Generally two or more standard deviations below the mean on a standardized measure of intelligence. This would refer to a person with an Intelligence Quotient (IQ) or 70 or lower (see below).
This means that approximately 2.28% of a population would have mental retardation.
Children with mental retardation must have deficits in two or more adaptive behaviors. Adaptive behaviors are those skills and abilities that allow a person to adapt to the demands of their environment. These skills or abilities are referenced to the person's age and cultural or linguistic background and take into account the community in which the person lives. Adaptive behaviors vary as a function of each of these variables. Some examples of adaptive behaviors include, but are not limited to:
Put a little differently, adaptive behaviors are the skills or abilities that help us to function in our everyday lives. The adaptive behavior criteria has been added to prevent children from being identified as having mental retardation simply from the results of IQ tests (Diana and the Larry P. cases) - the "six hour retarded child" syndrome.
This criterion is generally assumed to be between birth and the child's 18th birthday.
Any educational diagnosis of mental retardation must demonstrate that the intellectual impairment and adaptive skills deficits interfere with the child's educational progress.