The end-state comfort effect facilitates joint action

Motor experts can accurately predict the future actions of others by observing their movements. This report describes three experiments that investigate such predictions in everyday object manipulations and test whether these predictions facilitate responses to the actions of others. Observing video excerpts showing an actor reaching for a vertically mounted dial, participants in Experiment 1 needed to predict how the actor would rotate it. Their predictions were specific to the direction and extent of the dial rotation and improved proportionate to the length of the video clip shown. Testing whether such predictions facilitate responses, in the subsequent experiments responders had to undo an actor's actions, back-rotating a dial (Exp 2) and a bar (Exp 3). The responders' actions were initiated faster when the actors' movements obeyed the so-called end-state comfort principle than when they did not. Our experiments show that humans exploit the end-state comfort effect to tweak their predictions of the future actions of others. The results moreover suggest that the precision of these predictions is mediated by perceptual learning rather than by motor simulation.