ABSTRACT
There is a general consensus on the concept of (LD). It requires the presence of three criteria based on the definition derived after extensive consultation in the USA (American Association on Mental Retardation, 1992):
Significant intellectual impairment is usually defined as an intelligence quotient (IQ) more than two standard deviations below the general population mean (originally fixed at 100). This is an IQ below 70 on recognized IQ tests. 2% of the population have an IQ below this level.
