According to the American Association on Intellectual and Developmental Disabilities (AAIDD), an individual is considered to have intellectual disabilities based on the following three criteria: intellectual functioning level (IQ) is below 70-75; significant limitations exist in two or more adaptive skills areas, such as communication, self-care, home living, social skills, leisure, health and safety, self-direction, functional academics (reading, writing, basic math), community participation and employment; and the condition is present from childhood (defined as age 18 or less).
Intellectual disabilities can be caused by any condition that impairs development of the brain before birth, during birth or during 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 disabilities are Down syndrome, fetal alcohol syndrome, and fragile X.
According to the Administration on Developmental Disabilities (ADD), developmental disabilities are severe disabilities attributable to cognitive and/or physical impairments which appear before the age of 22 and are likely to be lifelong. They result in substantial limitations in three or more of the following areas: self-care, comprehension and language skills, learning, mobility, self-direction, capacity for independent living, economic self-sufficiency, and ability to function independently without coordinated services.
How Intellectual and Developmental Disabilities Differ
According to the AAIDD, intellectual disabilities fall under the umbrella of developmental disabilities, but the boundaries often blur as many individuals fall into both categories to differing degrees and for different reasons.
Some developmental disabilities are purely physical, such as congenital deafness or visual impairment resulting from the individual’s mother contracting rubella while pregnant. These are not intellectual disabilities. Other developmental disabilities can be caused by cerebral palsy, epilepsy, autism, or other disabling conditions. These conditions might or might not include intellectual disabilities.
Still other developmental disabilities can result from chromosomal disorders, such as Down syndrome, fetal alcohol syndrome, and fragile X syndrome. These instances could well include intellectual disabilities—but not always. For example, according to the Centers for Disease Control, males with fragile X syndrome generally have mild to severe intellectual disabilities, whereas females can have average intelligence.
On the other hand, some causes of intellectual disabilities are not physical. These include social factors, such as the level of child stimulation and adult responsiveness, and educational factors, such as the availability of family and educational supports that can promote mental development and greater adaptive skills.
According to the President’s Committee for People with Intellectual Disabilities (PCPID), it is estimated that there are seven to eight million Americans of all ages (about 2.5-3% of the U.S. population) who experience Intellectual and developmental disabilities (IDD). IDD affect about one in ten families in the U.S.