The naïve utility calculus theory of early social cognitionargues that by relating an agent’s incurred effort to the expectedvalue of a goal state, young children and infants can reasonabout observed behaviors. Here we report a series ofexperiments that tested the scope of such utility-basedreasoning adopted to choice situations in the first year of life.We found that 10-month-olds (1) did not expect an agent toprefer a higher quantity of goal objects, given equal action cost(Experiment 1) and (2) did not expect an agent to prefer a goalitem that can be reached at lower cost, given equal rewards(Experiment 2a and 2b). Our results thus suggest that younginfants’ utility calculus for action understanding may be morelimited than previously thought in situations where an agentfaces a choice between outcome options.

People may engage in a joint activity (JA) to accrue material rewards or to help others. From a third-party perspective, the occurrence of JAs is thus ambiguous about the goals of the participating agents. We argue that the payoff structure of a JA (how costs and rewards are distributed) may help disambiguate these goals. Specifically, we hypothesize that an agent’s participation in a JA should be interpreted as prosocially motivated when its costs cannot be recouped by material rewards (disadvantageous payoff). We tested this hypothesis across three looking-time experiments with 12-month-olds. As predicted, infants expected a disadvantaged agent to behave altruistically towards her JA partner (Exp. 1). However, this expectation might be explained by a sensitivity to changes in overall reward distribution from familiarization to test (Exps. 2 & 3). Our results call for a re-evaluation of the role that payoff information plays in early goal attribution within JA contexts.