Mild mental retardation occurs in about 3 in 100 in the population; moderate, severe, and profound mental retardation occur in 3 to 4 in 1000. Worldwide, about 120 million people are estimated to be affected. During the school years the prevalence of mental retardation within the population will be defined by the number of children with special educational needs in the category learning difficulty or severe learning difficulty.
Although there is now a trend towards mainstreaming children with special needs, and providing extra support, separate special schools for children with moderate and for severe learning difficulties are still provided in many places. At school-leaving age many young people with mild or moderate learning difficulties (roughly equivalent to IQ over 50) will not receive special services; only people who have severe mental retardation, and those with additional disabilities, including epilepsy, autism, mental illness, and/or behavioural problems will be referred to adult specialist services. The administrative prevalence of adults with mental retardation is thus much lower in adulthood as it is a measure of those in contact with services, that is typically 3 to 4 per 1000 of the population. (2) The administrative prevalence rates should not be confused with true prevalence rates, which are far more difficult to assess.
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