What are Intellectual and Developmental Disabilities?

An individual is considered to have intellectual and developmental disabilities based upon the following three criteria: his/her intellectual functioning level as measured by an IQ test is below 70; he/she has significant limitations in two or more adaptive skill areas (which might include communication, self-care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure and work); and the condition is present from childhood (defined as age 18 or earlier). Intellectual and developmental disability is not a disease, nor should it be confused with so-called mental illness. About 85 percent of people with the condition fall within the mild range of disability, whereas the remaining 15 percent have disabilities of varying severity.

The prevalence rate of intellectual and developmental disability in the United States is between 2.5 and 3 percent of the general population, or an estimated 6.2 to 7.5 million people based on the 1990 census. Intellectual and developmental disability is 12 times more common than cerebral palsy and affects 100 times as many people as total blindness. One out of every ten families is directly affected.

There are currently hundreds of known organic and acquired causes of intellectual and developmental disabilities.  For instance, the most common chromosomal cause is Down syndrome, whereas the most commonly known single gene cause is Fragile X syndrome. But these are only two of many different etiologies and make up a small percent of those diagnosed with the condition. In addition to the long list of organic causes, adverse environmental and social factors (which include poverty, lack of prenatal care, drug or alcohol abuse during pregnancy resulting in FAS – fetal alcohol syndrome) are placing greater and greater numbers of individuals at-risk for intellectual and developmental disability. In spite of all that is known, in 50 percent of all cases of intellectual and developmental disability there is no known specific cause.

For more information on services offered for individuals with Developmental Disabilities, please visit the Services & Supports section of this website.