Intellectual Disability

Adevelopmental disability that first appears in children under the age of 18. It is defined as a level of intellectual functioning (as measured by standard intelligence tests) that is well below average and results in significant limitations in daily living skills (adaptive functioning).

Intellectual disability (IDD) is not a mental illness in the usual sense but rather a disorder defined in order to identify groups of people who need social support and special educational services to carry out tasks of everyday living. On October 5, 2010, “Rosa's Law” went into effect in the United States. Named for a child with IDD living in Maryland, the law requires federal agencies to remove the terms mentally retarded or mental retardation from official documents and replace them with “individual with an intellectual disability” and “intellectual disability.”

The prevalence of intellectual disability in North America is a subject of heated debate. It is thought to be between 1% and 3% depending upon the population, methods of assessment, and criteria of assessment that are used. Many people believe that the actual prevalence is probably closer to 1%, and that the 3% figure is based on misleading mortality rates; cases that are diagnosed in early infancy; and the instability of the diagnosis across the age span. If the 1% figure is accepted, however, it means that about 3 million people with intellectual disability live in the United States. The three most common causes of intellectual disability, accounting for about 30% of cases, are Down syndrome, fragile X syndrome, and fetal alcohol syndrome. Males are more likely than females to have an intellectual disability in a 1.5:1 ratio, primarily because of the association with fragile X.

Intellectual developmental disorder begins in childhood or adolescence before the age of 18. In most cases, it persists throughout adult life. A diagnosis of IDD is made if an individual has an intellectual functioning level well below average, as well as significant limitations in two or more adaptive skill areas. Intellectual functioning level is defined by standardized tests that measure the ability to reason in terms of mental age (intelligence quotient or IQ). Intellectual disability is generally defined as an IQ score below 70–75. Adaptive skills is a term that refers to skills needed for daily life. Such skills include the ability to produce and understand language (communication); practical skills needed for activities of daily living (ADLs); use of community resources; health, safety, leisure, self-care, and social skills; self-direction; functional academic skills (reading, writing, and arithmetic); and job-related skills.

In general, children with intellectual and developmental impairments reach such developmental milestones as walking and talking much later than children in the general population. Symptoms of intellectual disability may appear at birth or later in childhood. The child's age at onset depends on the suspected cause of the disability. Some people with a mild intellectual disability might not be diagnosed before entering pre-school or kindergarten. These children typically have difficulties with social, communication, and functional academic skills. Children who have a neurological disorder or illness such as encephalitis or meningitis may suddenly show signs of cognitive impairment and adaptive difficulties.

The cause of IDD cannot be identified in about 40% of cases. The following sections discuss known biological and environmental factors that can cause IDD.

Hereditary factors are the most common single cause of IDD and are involved in about 30% of cases. The disorder may be caused by an inherited genetic abnormality, such as fragile X syndrome. Fragile X, a defect in the X chromosome in which a repeated group of letters in the DNA sequence reaches a certain threshold number that results in impairment, is the most common inherited cause of IDD. Such single-gene disorders as phenylketonuria (PKU) and other inborn errors of metabolism may also cause IDD if they are not discovered and treated early, although testing of infants for PKU is required at birth, and problems associated with this disorder can be avoided through dietary measures. Abnormalities in chromosome number can also be the cause of IDD. The presence of an extra chromosome 18 (trisomy 18) or chromosome 21 (trisomy 21 or Down syndrome) will result in some level of intellectual disability. In addition, there may be only a partial extra chromosome as a result of accidents at the cellular level, which sometimes results in milder forms of IDD compared to complete trisomies.

Fetal alcohol syndrome (FAS) affects one in 3,000 children in Western countries. It is caused when mothers drink heavily during the first twelve weeks (trimester) of pregnancy. Some studies have shown that even moderate alcohol use during pregnancy may cause learning disabilities in children. Drug abuse and cigarette smoking during pregnancy have also been linked to intellectual disability.

Maternal infections and such illnesses as glandular disorders, rubella, toxoplasmosis, and cytomegalovirus (CMV) infection can result in IDD in the child, among many other problems, if the developing fetus is exposed. When the mother has high blood pressure (hypertension) or develops toxemia (also called pregnancy-induced hypertension or preeclampsia) during pregnancy, the flow of oxygen to the fetus may in some cases be reduced, potentially resulting in brain damage and IDD.

Birth defects that cause physical deformities of the head, brain, and central nervous system frequently cause IDD. A neural tube defect, for example, is a birth defect in which the neural tube that forms the spinal cord does not close completely. This defect, which occurs with varying levels of severity, may cause children to develop an accumulation of cerebrospinal fluid inside the skull (hydrocephalus). The pressure on the brain resulting from hydrocephalus can lead to changes that cause learning impairment.

Hyperthyroidism, whooping cough, chicken pox, measles, and Hib disease (a bacterial infection caused by Haemophilus influenzae type B) may cause intellectual disability if they are not treated adequately. An infection of the membrane covering the brain (meningitis) or an inflammation of the brain itself (encephalitis) can cause swelling that in turn may cause brain damage and IDD. Traumatic brain injury caused by a blow to the head or by violent shaking of the upper body (shaken baby syndrome) may also cause brain damage and IDD in children.

Neglected infants who are not provided with the mental and physical stimulation required for normal development may suffer irreversible learning impairment. Children who live in poverty and/or suffer from malnutrition, unhealthy living conditions, abuse, and improper or inadequate medical care are at higher risk. Exposure to lead or mercury can also cause IDD. Many children have developed lead poisoning from eating the flaking lead-based paint often found in older buildings.

Low IQ scores and limitations in adaptive skills are the hallmarks of IDD, as is failure to reach normal developmental milestones. Aggression, selfinjury, and mood disorders are sometimes associated with the disability. The severity of the symptoms and the age at which they first appear depend on the cause. Children with IDD reach developmental milestones significantly later than expected, if at all. If the disorder is caused by chromosomal or other genetic disorders, it is often apparent from infancy. If IDD is caused by childhood illnesses or injuries, learning and adaptive skills that were once easy may suddenly become difficult or impossible to master.

If IDD is suspected, a comprehensive physical examination and medical history should be done immediately to discover any organic cause of symptoms. Such conditions as hyperthyroidism and PKU are treatable if discovered early enough when the progression of IDD can be stopped and, in some cases, partially reversed. If a neurological cause, such as brain injury or epilepsy, is suspected, the child may be referred to a neurologist or neuropsychologist for testing.

A complete medical, family, social, and educational history is compiled from existing medical and school records (if applicable) and from interviews with parents.

In many cases of mild IDD, the disorder is not identified until the child starts school. Schoolchildren are given intelligence tests to measure their learning abilities and intellectual functioning. Such tests include the Stanford-Binet Intelligence Scale, the Wechsler Intelligence Scales, the Wechsler Preschool and Primary Scale of Intelligence, and the Kaufman Assessment Battery for Children. For infants, the Bayley Scales of Infant Development may be used to assess motor, language, and problem-solving skills. Interviews with parents or other caregivers are used to assess the child's daily living, muscle control, communication, and social skills. The Woodcock-Johnson Scales of Independent Behavior and the Vineland Adaptive Behavior Scales (VABS) are frequently used to evaluate these skills.


Activities of daily living (ADLs)—
A general term for routine activities of self-care (e.g., brushing teeth, dressing oneself, bathing) and functioning independently (e.g., managing money, preparing meals, cleaning house)
Adaptive functioning—
A term used to describe a person's having the intellectual, social, and practical skills needed to live independently.
The branch of genetics concerned with the structure and function of the individual cell, particularly its chromosome content.
Any personal attribute that causes a person to be socially shamed, avoided, or discredited. Mental disorders are a common cause of stigma.

Recent advances in cytogenetics and microscopy have made the detection of chromosomal abnormalities after birth much more accurate. To investigate the possibility of a genetic disorder, the doctor will obtain a sample of cells from the child's blood or skin. The cells are cultured in the laboratory and prepared for examination under a microscope by one of several purification, preservation, and staining techniques. Analysis of the chromosomes in the cell must be done by a board-certified specialist in cytogenetics.

In addition, amniocentesis (often called amnio) can be performed on fetal cells found in the amniotic fluid in the early second trimester of pregnancy. A sample of the amniotic fluid that surrounds a fetus in the womb is collected through a pregnant woman's abdomen using a needle and syringe. Tests performed on fetal cells found in the amniotic fluid can reveal the presence of Down syndrome as well as many types of genetic disorders. Amniocentesis is recommended for women who will be older than 35 on their due date.

Federal legislation entitles children with intellectual impairments and developmental disabilities to free testing and appropriate, individualized education and skills training within the school system from ages 3 to 21. For children under the age of three, many states have established early intervention programs that assess children, make recommendations, and begin treatment programs. Many day schools are available to help train children with developmental and intellectual impairments in skills such as bathing and feeding themselves. Extracurricular activities and social programs are also important in helping children and adolescents who have developmental and intellectual impairments gain self-esteem. Special interventions (usually some form of behavioral therapy) may be needed for children with IDD who have problems with aggression.

Training in independent living and job skills is often begun in late adolescence or early adulthood. The level of training depends on the degree of impairment. People with mild levels of functional and intellectual impairment can often learn the skills needed to live independently and hold outside jobs. People with a great level of impairment may require supervised community living in group homes or other residential settings.

Family therapy can help relatives of people with intellectual disabilities develop coping skills. It can also help parents deal with feelings of guilt or anger. A supportive and warm home environment is essential to help people with intellectual disabilities reach their full potential.

People with mild to moderate intellectual disability are frequently able to achieve some self-sufficiency and to lead happy and fulfilling lives. To reach these goals, they need appropriate and consistent educational, community, social, family, and vocational supports. The outlook is less promising for those with severe to profound disability. Studies have shown that these people have a shortened life expectancy. The diseases that are usually associated with severe IDD may cause a shorter life span. People with Down syndrome will develop the brain changes that characterize Alzheimer's disease in later life and may develop the clinical symptoms of this disease as well. One additional complication in assessing the future health status and life expectancy of children with IDD is their unequal access to high-quality health care.

Immunization against diseases such as measles and Hib prevents many of the illnesses that can cause IDD. In addition, all children should undergo routine developmental screening as part of their pediatric care. Screening is particularly critical for those children who may be neglected or undernourished or may live in disease-producing conditions. Newborn screening and immediate treatment for PKU and hyperthyroidism can usually catch these disorders early enough to prevent adverse intellectual and developmental effects.

Good prenatal care can also be preventive. Pregnant women should be educated about the risks of alcohol and drug consumption and the need to maintain good nutrition during pregnancy. Tests such as an ultrasonography can determine whether a fetus is developing normally.



American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 4th ed., text rev. Washington, DC: American Psychiatric Press, 2000.

Burack, Jacob A., et al., eds. The Oxford Handbook of Intellectual Disability and Development. 2nd ed. New York: Oxford University Press, 2011.

Knight, Samantha J.L., ed. Genetics of Mental Retardation: An Overview Encompassing Learning Disability and Intellectual Disability. New York: Karger, 2010.

Schalock, Robert L., James F. Gardner, and Valerie J. Bradley. Quality of Life for People with Intellectual and Other Developmental Disabilities: Applications across Individuals, Organizations, Communities, and Systems. Washington, DC: American Association on Intellectual and Developmental Disabilities, 2007.


Brosnan, J., and O. Healy. “A Review of Behavioral Interventions for the Treatment of Aggression in Individuals with Developmental Disabilities.” Research in Developmental Disabilities 32 (March-April 2011): 437–446.

Gray-Stanley, J.A., and N. Muramatsu. “Work Stress, Burnout, and Social and Personal Resources among Direct Care Workers.” Research in Developmental Disabilities 32 (May-June 2011): 1065–1074.

McLaughlin, M.R. “Speech and Language Delay in Children.” American Family Physician 83 (May 15, 2011): 1183–1188.

Prince, E., and H. Ring. “Causes of Learning Disability and Epilepsy: A Review.” Current Opinion in Neurology 24 (April 2011): 154–158.

Schalock, R.L., et al. “Evidence-based Practices in the Field of Intellectual and Developmental Disabilities: An International Consensus Approach.” Evaluation and Program Planning 34 (August 2011): 273–282.

Schieve, L.A., et al. “Risk for Cognitive Deficit in a Population-based Sample of U.S. Children with Autism Spectrum Disorders: Variation by Perinatal Health Factors.” Disability and Health Journal 3 (July 2010): 202–212.

Shaffer, L.G., and B.A. Bejjani. “Development of New Postnatal Diagnostic Methods for Chromosome Disorders.” Seminars in Fetal and Neonatal Medicine 16 (April 2011): 114–118.

Ward, R.L., et al. “Uncovering Health Care Inequalities among Adults with Intellectual and Developmental Disabilities.” Health and Social Work 35 (November 2010): 280–290.


American Association on Intellectual and Developmental Disabilities (AAIDD). “Definition of Intellectual Disability.” (accessed May 18, 2011).

American Psychiatric Association. DSM-5 Development. “A 00 Intellectual Developmental Disorder.” (accessed May 18, 2011).

Medscape. “Pediatric Mental Retardation.” (accessed May 18, 2011).

National Dissemination Center for Children with Disabilities (NICHCY). NICHCY Disability Fact Sheet #8. “Intellectual Disability.” (accessed May 18, 2011).


American Association on Intellectual and Developmental Disabilities (AAIDD), 501 3rd St., NW, Ste. 200, Washington, DC, United States, 20001, (202) 387-1968, Fax: (202) 387-2193, (800) 424-3688, .

The Arc, 1660 L St. NW, Ste. 301, Washington, DC, United States, 20036, (202) 534-3700, Fax: (202) 534-3731, (800) 433-5255,, .

National Dissemination Center for Children with Disabilities (NICHCY) (formerly National Information Center for Children and Youth and Disabilities), 1825 Connecticut Ave. NW, Ste. 700, Washington, DC, United States, 20009, (202) 884-8200, Fax: (202) 884-8441, (800) 6950285,, .

President's Committee for People with Intellectual Disabilities (AAIDD), Aerospace Center, Ste. 210, 370 L'Enfant Promenade SW, Washington, DC, United States, 20447, (202) 619-0634, Fax: (202) 205-9519, (800) 424-3688, .