardation. Use of prescription, nonprescription, or illegal drugs can also harm a developing baby’s brain. Exposure before birth to certain infections, including rubella (also called German measles) and HIV, can also cause intellectual disability.
Events at birth can also harm the brain. Premature birth, injury during delivery, and failure of the newborn to breathe properly can all lead to intellectual disability. During childhood, intellectual disability can result from such causes as brain infections, head injuries, prolonged high fevers, or lead poisoning.
Prevention.
Good health habits and professional medical care during pregnancy can prevent many cases of intellectual disability. Skilled delivery and care of sick or premature infants also help avoid some cases. Damage resulting from PKU and a few other disorders can be controlled after birth by a special diet or medication.
Some couples know that such factors as their age, family history, or ethnic background increase their risk of having children with certain genetic conditions. In some cases, genetic tests are available to determine if the parents or the unborn baby carry damaged genes. Some couples decide to have such tests and take the results into account in planning or continuing a pregnancy. See Genetic counseling.
Treatment
of intellectual disability focuses on identifying each individual’s strengths and limitations and devising approaches that encourage the greatest possible intellectual and social development. One key part of treatment is appropriate education or training. In the United States and many other countries, it is the legal right of every school-age child with intellectual disability, no matter how serious, to have the opportunity to learn to function to the best of his or her ability. In some cases, training can begin in infancy. It may continue until the individual is well established in an adult role.
As children with intellectual disability grow up, their education tends to center increasingly on the skills they will need as adults. Many adults with mild disability are able to work and live with some independence. Some adults with moderate to severe intellectual disability may work in a sheltered workshop, a center that employs people with disabilities. Most such individuals live with their families or in group homes in the community. Only people with the most severe intellectual disability require constant care.
