Infants do not use payoff information to infer individual goals in joint-action events

For observers the occurrence of a joint action (JA) is inherently ambiguous with respect to the goals of the individuals involved. People may work together, for instance, because they are motivated to obtain material rewards or to help others. We hypothesized that to solve this interpretive ambiguity observers leverage information about the JA's payoff structure. Specifically, when a JA yields material rewards for a participating agent (as well as their partner), their behavior can be straightforwardly explained as instrumental to the obtainment of these rewards. Conversely, when a JA does not yield material rewards for the agent (but does so for her partner), the unrewarded agent's contribution needs to be accounted for by positing other types of goals, such as assisting her partner in obtaining her rewards. We examined this hypothesis across three looking-time experiments with 12-month-olds: specifically, we tested whether the absence of material rewards for an agent participating in a JA would prompt infants to interpret her participation as prosocially motivated. Consistent with this hypothesis, Experiment 1 showed that, after having been familiarized to two dyadic JA events resulting in one or both agents being rewarded, infants selectively expected the unrewarded agent to act altruistically towards her former JA partner by giving her a resource. Experiments 2–3 examined whether this expectation was driven by the prosocial interpretation of the unrewarded agent's behavior or by changes in the number or distribution of resources between familiarization and test. Contradicting the hypothesis that infants interpreted the agent's behavior as prosocially motivated, in Experiment 2 we observed similar looking times when the unrewarded agent performed a prosocial action (giving) or an antisocial action (taking) towards her partner at test. Further, in a close replication of the original experiment in which the change in the number of familiarized objects occurred in the test event featuring the rewarded agent rather than the unrewarded one (Experiment 3), infants produced a looking-time pattern opposite to the one first obtained. Taken together, these findings suggest that infants encoded the payoff structure of JA events (i.e., the number/distribution of resources that the interaction brought about) but did not leverage this information to infer the individual goals of participating agents. The present evidence calls for a critical re-evaluation of our original hypothesis and for further research into the mechanisms by which infants disambiguate the motives of agents involved in joint actions.