Understanding Mental Retardation

by CDC

Mental retardation is characterized both by a significantly below-average score on a test of mental ability or intelligence and by limitations in the ability to function in areas of daily life, such as communication, self-care, and getting along in social situations and school activities. Mental retardation is sometimes referred to as a cognitive or intellectual disability.

Children with mental retardation can and do learn new skills, but they develop more slowly than children with average intelligence and adaptive skills. There are different degrees of mental retardation, ranging from mild to profound. A person's level of mental retardation can be defined by their intelligence quotient (IQ), or by the types and amount of support they need.

People with mental retardation may have other disabilities as well. Examples of these coexisting conditions include cerebral palsy, seizure disorders, vision impairment, hearing loss, and attention-deficit/hyperactivity disorder (ADHD). Children with severe mental retardation are more likely to have additional disabilities than are children with mild mental retardation.

How common is mental retardation?

Mental retardation is the most common developmental disorder. To learn just how common it is, CDC is tracking the number of children with mental retardation in a five-county area in metropolitan Atlanta (Georgia). This activity is part of the Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP). In 1991-1994, we found that, on average, about 1% of children ages 3-10 years had mental retardation. We also found that mental retardation was more common in older children (ages 6-10 years) than in younger children (ages 3-5 years). Mental retardation was also more common in boys than in girls, and more common in black children than in white children.

CDC also studied how many children in metropolitan Atlanta had mental retardation in the mid-1980s. This project was done as part of the Metropolitan Atlanta Developmental Disabilities Study (MADDS), which studied how common certain disabilities were in 10-year-old children. We found that 12 of every 1,000 10-year-old children had mental retardation. Mild mental retardation was 3 times more common than severe mental retardation. As in MADDSP, mental retardation was more common in boys than in girls, and more common in black children than in white children.

In another study, CDC used data from the U.S. Department of Education and the Social Security Administration to find the number of people with mental retardation in the United States in 1993. The study showed that about 1.5 million children and adults (ages 6-64 years) had mental retardation. The highest rate of mental retardation was found in West Virginia and the lowest rate was found in Alaska.

What causes mental retardation? Can it be prevented?

Mental retardation can start anytime before a child reaches the age of 18 years. It can be caused by injury, disease, or a brain abnormality. These causes can happen before a child is born or during childhood. For many children, the cause of their mental retardation is not known. Some of the most common known causes of mental retardation are Down syndrome, fetal alcohol syndrome, and fragile X syndrome, all of which occur before birth. Other causes that take place before a child is born include genetic conditions (such as Cri-du-chat syndrome or Prader-Willi syndrome), infections (such as congenital cytomegalovirus), or birth defects that affect the brain (such as hydrocephalus or cortical atrophy). Other causes of mental retardation (such as asphyxia) happen while a baby is being born or soon after birth. Still other causes of mental retardation do not happen until a child is older. These may include serious head injury, stroke, or certain infections such as meningitis.

If you would like to learn more about a specific genetic condition that you think could cause mental retardation, you can go to the National Library of Medicine's Genetics Home Reference Web site. Information on each genetic condition includes symptoms, how common it is, related genes, treatments, and links to resources where you can learn more about the condition. The Genetics Home Reference also can help you learn more about genetics, including genetic testing, genetic counseling, and gene therapy.

Right now, we do not know how to prevent most conditions that cause mental retardation. However, there are some causes that can be prevented. Fetal alcohol syndrome (FAS) is one such cause. A woman can prevent FAS by not drinking when she is pregnant. CDC funds several projects to study how common FAS is, how to encourage women not to drink during pregnancy, and how to help people with FAS and their families.

Some metabolic conditions, such as phenylketonuria (PKU), galactosemia, and congenital hypothyroidism, can cause mental retardation and other problems if babies with these conditions do not begin treatment soon after birth. Parents and doctors can find out if a child has one of these conditions through a simple blood test or heel prick. Newborns in the United States are tested soon after birth, but different states test for different conditions. Parents can request that their baby be tested for all conditions that have tests. Children that do have these conditions are usually treated with medicine or put on a special diet. If the correct treatment is started soon enough after the child is born and continues as long as needed, the child will not have mental retardation.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.