Fig. 2: Effort-based decision-making task with rewards for self or other.

a, Before any further instructions, the participants squeezed as hard as they could to measure their maximum voluntary contraction (MVC) on a handheld dynamometer to threshold each effort level to their strength. After thresholding and practice, the participants chose on each trial between a ‘rest’ option, which required no effort (0% MVC, one segment of the pie chart) for a low reward of one credit, and a ‘work’ option, which required more effort (30–70% MVC, two to six pie chart segments) for more reward (two to ten credits). The reward available and the effort required were manipulated independently. After making their choice, the participants had to exert the required force (shown by the yellow line) for at least 1 s of a 3 s window to receive the reward. Visual feedback on the amount of force used was displayed on the screen. The participants then saw the outcome, depending on the offer they had chosen and whether they were successful. If they did not meet the required force level, ‘0 credits’ was displayed. Crucially, on self trials, the participants made the choice, exerted the effort and received the reward themselves, whereas on other trials (‘AMY’ in this example), the participants made the choice and exerted the effort, but the other participant received the reward. b, The participants were designated as ‘Player 1’ (self) and told that their decisions impacted another player, ‘Player 2’ (other), who they met at the beginning of the testing session with their identity obscured (to control for influences of identity or reciprocity). The name used in the task was gender-matched to the participant. The procedure involved four people: two experimenters, EXP1 and EXP2, and two participants, self and other.