The Down syndrome adolescent with an IQ of around 50 points does not seem to progress beyond the grammatical level of the normally intelligent child at three years, with short sentences that are restricted in variety and complexity.
According to the Child Development Institute, it has been shown that children with Fragile X who are integrated into "normalized" educational and social environments "achieve above the level that would have been predicted from measured IQ".
Profoundly retarded individuals have IQ scores under 20 to 25.
Low IQ scores and limitations in adaptive skills are the hallmarks of mental retardation.
Their findings pointed to links between social class, race, and intelligence quotient (IQ) scores, despite questions by many about the validity of IQ tests as a measurement of intelligence or a predictor of achievement and success.