Abstract
A review of the mental retardation literature failed to reveal any empirical studies supporting the assumption that “the retardate having an extremely low I.Q. (below 40) is almost invariably of the physiologically defective type... (while familial retardates... are almost invariably mildly retarded)” (Zigler, 1967, p. 1). Thus, the two-group approach to mental retardation, despite its research and clinical implications, rests on assumption. We report herein on 20,218 institutionalized retardates on whom both IQ and etiologies were available. For institutionalized retardates, the criterion of “invariability” was not met. Since our sample was small relative to the total estimated retarded population, the two-group theory still retains research and practical merit, and suggestions for increasing its heuristic value are provided.
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Penrose, L. S. A clinical and genetic study of 1,280 cases of mental defect (Special Report Series, No. 229). London: Medical Research Council, 1938.
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The authors thank Edward Zigler, Sterling Professor of Psychology, Yale University, for his valuable suggestions. Thanks are also due the superintendents and staff of the participating institutions.
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Cleland, C.C., Case, J. & Manaster, G.J. IQs and etiologies: The two-group approach to mental retardation. Bull. Psychon. Soc. 15, 413–415 (1980). https://doi.org/10.3758/BF03334575
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DOI: https://doi.org/10.3758/BF03334575