Intellectual disability 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. Intellectual disability is sometimes referred to as a cognitive disability or mental retardation.
Children with intellectual disability can and do learn new skills, but they develop more slowly than children with average intelligence and adaptive skills. There are different degrees of Intellectual disability, ranging from mild to profound. A person's level of Intellectual disability can be defined by their intelligence quotient (IQ), or by the types and amount of support they need.
People with intellectual disability 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 intellectual disability are more likely to have additional disabilities than are children with mild Intellectual disability.
How common is intellectual disability?
Intellectual disability is the most common developmental disorder. To learn just how common it is, CDC is tracking the number of children with intellectual disability in a five-county area in metropolitan Atlanta (Georgia). This activity is part of the Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP). For the purposes of tracking, MADDSP defines intellectual disabilities in 8-year-old children by the presence of a score of 70 or below on the most recent test of intellectual functioning. In 1996, an estimated 16 per 1,000 8-year-old children in metro Atlanta, or about 1 in 63, had an intellectual disability. In 2000, the prevalence was an estimated 12 per 1,000, or 1 in 83, 8-year-olds. In 1996 and 2000, respectively, 29% and 35% of children with intellectual disabilities also had one or more other developmental disabilities tracked by MADDSP.
CDC also studied how many children in metropolitan Atlanta had intellectual disability 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 intellectual disability. Mild intellectual disability was 3 times more common than severe intellectual disability. As in MADDSP, intellectual disability 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 intellectual disability in the United States in 1993. The study showed that about 1.5 million children and adults (ages 6-64 years) had intellectual disability. The highest rate of intellectual disability was found in West Virginia and the lowest rate was found in Alaska.
- The Autism and Developmental Disabilities Monitoring (ADDM) Program tracked the number of children with autism spectrum disorders and intellectual disabilities in five sites in 2002 and four sites in 2006.
- Fetal alcohol syndrome surveillance programs track the number of children with FAS in five states.
- CDC works with vaccine programs to prevent infectious diseases that may cause intellectual disabilities.
- Community-based studies are exploring effective ways that parents can help improve developmental outcomes for their children.
What causes intellectual disability? Can it be prevented?
Intellectual disability 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 intellectual disability is not known. Some of the most common known causes of intellectual disability 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 intellectual disability (such as asphyxia) happen while a baby is being born or soon after birth. Still other causes of intellectual disability 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 intellectual disability, 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 intellectual disability. 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 intellectual disability 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 intellectual disability.
It's also important for women with PKU to follow a special diet when they are pregnant. If they do not follow their diets, their babies are very likely to be affected by intellectual disability and other birth defects.
Another cause of intellectual disability that can be prevented is kernicterus, a kind of brain damage that happens when a newborn baby has too much jaundice. In some newborn babies, the liver makes too much yellow pigment called bilirubin. If too much bilirubin builds up in a new baby's body, the skin and whites of the eyes turn yellow. This yellow coloring is called jaundice. A little jaundice is not a problem. It is actually very common in newborn babies and usually goes away by itself. Some babies, however, have too much jaundice. If not treated, these high levels of bilirubin can damage a baby's brain. Kernicterus most often causes cerebral palsy and hearing loss, but in some children it can also cause intellectual disability. Kernicterus can be prevented by using special lights (phototherapy) or other therapies to treat babies.
What is the cost or economic impact associated with intellectual disability?
Many people with [intellectual disability] need long-term services or care. The average lifetime cost for one person with mental retardation is estimated to be $1,014,000 (in 2003 dollars). This represents costs over and above those experienced by a person who does not have a disability.
It is estimated that the lifetime costs for all people with mental retardation who were born in 2000 will total $51.2 billion (in 2003 dollars). These costs include both direct and indirect costs. Direct medical costs, such as doctor visits, prescription drugs, and inpatient hospital stays, make up 14% of these costs. Direct nonmedical expenses, such as home modifications and special education, make up 10% of the costs. Indirect costs, which include the value of lost wages when a person dies early, cannot work, or is limited in the amount or type of work he or she can do, make up 76% of the costs.
These estimates do not include other expenses, such as hospital outpatient visits, emergency department visits, residential care, and family out-of-pocket expenses. The actual economic costs of mental retardation are, therefore, even higher than what is reported here.
Where can I go to learn more about intellectual disability and mental retardation?
National Information Center on Children and Youth with Disabilities (NICHCY)
NICHCY provides information on disabilities and disability-related issues for families, teachers, and other professionals. NICHCY has a fact sheet about mental retardation that includes general information on topics such as diagnosis and causes as well as tips for parents and teachers. NICHCY staff will also give information and referrals over the phone (800-695-0285) or by email (email@example.com). Website: General Information about Mental Retardation
MEDLINEplus: Developmental Disabilities
MEDLINEplus is an online service of the National Library of Medicine. MEDLINEplus is designed to link you to information on specific health topics, including developmental disabilities. Information about mental retardation is included on the Developmental Disabilities page. MEDLINEplus brings together information from many sources and is updated every day. This page includes information on the latest news, general overviews, clinical trials, coping, diagnosis and symptoms, research, specific conditions, law and policy, organizations, children, seniors. Some materials are in Spanish. Website: MEDLINEplus Health Information: Developmental Disabilities
National Center on Birth Defects and Developmental Disabilities (NCBDDD) Publications
Source: National Center on Birth Defects and Developmental Disabilities
NCBDDD staff have written many scientific articles on mental retardation. These articles examine such topics as how common mental retardation is, and factors such as low birth weight or smoking during pregnancy that increase the risk that a child will have mental retardation. You can see a list of these papers (starting in 1990) by using the keyword search on the NCBDDD publications Web page. Choose "mental retardation" in the keyword box on the search page. You can choose whether you want the list to be sorted by author or by date. You can also choose to have the list appear with or without graphics. Click on the Submit button. You will see a list of papers that are about mental retardation. The list will include the complete reference for each paper and a link to an abstract of the paper or to the full text, when available. Website: NCBDDD publications keyword search page
Page last modified or reviewed by AH on January 27, 2010