Intellectual disability is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills. This disability originates before the age of 18.

Intellectual functioning - also called intelligence - refers to general mental capacity, such as learning, reasoning, problem solving, and so on.

One criterion to measure intellectual functioning is an IQ test. Generally, an IQ test score of around 70 or as high as 75 indicates a limitation in intellectual functioning.

Standardized tests can also determine limitations in adaptive behavior, which comprises three skill types:

1. Conceptual skills - language and literacy; money, time, and number concepts; and self-direction.

2. Social skills - interpersonal skills, social responsibility, self-esteem, gullibility, naivete (i.e., wariness), social problem solving, and the ability to follow rules/obey laws and to avoid being victimized.

3. Practical skills - activities of daily living (personal care), occupational skills, healthcare, travel/transportation, schedules/routines, safety, use of money, use of the telephone.

On the basis of such many-sided evaluations, professionals can determine whether an individual has an intellectual disability and can tailor a support plan for each individual.

But in defining and assessing intellectual disability, professionals must take additional factors into account, such as the community environment typical of the individual's peers and culture. Professionals should also consider linguistic diversity and cultural differences in the way people communicate, move, and behave.

Finally, assessments must also assume that limitations in individuals often coexist with strengths, and that a person's level of life functioning will improve if appropriate personalized supports are provided over a sustained period.

-American Association on Intellectual and Developmental Disabilities, August 2009