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This study examined the facial affect expressions of autistic, mentally retarded and normal children. Affect was coded using the Maximally Discriminative Movement Coding System. Results indicated that the autistic children were more flat/neutral in their affect expression than the mentally retarded children. Moreover, they displayed a variety of ambiguous expressions not displayed by any of the other children. This unique pattern may be related to the difficulties that autistic children have in sharing affect, and to the difficulties that others experience in reading their affective signals.
Yirmiya et al. (Fri,) studied this question.