Mental retardation is a developmental disability that first appears in children under the age of 18 and, in most cases, persists throughout adulthood. A person is considered mentally retarded if he or she has an intellectual functioning level well below average and significant limitations in two or more adaptive skill areas. Intellectual functioning level is defined by standardized tests that measure the ability to reason in terms of mental age (intelligence quotient or IQ). Mental retardation is defined as IQ score below 70-75. Adaptive skills are the skills needed for daily life and include the ability to produce and understand language (communication); home-living skills; use of community resources; health, safety, leisure, self-care, and social skills; self-direction; functional academic skills (reading, writing, and arithmetic); and work skills. Mental retardation occurs in 2.5-3% of the general population, and about 6-7.5 million mentally retarded individuals live in the United States...