Subnormal intellectual functioning which originates during the developmental period. This has multiple potential etiologies, including genetic defects and perinatal insults. Intelligence quotient (IQ) scores are commonly used to determine whether an individual is mentally retarded. IQ scores between 70 and 79 are in the borderline mentally retarded range. Scores below 67 are in the retarded range. (Joynt, Clinical Neurology, 1992, Ch55, p28)