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.
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. [Read more about 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. [Read a summary of the article about intellectual disability in MADDS] [Read more about MADDS]
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. [Read the complete article on intellectual disability in the United States]
- 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. [Go to the Genetics Home Reference Web site]
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. [Read more about CDC's FAS program]
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. [Learn more about newborn screening in your state] [Learn more about the conditions screened for in newborn screening programs]
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. [Read the CDC press release about this topic] [Read the complete article]
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. [Read more about kernicterus]
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Murphy CC, Boyle C, Schendel D, Decouflé P, Yeargin-Allsopp M. Epidemiology of mental retardation in children. Mental Retardation and Developmental Disabilities Research Reviews 1998;4:6-13.
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Yeargin-Allsopp M, Murphy CC, Cordero JF, Decouflé P, Hollowell JG. Reported biomedical causes and associated medical conditions for mental retardation among 10-year-old children, metropolitan Atlanta, 1985 to 1987. Developmental Medicine & Child Neurology 1997;39:142-149. [Read abstract of this paper]
(From: http://www.cdc.gov/ncbddd/dd/mr3.htm#causes )