Efficiency as a principle for social preferences in infancy.

Two separate research lines have shown that (1) infants expect agents to move efficiently toward goal states and that (2) infants navigate the social world selectively, preferring some individuals to others and attributing social preferences to others' agents. Here, we studied how the expectation of efficient actions influences infants' looking preferences and their inferences about others' preferences. We presented 15-month-olds with a set of videos containing three geometric figures depicting social agents. One of them (observer) watched how the other two agents acted to obtain a reward. Critically, the efficiency of their actions was manipulated. One agent reached the reward taking a direct efficient path (efficient agent), whereas the other agent took a curvilinear inefficient path (inefficient agent). At test, the observer approached each of them in two separate trials. Infants looked longer at the screen when the observer approached the inefficient agent rather than the efficient agent. In addition, infants showed a bias to track the actions of the efficient agent when efficient and inefficient agents acted simultaneously. In a second experiment, we rejected the possibility that infants' expectations in Experiment 1 resulted from differences in the movement repertoire of the agents. The results suggest that the principle of efficiency tunes infants' attention towards agents who previously acted efficiently rather than inefficiently and it guides infants' expectations in third-party scenarios-infants are surprised when a third agent approaches an agent who in the past acted inefficiently.