WHAT DO WE MEAN BY ‘MENTAL RETARDATION’?
As mentioned previously, mental retardation is a diagnostic category applied to individuals with an IQ below 70 in the presence of other limitations in functional skills, such as communication, self-care and social skills. People with mental retardation make up approximately 3 per cent of the general population. Interestingly, there are more severely mentally retarded individuals (i.e. with IQs under 50) than the normal distribution of IQ strictly ought to allow. There is relatively high heritability of IQ (Bouchard et al., 1990), and it is likely that low-g is the major form of inherited mental retardation (Spitz, 1992). In addition, there are specific clinical etiologies, including Down’s syndrome, Fragile-X, autism and Praeder-Willi syndrome that have little in common other than mental retardation (Simonoff, Bolton & Rutter, 1996). These characteristics support the idea that there are two distinct groups of people with mental retardation – those with known organic etiology and those who represent the low end of the normal distribution of general intelligence (Zigler, 1967, 1969). While there is a clear theoretical value in distinguishing these groups, its importance for predicting everyday behavioural competence is disputed (Burack, Hodapp & Zigler, 1990; Goodman, 1990). To put it another way, IQ is a good predictor of functional abilities no matter how it comes about, but each low-IQ group may require different approaches to education and home care.
As mentioned previously, mental retardation is a diagnostic category applied to individuals with an IQ below 70 in the presence of other limitations in functional skills, such as communication, self-care and social skills. People with mental retardation make up approximately 3 per cent of the general population. Interestingly, there are more severely mentally retarded individuals (i.e. with IQs under 50) than the normal distribution of IQ strictly ought to allow. There is relatively high heritability of IQ (Bouchard et al., 1990), and it is likely that low-g is the major form of inherited mental retardation (Spitz, 1992). In addition, there are specific clinical etiologies, including Down’s syndrome, Fragile-X, autism and Praeder-Willi syndrome that have little in common other than mental retardation (Simonoff, Bolton & Rutter, 1996). These characteristics support the idea that there are two distinct groups of people with mental retardation – those with known organic etiology and those who represent the low end of the normal distribution of general intelligence (Zigler, 1967, 1969). While there is a clear theoretical value in distinguishing these groups, its importance for predicting everyday behavioural competence is disputed (Burack, Hodapp & Zigler, 1990; Goodman, 1990). To put it another way, IQ is a good predictor of functional abilities no matter how it comes about, but each low-IQ group may require different approaches to education and home care.
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