When people perform simple actions, they often behave efficiently, minimizing the costs of movement for the expected benefit. The present study addressed the question of whether this efficiency scales up to dyads working together to achieve a shared goal: Do people act efficiently as a group (i.e., coefficiently), or do they minimize their own or their partner’s individual costs even if this increases the overall cost for the group? We devised a novel, touch-screen-based, sequential object-transfer task to measure how people choose between different paths to coordinate with a partner. Across multiple experiments, we found that participants did not simply minimize their own or their partner’s movement costs but made coefficient decisions about paths, which ensured that the aggregate costs of movement for the dyad were minimized. These results suggest that people are able and motivated to make coefficient, collectively rational decisions when acting together.

Successful performance in cooperative activities relies on efficient task distribution between co-actors. Previous research found that people often forgo individual efficiency in favor of co-efficiency (i.e., joint-cost minimization) when planning a joint action. The present study investigated the cost computations underlying co-efficient decisions. We report a series of experiments that tested the hypothesis that people compute the joint costs of a cooperative action sequence by summing the individual action costs of their co-actor and themselves. We independently manipulated the parameters quantifying individual and joint action costs and tested their effects on decision-making by fitting and comparing Bayesian logistic regression models. Our hypothesis was confirmed: people weighed their own and their partner’s costs similarly to estimate the joint action costs as the sum of the two individual parameters. Participants minimized the aggregate cost to ensure co-efficiency. The results provide empirical support for behavioral economics and computational approaches that formalize cooperation as joint utility maximization based on a weighted sum of individual action costs.

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Previous research shows that children evaluate the competence of others based on how effectively someone accomplished a goal, that is, based on the observed outcome of an action (e.g., number of attempts needed). Here, we investigate whether 5-to 10-year-old children and adults infer competence from how efficiently someone solves a task by implementing question-asking strategies of varying expected information gains (EIG). Whereas the efficiency of a strategy defined as EIG is a reliable indicator of competence, the observed effectiveness of actions may depend on unrelated external factors, such as luck. Across two experiments conducted in Germany, we varied how efficiently and how effectively different agents solved a 20-questions game (Experiments 1 and 2) and a maze-exploration game (Experiment 2). In Experiment 1 (N = 121), only adults identified a more efficient agent as more competent, and all participants attributed higher competence to agents needing fewer questions even when they employed the same inefficient strategy. In Experiment 2 (N = 220), adults and children from about 8 years onward successfully identified the agents using the more efficient strategy as more competent. Overall, our results suggest that observed effectiveness is a powerful cue for competence even when such an inference may not be warranted and that the ability to make explicit competence judgments based on the efficiency of a strategy alone emerges around 8 years of age, although, as shown in previous work, a more implicit understanding of competence may already be present during the preschool years. Public Significance StatementThis study investigated the cues young children and adults use to evaluate the competence of someone they observed solving a problem. Children struggle to evaluate competence based on the efficiency of the strategy used until around age 8. Instead, the observed number of steps taken to reach a solution remains a powerful cue for competence throughout childhood and even adulthood. Children's accuracy in making inferences about other people's competence is relevant for their skill growth, socio-cognitive development, and academic achievement. Therefore, exploring and promoting an unbiased understanding of competence, stressing the importance of strategy efficiency, can support children's development in classroom settings and beyond.

In order to sustain cooperation, it is important that we have a sense that the distribution of efforts is fair. But how proficient are we at comparing our effort relative to that of others? Does the perception of our effort differ in individual and joint action contexts? To address these questions, we asked participants to squeeze a hand dynamometer at varying degrees of force to meet three target levels alone and with a partner. The results do not reveal a significant difference in the perception of effort between the two conditions. However, participants’ estimation of their effort skewed towards half when they made partial contribution to the target and this effect was more pronounced in the joint action. Taken together, the findings suggest that participants might have applied heuristics when perceiving their effort and in addition, expected fairness in the effortcontribution in a joint action context.