Intellectual disability is a condition that may be present from birth or childhood that is marked by significantly lower intellectual functioning than the average for individuals of the same age and by delays in developing social skills, communication skills, and the ability to care for oneself and live independently. Its effects range from mild to profound.
Intellectual disability is characterized by considerable delays in cognitive, social, practical, and abstract learning skills. People with intellectual disabilities may have problems with adaptive skills such as communication, personal care, social skills, community interaction, health and safety, leisure activities, school, and work. Intellectual disabilities are often classified based on practical and cognitive tests, as well as intelligence quotient tests * . Intellectual disability impacts nearly 6.5 million people in the United States.
The majority of children with below-average IQ scores, in the range of 70 to 89, are not considered to have intellectual disability. Those with mild disability account for about 85 percent of all cases of intellectual disabilities. They are identified as functioning two to four years behind their developmental stage or having an IQ range of 50 to 75. Children may go undiagnosed until they are well into their school years. They are often slower to walk, talk, and feed themselves than most other children, but they can learn practical skills, including reading and math, up to the sixth grade level. Mildly disabled adults usually build social and job skills and can live independently.
Children in this group have an IQ range of 35 to 49 and show noticeable delays in developing speech and motor skills * . Although they are unlikely to acquire academic skills such as reading or math, they can learn basic communication skills, some health and safety habits, and other essential skills. As adults, they usually cannot live independently, but they can master some simple tasks and travel alone in familiar places.
People in this category account for 3 to 5 percent of all cases of intellectual disabilities. Severely disabled children have an IQ range of 20 to 34 and are often diagnosed at birth or soon after. These children have difficulty with motor development and communication. With training, they may learn some self-help skills, such as how to feed and bathe themselves. They usually learn to walk and gain a basic understanding of speech as they age. As adults, they may be able to follow daily routines and perform simple tasks, but they need to be directed and live in a protected environment.
People in this category account for 1 to 2 percent of all cases of intellectual disabilities. Children with profound disabilities have an IQ of less than 20 and are usually diagnosed at birth. These children often have additional medical problems and require nursing care. Children who are profoundly disabled need to be continuously supervised. They show delays in all aspects of development. With training, however, they may learn to use their legs, hands, and jaws. Adults who are profoundly disabled usually learn some speech, and they may learn to walk, but they cannot care for themselves and need complete support in daily living.
Intellectual disabilities sometimes have a genetic cause, resulting from one or more chromosomal abnormalities. At other times, though, intellectual disability may be the result of problems during pregnancy or childbirth that affect the development of the fetal brain and central nervous system * . Babies may be born with intellectual disabilities if their mothers do not receive proper nutrition and medical care during pregnancy, if their mothers have infections during pregnancy, or if their mothers used alcohol or drugs or were exposed to environmental toxins * during pregnancy. Many different causes of intellectual disability have been identified, but a specific cause can only be pinpointed in about 25 percent of cases.
Chromosomes are threadlike structures in cells that carry genetic information. Most cells in the human body have 23 pairs of chromosomes. Chromosome pair 23 determines whether a person is female (two X chromosomes) or male (one X chromosome and one Y chromosome). Chromosomes 1 through 22 determine all of the other traits and characteristics. Intellectual disabilities may occur when a baby has an extra chromosome, an abnormal or partially missing chromosome, or a mislocated chromosome. Often, embryos that have been assigned the wrong number of chromosomes do not survive, and the pregnancy results in miscarriage * . Nearly 75 percent of early miscarriages occur due to these chromosomal abnormalities. The chromosomal abnormalities most often linked to intellectual disability are Down syndrome, Fragile X syndrome, Prader-Willi syndrome, and Williams syndrome.
Down syndrome results from an abnormality on chromosome 21. These abnormalities can occur in several ways:
The physical differences that may result from chromosome 21 errors include a flat facial profile, an upward slant to the eyes, a short neck, a single deep crease on the palm, hearing loss, an enlarged tongue, vision problems, thyroid disorders, and heart disease. Chromosome 21 errors have been linked to a mother's age. The incidence of Down syndrome in the children of mothers older than age 45 is 1 in 30, whereas for children of mothers younger than age 30 the incidence is less than 1 in 1,000.
Fragile X syndrome results from an abnormality on the X chromosome. It is the most common cause of inherited intellectual disability, including autism spectrum disorder. The symptoms of Fragile X syndrome tend to be much more severe in boys than girls. Only about one-third of girls with the syndrome have severe intellectual disabilities. About 20 percent of boys with Fragile X are considered to have an autism spectrum disorder. Fragile X can also cause a large head, protruding ears, flat feet, a prominent jaw, hyperextensible (able to extend to a greater than normal degree) joints, ADHD * , anxiety, and seizures * .
Williams syndrome is caused by a slight abnormality on chromosome 7. Children who are born with this syndrome have deep eyes and thin, high ears. As they grow older, they are vulnerable to heart problems and other health complications. Most are hypersensitive to sounds. They tend to have sociable and friendly personalities. They approach strangers without concern. However, they are fearful in certain ordinary situations. They are limited in tasks requiring visual detail or spatial judgment. Their relative strength is in language skills, although most are not at age level in this area. They tend to be especially good at accurate, precise auditory perception, which makes them likely to have a particular appreciation of music. Not all children with Williams syndrome are musicians, but those who are tend to be quite proficient.
Sometimes called “inborn errors of metabolism,” these conditions result from abnormalities in the genes * that govern how the body produces and handles amino acids * , proteins, enzymes, hormones, and nutrients. One metabolic disorder that can cause intellectual disability is phenylketonuria (FEN-il-KEE-toe-NOOR-ee-a) (PKU), which is linked to a lack of the enzyme needed to process the amino acid phenylalanine (FEN-ill-AL-a-neen). Another disorder that causes intellectual disability is hypothyroidism (HY-poe-THY-royd-izum), which is linked to an underdeveloped, underactive, or damaged thyroid gland * . The thyroid gland is needed to produce hormones essential for normal growth and brain development. Many other metabolic disorders can be detected at birth by a blood test and treated through special diets, medications, and hormone therapy.
When a pregnant woman drinks alcohol or takes drugs (legal or illegal), these substances are transmitted directly to the fetus. Intellectual disability that is linked to a mother's drinking is known as fetal alcohol syndrome (FAS). Children with FAS may have skeletal problems and distinctive facial characteristics, including widely spaced eyes, a shortened or flattened nose, and abnormalities in the shape and placement of the ears. In addition, children with FAS may have growth problems, central nervous system disorders, learning disabilities, vision or hearing problems, and behavioral problems.
Serious infections can harm a baby's developing brain before birth or early in life. Viral infections linked to intellectual disability include the cytomegalovirus (SIE-toe-MEG-a-lo-VY-rus) and the rubella virus, which causes German measles encephalitis * and meningitis * (MENin-JY-tis), two infections that result in inflammation of the brain that can also cause intellectual disability.
Other possible causes of intellectual disability include premature or difficult birth, severe head injury, malnutrition, mercury poisoning, and lead poisoning.
Chromosomal abnormalities and metabolic disorders are often diagnosed by doctors during prenatal testing or at birth. In other cases, however, a parent, caregiver, or teacher may be the first to notice that a baby or young child is not demonstrating new skills at the same pace as his or her peers. For example, the child may not crawl, walk, or talk by the expected age. Because parents may not notice milder intellectual disabilities, doctors routinely give parents questionnaires to fill out during the child's checkups. These questionnaires help the doctor to assess the child's developmental skills. Children who are not executing tasks at the same rate as their peers may be referred for more formal testing to determine if any disability is present.
Pediatricians use blood tests, brain scans, genetic testing, and other medical tests to detect underlying physical disorders. Psychologists use developmental tests to determine whether babies and children are developing at a slower pace than their peers, and they use standardized intelligence quotient tests to compare the abilities of school-aged children to those of average children in the same age group. Psychologists may also observe the child at play, in school, and interacting with family members before making a diagnosis of intellectual disability. Because developmental delays are not always linked to intellectual disability and because they may improve with physical treatment or changes in the child's environment, psychologists often schedule repeated evaluations to measure delays and assess improvements over time in intelligence and adaptive behaviors.
Parents who learn that an infant or child has an intellectual disability are often shocked, and they may be overwhelmed by such feelings as sadness, guilt, helplessness, and anger until they adjust to the news. Family counselors and support groups are resources that can help parents learn how to meet the special needs of intellectually disabled children and balance those needs with other family responsibilities, including helping siblings to adjust to the situation. To help children with intellectual disabilities, many families work with a team of specialists that includes psychologists, speech-language pathologists, physical and occupational therapists, social workers, and special educators.
Children with intellectual disabilities face many physical and emotional challenges. They may know that they are different from their peers in ways that they may not understand and feel frustrated, depressed, and anxious. They may develop behavioral problems to express feelings that they cannot verbalize. Children with intellectual disabilities can benefit from treatment and support in learning academic skills and adaptive behaviors needed for everyday living. They may also get a boost in selfesteem by realizing that they, like other children, are unique and valuable individuals.
Adults with severe or profound intellectual disabilities requiring constant supervision often enter nursing homes or other residential facilities that offer skilled 24-hour care. However, the majority of adults with mild to moderate intellectual disabilities can achieve varying degrees of independence. Because they may want or need some support and guidance, many continue to live with family members or in group homes or apartment clusters designed especially for people with special needs. Many are able to succeed in jobs and participate in community events. Some even participate in the Special Olympics. These activities help them develop a greater sense of self-worth. Others are able to get married and start their own families.
See also Attention Deficit Hyperactivity Disorder (ADHD) • Autism Spectrum Disorder • Birth Defects: Overview • Disability • Down Syndrome • Fetal Alcohol Spectrum Disorders (FASD) • Genetic Diseases: Overview • Learning Disabilities: Overview • Microcephaly • Phenylketonuria (PKU) • Testing and Evaluation
Dlouhy, Susan, and Patty Mitchell. Upcycling Sheltered Workshops: A Revolutionary Approach to Transforming Workshops into Creative Spaces. Athens, OH: Swallow, 2015.
Meyer, Don, and Emily Holl. The Sibling Survival Guide: Indispensable Information for Brothers and Sisters of Adults with Disabilities. Bethesda, MD: Woodbine House, 2014.
Wehmeyer, Michael L. The Story of Intellectual Disability: An Evolution of Meaning, Understanding, and Public Perception. Baltimore, MD: Brookes, 2013.
Healthy Place. “Mild, Moderate, Severe Intellectual Disability Differences.” http://www.healthyplace.com/neurodevelopmental-disorders/intellectual-disability/mild-moderate-severe-intellectual-disabilitydifferences (accessed July 20, 2015).
MedlinePlus. “Intellectual Disability.” U.S. National Library of Medicine. http://www.nlm.nih.gov/medlineplus/ency/article/001523.htm (accessed July 20, 2015).
American Association on Intellectual and Developmental Disabilities. 444 N. Capitol St. NW, Suite 846, Washington, DC 20001-1512. Toll-free: 800-424-3688. Website: http://www.aaidd.org (accessed July 20, 2015).
Arc of the United States. 1010 Wayne Ave., Suite 650, Silver Spring, MD 20910. Telephone: 301-565-3842. Website: http://www.thearc.org (accessed July 20, 2015).
Dolan DNA Learning Center. 334 Main St., Cold Spring Harbor, NY 11724. Telephone: 516-367-5170. Website: http://www.ygyh.org (accessed July 20, 2015).
National Dissemination Center for Children with Disabilities. PO Box 1492, Washington, DC 20013. Toll-free: 800-695-0285. Website: http://www.nichcy.org (accessed July 20, 2015).
National Down Syndrome Society. 666 Broadway, 8th Floor, New York, NY 10012. Toll-free: 800-221-4602. Website: http://www.ndss.org (accessed July 20, 2015).
National Fragile X Foundation. PO Box 37, 1615 Bonanza St., Suite 202, Walnut Creek, CA 94597. Toll-free: 800-688-8765. Website: http://www.fragilex.org (accessed July 20, 2015).
Prader-Willi Syndrome Association. 8588 Potter Park Dr., Suite 500, Sarasota, FL 34238. Toll-free: 800-926-4797. Website: http://www.pwsausa.org (accessed July 20, 2015).
Williams Syndrome Association. PO Box 297, Clawson, MI 48017. Toll-free: 800-806-1871. Website: http://www.williams-syndrome.org (accessed July 20, 2015).
* intelligence quotient test, also known as an IQ test, refers to a test designed to estimate a person's intellectual potential.
* motor skills are muscular movements or actions.
* central nervous system (SEN-trul NER-vus SIS-tem) is the part of the nervous system that includes the brain and spinal cord.
* toxins are substances that cause harm to the body.
* miscarriage (MIS-kare-ij) is the end of a pregnancy through the death of the embryo or fetus before birth.
* seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.
* ADHD or Attention Deficit Hyperactivity Disorder is a condition that makes it hard for a person to pay attention, sit still, or think before acting.
* genes (JEENS) are chemical structures composed of deoxyribonucleic acid (DNA) that help determine a person's body structure and physical characteristics. Inherited from a person's parents, genes are contained in the chromosomes found in the body's cells.
* amino acids (a-MEE-no acids) are the chief building blocks of proteins. In humans, certain amino acids are required to sustain life.
* thyroid gland (THY-roid) is located in the lower part of the front of the neck. The thyroid produces hormones that regulate the body's metabolism (me-TABo-LIZ-um), the processes the body uses to produce energy, to grow, and to maintain body tissues.
* anencephaly (AN-en-SEF-uhlee) is a condition present at birth in which most of the brain is missing.
* hydrocephalus (HY-droe-SEFuh-lus) is a condition, sometimes present at birth, in which there is an abnormal buildup of fluid within the skull, leading to enlargement of the skull and pressure on the brain.
* spina bifida (SPY-nuh BIF-ih-duh) is a condition present at birth in which the spinal column is imperfectly closed, leaving part of the spinal cord exposed and often leading to neurological and other problems.
* encephalitis (en-seh-fuh-LYEtis) is an inflammation of the brain, usually caused by a viral infection.
* meningitis (meh-nin-JY-tis) is an inflammation of the meninges, the membranes that surround the brain and the spinal cord. Meningitis is most often caused by infection with a virus or a bacterium.