1) 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.

2) Subaverage intellectual ability that is equivalent to or less than an IQ of 70, is present from birth or infancy, and is manifested especially by abnormal development, by learning difficulties, and by problems in social adjustment