Historically, no differentiation was made between mental retardation and psychopathological conditions (see, e.g., Kraepelin, 1919). In more recent years, both research and clinical services have often been guided by the belief that mental retardation exists with an etiology, diagnosis, prognosis, and treatment distinct from other forms of mental or behavioral disorders. The two initial questions
... [Show full abstract] must be: (1) Are there diagnostic entities that can be identified as types of mental retardation? (2) Is mental retardation different from other forms of behavioral and emotional disorders? The answer to one question influences the other. Together, they raise a third issue: (3) Are mental retardation and other forms of behavioral and emotional disorder independent or overlapping?