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.

Effort perception is a crucial capacity underpinning characteristically human forms of sociality, allowing us to learn about others’ mental states and about the value of opportunities afforded by our environment, and supporting our ability to cooperate efficiently and fairly. Despite the crucial importance and prevalence of effort perception, little is known about the mechanisms underpinning it. Across two online experiments (N= 462), we tested whether adults estimate others' cognitive effort costs by tracking perceptible properties of movement such as path length, time and speed. The results showed that only time had a consistent effect on effort perception, i.e., participants rated longer time as more effortful. Taken together, our results suggest that within the context of our task – observing an agent deciphering a captcha – people rely on the time of others’ actions to estimate their cognitive effort costs.