Prevalence and Definitions of Mental Retardation
Individuals classified as mentally retarded represent 10.3% of the students ages 6–21 served under IDEA (U.S. Department of Education, 2002a) or about .9% of the population in general. Although this number includes all individuals with mental retardation served under IDEA, as many as 85% have mild or moderate mental retardation, as opposed to severe disabilities discussed in chapter 4 (Drew & Hardman, 2004).
Although mental retardation is commonly used, other terms are used to describe this condition, including intellectual disability, cognitive disability, mental deficiency, mental subnormality, mentally handicapped, or intellectually challenged. Mental retardation is also referred to as one type of the more general term, developmental disability (Beirne-Smith, Patton, & Kim, 2006).
The definition of the American Association on Mental Retardation (AAMR, 2002) states that:
Mental retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before age 18. (p. 1)
This definition also includes five assumptions to be used in applying the definition: (1) consideration of the context of community, peers, and culture; (2) consideration of cultural and linguistic diversity; (3) consideration of strengths as well as weaknesses; (4) the necessity of developing a profile of needed supports; (5) the expectation that the individual’s functioning will improve over time with appropriate supports (AAMR, 2002; see also Drew & Hardman, 2004).
According to the American Psychological Association (APA), mild mental retardation represents the upper range of functioning within the mental retardation classification with IQ scores between 55 to 70. Scores between 35 and 54 are considered moderate mental retardation, scores of 20 to 34 are severe, and scores below 20 are associated with profound mental retardation (Jacobson & Mulick, 1996).
The American Association on Mental Retardation (AAMR, 2002) does not employ a classification system based on IQ level. Rather the AAMR definition suggested that individuals could be evaluated relative to a system of services and supports. These include support areas (e.g., human development, teaching and education, home and community living), relevant support activities (e.g., individual’s interest, activities and settings for participation), and levels and intensity of supports (intermittent, limited, extensive or pervasive). These levels were intended to replace previous classification systems of mild to severe mental retardation, although these latter terms are still widely used (Berine-Smith, Patton, & Kim, 2006)
Causes of Mental Retardation
The vast majority of causes of mental retardation are unknown, and some speculate that known causes account for only 10% to 15% of the cases of retardation (Beirne-Smith, Patton, & Kim, 2006). The causes of mild mental retardation are more difficult to determine than causes for severe and profound mental retardation. Known causes can be classified into genetic factors, brain factors, and environmental factors.
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