Truth is in the head. A nod and shake compatibility effect.

Studies from the embodiment perspective on language processing have shown facilitation or interference effects depending on the compatibility between verbal contents, concrete or abstract, and the motion of various parts of the body. The aim of the present study was to test whether such compatibility effects can be found when a higher cognitive process like truth evaluation is accomplished with head movements. Since nodding is a vertical head gesture typically performed with positive and affirmative responses, and shaking is a horizontal head gesture associated with negative and dissenting contents, faster response times can be expected when true information is evaluated by making a vertical head movement and false information by making a horizontal head movement. Three experiments were designed in order to test this motor compatibility effect. In the first experiment a series of very simple sentences were asked to be evaluated as true or false by dragging them vertically and horizontally with the head. It resulted that truth-value was assessed faster when it was compatible with the direction of the head movement, compared to when it was incompatible. In the second experiment participants were asked to evaluate the same sentences as the first experiment but by moving them with the mouse. In the third experiment, a non-evaluative classification task was given, where sentences concerning animals or objects were to be dragged by vertical and horizontal head movements. In the second and third experiment no compatibility effect was observed. Overall results support the hypothesis of an embodiment effect between the abstract processing of truth evaluation and the direction of the two head movements of nodding and shaking. Cultural aspects, cognitive implications, and the limits of these findings are discussed.