An individual is considered to have an intellectual disability based on three criteria:
Adaptive skill areas are those daily living skills needed to live work and play in the community. The definition includes ten adaptive skills:
Communication Social skills Self-direction
Self-care Leisure Functional academics
Home living Health and safety Community use
Adaptive skills are assessed in the person's typical environment across all aspects of an individual's life. A person with limits in intellectual functioning (IQ) who does not have limits in adaptive skill areas may not be diagnosed as having an intellectual disability.
Various studies have been conducted in local communities to determine the prevalence of people with intellectual disability. The Arc reviewed many of these prevalence studies in the early 1980s and concluded that 2.5 to 3 percent of the general population has intellectual disability (The Arc, 1982.) A 1993 review of prevalence studies generally confirms this distribution (Fryers, 1993.)
Based on the 1990 census, an estimated 6.2 to 7.5 million people have intellectual disability. Intellectual Disability is 12 times more common than cerebral palsy and 30 times more prevalent than neural tube defects such as spina bifida. It affects 100 times as many people as total blindness (Batshaw & Perret, 1992).
Intellectual disability cuts across the lines of racial, ethnic, educational, social and economic background. It can occur in any family. One in ten American families is directly affected by intellectual disability.
The effects of intellectual disability vary considerably among people, just as the range of abilities varies considerably among people who do not have this disability. About 87 percent of individuals will be mildly affected and will be only a little slower than average in learning new information and skills. As children, disability is not readily apparent and may not be identified until school age. As adults, many people will be able to lead independent lives in the community and will no longer be viewed as having an intellectual disability.
The remaining 13 percent of people with intellectual disability, those with IQs under 50, may have serious limitations in functioning. However, with early intervention, a functional education and appropriate supports as an adult, all individuals can lead satisfying lives in the community.
AAIDD’s definition no longer labels individuals according to the categories of mild, moderate, severe and profound intellectual disability based on IQ level. Instead, it looks at the intensity and pattern of changing supports needed by an individual over a lifetime.
The AAIDD process for diagnosing and classifying a person as having intellectual disability contains three steps and describes the system of supports a person needs to overcome limits in adaptive skills.
The first step in diagnosis is to have a qualified person give one or more standardized intelligence tests and a standardized adaptive skills test, on an individual basis.
The second step is to describe the person's strengths and weaknesses across four dimensions. These four dimensions are:
The term mental age is used in intelligence testing. It means that the individual received the same number of correct responses on a standardized IQ test as the average person of that age in the sample population.
Saying that an older person with intellectual disability is like a person of a younger age or has the "mind" or "understanding " of a younger person is incorrect usage of the term. The mental age only refers to the intelligence test score. It does not describe the level and nature of the person's experience and functioning in aspects of community life.
Intellectual disability can be caused by any condition which impairs development of the brain before birth, during birth or in the childhood years. Several hundred causes have been discovered, but in about one-third of the people affected, the cause remains unknown. The three major known causes of intellectual disability are Down syndrome, fetal alcohol syndrome and fragile X.
The causes can be categorized as follows:
Genetic conditions - These result from abnormality of genes inherited from parents, errors when genes combine, or from other disorders of the genes caused during pregnancy by infections, overexposure to x-rays and other factors. Inborn errors of metabolism which may produce intellectual disability, such as PKU (phenylketonuria), fall in this category. Chromosomal abnormalities have likewise been related to some forms of intellectual disability, such as Down syndrome and fragile X syndrome.
Problems during pregnancy - Use of alcohol or drugs by the pregnant mother can cause disabilities. Malnutrition, rubella, glandular disorders and diabetes, cytomegalovirus, and many other illnesses of the mother during pregnancy may result in a child being born with intellectual disability. Physical malformations of the brain and HIV infection originating in prenatal life may also result in intellectual disability.
Problems at birth -Although any birth condition of unusual stress may injure the infant's brain, prematurity and low birth weight predict serious problems more often than any other conditions.
Problems after birth - Childhood diseases such as whooping cough, chicken pox, measles, and Hib disease which may lead to meningitis and encephalitis can damage the brain, as can accidents such as a blow to the head or near drowning. Substances such as lead and mercury can cause irreparable damage to the brain and nervous system.
Poverty and cultural deprivation - Children in poor families may become disabled because of malnutrition, disease-producing conditions, inadequate medical care and environmental health hazards. Also, children in disadvantaged areas may be deprived of many common cultural and day-to-day experiences provided to other youngsters. Research suggests that such under-stimulation can result in irreversible damage and can serve as a cause of intellectual and related developmental disabilities.
During the past 30 years, significant advances in research have prevented many cases of intellectual disability. For example, every year in the United States, we prevent:
New attempts at treatment of a variety of causes are being developed. There are now unproved ways to manage head trauma, asphyxia (lack of oxygen) and infectious diseases to reduce their adverse effects on the brain. Early intervention programs with high-risk infants and children have shown remarkable results in reducing the predicted incidence of subnormal intellectual functioning.
Finally, early comprehensive prenatal care and preventive measures prior to and during pregnancy increase a woman's chances of preventing intellectual disability.
For more information about the new definition and classification system for intellectual disability, contact AAIDD at:
www.aaidd.org or 1-800-424-3688
for phone numbers of the authors who are available to answer specific questions.
Staff at the national headquarters of The Arc can also help you with a variety of other topics related to intellectual and related developmental disabilities.
Contact information: firstname.lastname@example.org or (800) 433-5255
Or, contact your local chapter of The Arc, The Arc New London County (CT) at:
www.thearcnlc.org or (860) 889-4435