2.5% of the population has an IQ of less than 70, of which 80% have mild mental retardation with an IQ of less than 70 but greater than 50.
Severe to moderate mental retardation (IQ less than 50) occurs in about 0.5% of the population or for about 12% of those with mental retardation.
Individuals with mild mental retardation show no significant increase in incidence of psychiatric symptoms and syndromes. However, individuals with severe mental retardation (IQ below 35) do show a higher relative incidence of different kinds of abnormal behaviour like hyperkinetic syndromes, autism, stereotyped movements, pica and self-mutilation. About 7% of patients with mental retardation have a severe condition and about 1% of patients with severe conditions have profound mental retardation, ie. IQ below 20%.
2.5 to 3% of children are mildly retarded, and their backwardness usually becomes apparent only on starting school at the age of five. Social factors are as important as biological ones in the aetiology of their retardation.
3 to 4 children per thousand population are severely mentally retarded. Nearly all of these are children who have organic brain disease.
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