Intellectual disability, formerly known as mental retardation, is a condition that is apparent before the age of 18 and consists of both below-average intellectual functioning and adaptive behavior. There are a number of causes of intellectual disability. Our understanding of the causes of intellectual disability focuses on the types of risk factors (biomedical, social, behavioral, and educational) and the timing of exposure (before, during, or after birth) to those factors.
Intellectual disability is often diagnosed during early childhood or the early school years. To make a diagnosis of intellectual disability, a psychologist or physician will use measures of intellectual functioning (mental capacity for learning, reasoning, problem solving, and so on, often known as an “IQ test”) and adaptive behavior, including skills related to using concepts (such as language, money, or time), social skills, and those practical skills needed to take care of one’s self.
There are many sources of information and support for people with intellectual disability and their families.
Source: American Association on Intellectual and Developmental Disabilities