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Individual components of cognitive disability were assessed in the siblings of 41 autistic probands and compared with similar measures from a control group of siblings of Down's syndrome individuals. The results showed a significant familial clustering of cognitive disabilities in the siblings of autistic probands. These cognitive impairments included disturbances in expressive and receptive language, specific learning disabilities, and varying degrees of mental subnormality.
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August et al. (1981) studied this question.
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