Factors that cause intellectual disability can be genetic, developmental, environmental, or a combination. Genetic causes include chromosomal and inherited conditions. Developmental and environmental factors include prenatal exposure to infections and toxins. Environmental or acquired factors include prenatal trauma (e.g., prematurity) and sociocultural factors. Among chromosomal disorders, Down syndrome and fragile X syndrome are the most common disorders that usually produce at least mild intellectual disability.
A diagnosis of “unspecified intellectual disability” is given to children over 5 years of age when it is either too difficult or impossible to assess the degree of intellectual disability through locally available procedures because of sensory or physical impairments, such as blindness or deafness, locomotor disability, severe behavior problems, or a co-occurring mental disorder. This diagnosis is only used in exceptional circumstances and requires reassessment after a period of time.
According to the American Association on Intellectual and Developmental Disabilities, a child or adolescent with an intellectual disability needs a degree of “environmental support” to develop a specific set of adaptive behaviors. This includes support for succeeding in the areas of communication, self-care, home living, social or interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.