(a) Except as otherwise provided by statute, “intellectual disability” means a significant limitation in intellectual functioning existing concurrently with deficits in adaptive behavior that originated during the developmental period before eighteen years of age.

(b) As used in subsection (a) of this section, “significant limitation in intellectual functioning” means an intelligence quotient more than two standard deviations below the mean as measured by tests of general intellectual functioning that are individualized, standardized and clinically and culturally appropriate to the individual; and “adaptive behavior” means the effectiveness or degree with which an individual meets the standards of personal independence and social responsibility expected for the individual’s age and cultural group as measured by tests that are individualized, standardized and clinically and culturally appropriate to the individual.