CHAPTER 28
Intellectual Disability
Though both DSM-IV and ICD-10 use the termmental retardation,an
increasing number of professionals and the general public dislike this
term and avoid it. By contrast, the termintellectual disabilityis increasingly
widely used, particularly in the USA. In the UK, the termgeneralised
learning disabilityis still commonly used, but may lead to international
misunderstanding, since learning disabilityin the USA often refers to
individuals of normal intelligence who have specific reading or spelling
difficulties. We have opted for the less ambiguousintellectual disability.
Definition
At its simplest, intellectual disability is defined solely by a general deficit
in cognitive function that emerges during childhood. The threshold is
generally operationalised as having an IQ level below 70 on a standardised
IQ test where the population mean is 100 and the standard deviation is
- Thus, an IQ below 70 is more than two standard deviations below
average. Most definitions of intellectual disability also require impaired
social functioning, involving reduced personal independence or a need
for special care or protection. This double requirement for intellectual
and social impairment is found in the ICD-10 and DSM-IV definitions of
mental retardation. The same is generally true for legal and administrative
definitions of intellectual disability, for example, for the definitions of
mental impairment and severe mental impairment in English law. While
most individuals with intellectual disability have educational as well as
intellectual and social difficulties, these educational impairments are not
central to the definition of intellectual disability.
Prevalence
1 Mild intellectual disability, as defined by IQ criteria of 50–69, affects about
2% of the general population, which is about what would be expected
if IQ were normally distributed with a mean of 100 and a standard
Child and Adolescent Psychiatry, Third Edition. Robert Goodman and Stephen Scott.
©c2012 Robert Goodman and Stephen Scott. Published 2012 by John Wiley & Sons, Ltd.
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