Intellectual Disability

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Date: Mar. 19, 2014
From: Sick!
Publisher: Gale
Document Type: Disease/Disorder overview
Length: 1,595 words
Lexile Measure: 980L

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Intellectual disability is a developmental disability that is marked by lower-than-normal intelligence and limited daily living skills. intellectual disability is normally present at birth or develops early in life. The term mental retardation was once used to describe the condition of intellectual disability, but that term has largely been replaced and become obsolete today.


Intellectual disability is defined by two standards. The first standard is a person's level of intelligence. Intelligence levels are usually measured by special tests called intelligence tests. Intelligence tests provide a numerical ranking of a person's mental abilities. That ranking is called an intelligence quotient or, more commonly, an IQ. In general, a person with an IQ score of less than 75 is said to be intellectually disabled.

The second standard for intellectual disability is adaptive skills. The term"adaptive skills" means how well a person can deal with the tasks of everyday life. These tasks include the ability to speak and understand; home-living skills; use of community resources; leisure, self-care, and social skills; self-direction; basic academic skills (reading, writing, and arithmetic); and work skills. A person is regarded as intellectually disabled if he or she is unable to dress, feed, wash, or otherwise care for himself or herself; to hold a job; or to carry out most of the other tasks needed to get through an ordinary day.

Mental-health professionals classify patients into one of four levels of disability.

Mild intellectual disability

  • IQ scores from 50 to 75
  • Includes about eighty-five percent of the intellectually disabled population
  • Individuals in this group can often live on their own with community support.

Moderate intellectual disability

  • IQ scores between 35 and 50
  • Includes about ten percent of the intellectually disabled population
  • Individuals in this group can often lead relatively normal lives provided they receive some level of supervision. Such individuals often live in group homes with other intellectually disabled people.

Severe intellectual disability

  • IQ scores between 20 and 35
  • Includes about three to four percent of the intellectually disabled population
  • Individuals in this category can often master the most basic skills of living, such as cleaning and dressing themselves. They often live in group homes.

Profound intellectual disability

  • IQ scores of less than 20
  • Includes about one to two percent of the intellectually disabled population
  • Individuals at this level can often develop basic communication and self-care skills. They often have other mental disorders.


In about one-third of all cases, the cause of intellectual disability is not known. The remaining two-thirds of cases are thought to be caused by one of four factors: heredity, prenatal problems, childhood illnesses, and environmental factors.


About five percent of intellectual disability cases are caused by genetic factors. Genes are chemical units found in all cells. They carry the instructions that tell cells how they are to perform. In some cases, children inherit defective genes from their parents. These genes may interfere with the normal development of the child's brain, leading to intellectual disability.

Prenatal Problems

The daily choices a pregnant woman makes may affect the mental health of her fetus. For example, fetal alcohol syndrome (FAS) affects about one in 600 children in the United States. Fetal alcohol syndrome is caused by excessive drinking by the mother during pregnancy and can lead to intellectual disability in the fetus. Drug abuse and smoking during pregnancy may also cause intellectual disability in the fetus.

Infections in the mother may lead to intellectual disability of the fetus. The infections may spread to the fetus and damage its nervous system, including its brain. High blood pressure and blood poisoning in a pregnant woman may also cause brain damage in a fetus, leading to intellectual disability.

Fetal damage may occur naturally, for unknown reasons. An example is the problem known as neural tube defect. In this disorder, the fetus's spine does not close normally. Fluids may collect in its brain, producing a condition known as hydrocephalus ("water on the brain"; pronounced hi-dro-SEF-uh-luhs). One possible result of hydrocephalus is intellectual disability.

Childhood Illnesses

Children sometimes experience serious infections. These infections may spread to the brain and cause it to become inflamed and swollen. These changes can damage brain cells and bring about intellectual disability. Childhood injuries can also lead to intellectual disability. A blow to the head or a violent shaking by an adult may cause brain damage and intellectual disability.

Environmental Factors

Children who might otherwise develop normally sometimes become intellectually disabled because of the environment in which they live. Poverty, malnutrition, unhealthy living conditions, and inadequate medical care may all increase a child's risk for intellectual disability. Children who are neglected or abused often do not develop normally. Their native intelligence never gets a chance to express itself, and they become intellectually disabled.

Another important environmental factor is lead poisoning. Young children sometimes eat paint that has flaked off the walls of their home. This paint may contain the element lead. Lead has many harmful effects on growing children, one of which is damage to the brain.


The symptoms of intellectual disability usually appear early in life. Children with the disorder tend to develop more slowly than normal. They may learn to sit up, to walk, to talk, and to perform other simple tasks later than average. Intellectual disability is often accompanied by other symptoms as well. These symptoms include aggression, a tendency toward self-injury, and personality changes. As a child grows older, the best indication of intellectual disability is one or more of the standard intelligence tests.


The first step in diagnosing intellectual disability is a complete physical examination and medical history. Some forms of intellectual disability are caused by treatable illnesses. Hyperthyroidism is an example. Hyperthyroidism is a condition in which the thyroid gland produces too much thyroid hormone. A child's mental disorder may be treated, then, by first treating the medical problem that caused it.

If medical problems are ruled out, the patient may then be given a series of intelligence tests. These tests are designed to determine the child's intelligence quotient. Some tests that are commonly used include the Stanford-Binet Intelligence Scale, the Wechsler Intelligence Scales, and the Kaufmann Assessment Battery for Children.

Final diagnosis may also include a series of interviews between a mental-health professional, the child, and the child's family.


Some forms of intellectual disability can be treated. These are cases that are caused by medical problems, such as hyperthyroidism. In most cases, however, no treatment can change a person's basic intellectual capabilities.

The goal of most treatment programs is to help intellectually disabled individuals develop their intellectual and functional skills to the maximum possible level. The federal government has recognized the importance of this goal. It allows for all intellectually disabled children to have free testing and appropriate education and skills training from ages three to twenty-one.

Many intellectually disabled children now have the opportunity to attend special preschool programs and day schools. These programs and schools teach children basic skills, such as bathing and feeding themselves. They also provide educational programs, extracurricular activities, and social events developed especially for intellectually disabled children.

As intellectually disabled individuals approach adulthood, they may receive training in independent living and job skills. The level of training they receive depends on the degree of intellectual disability. Mildly disabled individuals can often acquire the skills needed to live on their own and hold an outside job. Moderate to profoundly disabled individuals usually require supervised community living.

Treatment may also include family therapy. The purpose of family therapy is to help family members understand the nature of intellectual disability. It also helps them develop skills for dealing with the special needs of a disabled child. Parents may also receive counseling to help them deal with feelings of anger or guilt.


The prognosis for individuals with mild to moderate intellectual disability is usually good. These individuals can often become self-sufficient to some degree. However, they may require some educational, community, social, family, and vocational support.

The outlook is less promising for those with severe to profound intellectual disability. These individuals tend to have a shortened life expectancy due to the medical problems that often accompany serious intellectual disability.


Many forms of intellectual disability can be prevented. Pregnant women can avoid using alcohol, drugs, and tobacco. Infections they develop should be treated promptly. Good nutrition during pregnancy is always an important factor in providing the best possible environment for the fetus.

There are a variety of ways in which young children can be protected from intellectual disability. For example, they should be vaccinated against infections that can lead to intellectual disability. In addition, young children should see a medical professional on a regular basis. In this way, any decline in a child's health that could lead to intellectual disability may be discovered.

Words to Know

Adaptive skills: The ability to carry out a large variety of ordinary tasks required to live successfully in the world.

Fetal alcohol syndrome (FAS): A medical problem that affects a fetus and is caused by excessive drinking by the mother during pregnancy.

Genes: Chemical units found in all cells that carry the instructions telling cells how they are to perform.

Hydrocephalus: Accumulation of fluid in the brain; also known as"water on the brain."

Hyperthyroidism: A condition in which the thyroid gland produces too much thyroid hormone, causing a variety of medical problems.

Intelligence: The ability to learn and understand.

Intelligence quotient (IQ): A numerical measure of a person's intelligence.

Neural tube defect: A medical disorder in which a fetus's spine does not close normally.

Source Citation

Source Citation   

Gale Document Number: GALE|CV2643900087