port, this opportunity rests to a substantial degree on the judgment of the examiner. Therefore, the use of uniform, valid procedures and criteria are essential for cases falling into this category, as well as for similar situations with adults.

Issues Associated with Life Span Development

Age-related considerations are crucial in making a diagnosis of mental retardation because key signs and symptoms, as well as appropriate assessment methods, are quite different across the life span. Abilities and socially appropriate expectations change dramatically with development, and these factors can complicate the evaluation process in distinct, age-specific ways. Because adaptive behavior changes and tends to increase with age (e.g., Hundert et al., 1997), most measures of adaptive behavior are structured so that items are presented in a developmental sequence. Declines in adaptive behavior can be associated with advanced aging, as it is in the general population, or at an earlier chronological age in the instance of some disorders, like Down syndrome (Kapell et al., 1998; Zigman et al., 1996), or in the presence of severe to profound mental retardation. Moreover, although there are systematic differences in the typical adaptive performance of people with mild mental retardation and their peers with more severe degrees of mental retardation, there is also overlap in adaptive behavior skills among people assessed with differing degrees of intellectual disability (Janicki & Jacobson, 1982). For example, it is possible for someone with mild mental retardation to have certain adaptive behavioral skills that are less advanced than those of another person with moderate mental retardation. These differences may reflect different experiences, opportunities, and participation in services. The following sections use four stages of maturation to identify and discuss key factors in making the differential diagnosis of mental retardation and determining SSI eligibility.

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