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ABSTRACT Asymmetries of the smiling facial movement were more frequent in deliberate imitations than spontaneous emotional expressions. When asymmetries did occur they were usually stronger on the left side of the face if the smile was deliberate. Asymmetrical emotional expressions, however, were about equally divided between those stronger on the left side of the face and those stronger on the right. Similar findings were obtained for the actions involved in negative emotions, but a small data base made these results tentative.
Ekman et al. (1981) studied this question.