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Humans interpret others’ behaviour as intentional and goal-directed, expecting others to take the most energy-efficient path to achieve their goals. Recent studies have shown that these expectations of efficient action provide a perceptual prediction of an ideal efficient trajectory, against which the observed action is evaluated, resulting in a distorted perceptual representation of unexpected inefficient actions. Here we show that these predictions rely on the inferred intentionality of the stimulus. Participants observed an actor reach for an object with a straight or arched trajectory. The actions were made efficient or inefficient by adding or removing an obstructing object. The action disappeared mid-trajectory and participants reported the last seen position of the hand on a touch screen. Replicating previous research, judgments of inefficient actions were biased toward the efficient prediction (straight trajectories upward to avoid the obstruction, arched trajectories downward towards the target) In two further experiments, we removed intentional cues by replacing the hand with a non-agentive ball (Exp 2), and by removing the biological profile of the motion by depicting it move at a constant speed (Exp 3). Perceptual biases were substantially reduced when intentional cues were removed. Predictions of efficient action are at least partially perceptually represented and influence perceptual judgments of others actions, biasing them towards these expectations. These predictions emerge from attributions of intentionality to the observed actor, triggered by the perception of agency and kinematics that follow biological motion profiles.
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