In it together: evidence of a preference for the fair distribution of effort in joint action

A growing body of theoretical and empirical work suggests that our sense of fairness implies a preference for divisions of rewards that are proportional to contributions. However, there has been no study testing whether people distribute effort costs according to the expected reward distribution. We hypothesized that when people expect to share the reward of the joint task equally, they will ensure fairness by calibrating their effort investment such as to reduce inequity with respect to joint action partners' effort investment. We developed a task in which participants traded off effort costs against reward. Before making their decision, they observed as their partner performed an effort task. We examined how the perception of the partner's effort modulated effort-based decision-making in joint action, depending on whether participants were in a joint or separate reward structure, and on whether the available reward was known or unknown. Across two lab-based, pre-registered experiments (N = 57), we found support for our hypothesis, and we controlled for other candidate explanations for the observed effort matching effect such as a preference for acting jointly, learning about the value of opportunities afforded by the environment, learning the value of effort and competition.