Below-average intellectual abilities that are present before the age of 18 and interfere with developmental processes and with the ability to function normally in daily life (adaptive behavior).

The term mental retardation is commonly used to refer to people with an intelligence quotient (IQ) below 70. An IQ of 80-130 is considered the normal range, and 100 is considered average. According to the definition in the American Psychiatric Association's Diagnostic and Statistical Manual (DSM-IV), a mentally retarded person is significantly limited in at least two of the following areas: self-care, communication, home living, social/inter-personal skills, self-direction, use of community resources, functional academic skills, work, leisure, health, and safety. Mental retardation affects roughly 1% of the American population. According to the U.S. Department of Education, about 11% of school-aged children were enrolled in special education programs for students with mental retardation.

There are four categories of mental retardation: mild, moderate...