When hands are used to communicate they are less susceptible to illusion than when they are used to estimate
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Description: When we use our hands to estimate the size of sticks in the Müller-Lyer illusion, we are highly susceptible to the illusion. But when we prepare to act on sticks under the same conditions, we are significantly less susceptible to the illusion. Here we ask whether our hands are susceptible to illusion when used, not to act on objects, but to describe them in spontaneous co-speech gestures or in conventional sign languages of the Deaf. Thirty-two English-speakers and 13 ASL-signers used their hands to act on, estimate, and describe sticks eliciting the Müller-Lyer illusion. For both gesture and sign, the magnitude of illusion for description was smaller than the magnitude of illusion for estimation, and not different from the magnitude of illusion for action. The mechanisms responsible for producing these non-codified gestures and codified signs thus appear to operate, not on percepts involved in estimation, but are rather derived from the way we act on objects.