Mental Retardation
From Grolier's The New Book of Knowledge

No two people are exactly alike. We each develop at different rates and have our own unique characteristics. Most of these differences do not limit our ability to deal with the everyday tasks of life. In some cases, however, a person may develop so slowly or may fail to master so many basic skills that parents and professionals decide that the person would benefit from extra help. A decision about what type of challenges the person experiences must be made. This is called a diagnosis. One possible diagnosis is mental retardation.


The diagnosis of mental retardation requires that a person meet three criteria. First, the person must be evaluated using a comprehensive test of intelligence. Such tests provide an IQ score that indicates how the person compares in intelligence to other people of the same age. Most people receive an IQ score near 100, which indicates average intelligence. A diagnosis of mental retardation requires that a person has an IQ of 70 or below. Second, the person must have challenges in adaptive behavior--the ability to succeed at the activities that are typical for most people his or her age, such as taking care of personal hygiene, communicating effectively, or mastering basic school or work tasks. Third, these challenges in intelligence and adaptive behavior must be apparent before the person reaches the age of 18. (A person who has problems that occurred after age 18 would receive a different diagnosis.)

Differences in Severity

Not all people with mental retardation are affected to the same degree or need the same amount of help. For this reason, professionals classify people with mental retardation according to whether they require intermittent (occasional), limited, extensive, or pervasive (constant or total) support. Those who are mildly affected by mental retardation may need help only when they are learning new skills or entering new settings. People with more severe mental retardation may require constant supervision and assistance.

Frequency of Occurrence

The exact number of people who have mental retardation is difficult to estimate, but it is probably between 1 and 2 percent of the population. The occurrence of mental retardation is 11/2 to 2 times more frequent in males than in females. Although children with the most severe forms of mental retardation are typically identified at very young ages, often at birth, many children are not diagnosed until they are between 10 and 14 years old.

Causes of Mental Retardation

Mental retardation has many causes, including genetic problems and environmental factors. Scientists have identified more than 750 genetic problems that cause mental retardation. The most common of these is Down syndrome, which is caused by the presence of an extra chromosome. The chromosomes in our body are made up of genes, which contain the chemical instructions for growth and development. Most people have 46 chromosomes in each of their cells. People with Down syndrome have 47 chromosomes. This gives them a distinctive physical appearance, including a small face and eyes that slant upward. The presence of the extra chromosome can also cause health problems, such as heart defects. Language is a special problem for these individuals, and their speech can be difficult to understand.

Fragile X syndrome is the second most common known genetic cause of mental retardation. In fragile X syndrome, people inherit a gene that fails to make the necessary chemical instructions for typical growth and development. This gene is on the X chromosome. Males have one X chromosome in each of their cells, whereas females have two X chromosomes. As a result, males with fragile X syndrome are more likely than females to receive a diagnosis of mental retardation because males do not have a healthy "backup" copy of the gene. Some females with fragile X syndrome may have mental retardation, while others have learning disabilities and still others are not affected at all.

A third genetic cause of mental retardation is phenylketonuria (PKU). Children with PKU inherit a gene that causes problems with an enzyme that helps the body break down certain foods into the substances it needs to grow and develop. This leads to a buildup of chemicals that poison the body. If left untreated, children with PKU suffer the effects of this gradual poisoning and by middle childhood are diagnosed as having mental retardation.

Environmental factors can also cause mental retardation. Consuming alcohol, smoking, or using illegal drugs during pregnancy can hurt a developing baby. Alcohol use during pregnancy can hurt the fetus by causing fetal alcohol syndrome (FAS). Children with FAS have a distinctive appearance, including a small head, widely spaced eyes, and thin lips. In addition to mental retardation, they can have severe emotional problems. Maternal infection with certain viruses, such as rubella, during early pregnancy can also lead to mental retardation.

Other environmental factors can cause mental retardation after birth. These include exposure to lead and other poisons in the environment. Although lead is no longer allowed to be present in products such as gasoline and paint, it can still be found in the environment, particularly in the paint on homes built before the early 1970's. Children can ingest this lead by eating peeling paint or by swallowing paint dust. Mental retardation can also be caused after birth by head trauma from an accident, by a stroke, and by serious diseases, such as meningitis and encephalitis.

Prevention and Identification

Many cases of mental retardation are preventable. For example, infants can be screened for PKU using a simple blood test. Infants who are found to have PKU can be put on a special diet that prevents the poisonous buildup that causes mental retardation. Good prenatal care, including monitoring of a pregnant woman's diet, can also help prevent mental retardation. Fetal alcohol syndrome can be prevented by avoiding the use of alcohol during pregnancy. Limiting exposure to lead and other environmental poisons and vaccinating children against certain diseases are additional preventive measures. Also, the use of bicycle helmets, car seats, and seat belts can reduce the risk of brain trauma from accidents that can lead to mental retardation.

There are also several tests that can determine if unborn children have genetic problems, such as Down syndrome or fragile X syndrome. Amniocentesis and chorionic villus sampling are two such techniques. Each test involves obtaining cells from the unborn baby and examining the genetic material of the cells.


For much of the 1900's, people with mental retardation were not treated very well. They were often locked away in institutions that did little more than keep them alive. That situation has changed dramatically, due in part to changes in society's attitudes and the passage of some important laws.

Public Law 94-142, the Education for All Handicapped Children Act, guarantees all children with special needs, including mental retardation, between the ages of 3 and 21, the right to an appropriate public education. This law is the basis for inclusion, which means that children with mental retardation or other special needs attend regular education classes for at least part of the school day. Students with mental retardation who attend regular education classes may do so with special assistance in the form of tutors or aides or with changes in the curriculum designed to overcome the students' challenges. Inclusion can have benefits for both students with mental retardation and those without mental retardation, who can learn patience and respect for individual differences.

Public Law 99-457 requires education, health, and social services for infants and toddlers with special needs and help for their parents. These early intervention services help ensure that young children with mental retardation start off on the right foot and that families develop the skills and resources they need to provide healthy, caring, and supportive homes for their children.

Public Law 93-112 helps people with mental retardation and other special needs prepare to finish school and live as productive and independent adults by assisting with job skills and finding employment. It also helps families arrange care for their adult children with mental retardation in their homes or in community-based housing rather than in hospitals and other institutions.

Finding an Identity

Most teenagers and young adults struggle to figure out who they are, what they want to be, and how to be independent from their parents. In short, they search for their identities. People with mental retardation also have a sense of identity and are influenced by the world around them. They can be aware of the challenges that keep them from doing the same things as other people their own age, such as having a driver's license or living independently. As a result, the person with mental retardation may experience frustration and sadness. Fortunately, with the help of parents, teachers, other professionals, and their typically developing peers, many individuals with mental retardation are able to graduate from high school, live away from their parents, have jobs, and start their own families.

Leonard Abbeduto and Melissa M. Pavetto
Department of Educational Psychology
Waisman Center on Mental Retardation and Human Development
University of Wisconsin-Madison

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