Fig. 1: Behavioral design and results.

a The time course of a trial is shown proceeding from top-left to bottom-right. The animal initiated the trial by directing its gaze toward an open circle, or fixation point (FP), and held fixation for a variable interval. An offer was presented as an array of icons (0, 1, 2, 4, or 8 yellow icons, selected randomly across trials) with each icon indicating a drop of juice in exchange for effortful work; simultaneously, the FP filled-in to a solid circle (providing a timing cue on 0-reward offers). To accept the offer, the animal sustained fixation through the work period (4 or 6.5 s, monkey N or K, respectively), after which the promised drops of juice were delivered in rapid succession. To reject the offer (large oblique gray arrow), the animal averted its gaze during the offer or work period. The trial then entered a timeout period equivalent in duration to the time that would have elapsed had the animal accepted the offer. The trial was followed by an inter-trial interval before advancing to the next trial (solid curve). b The proportion of offers accepted is shown as a function of offer size, which was conveyed to the animal either by the number of yellow icons (non-singleton offers; filled circles) or as single purple icon worth 8 drops (singleton offer; open circle), which controlled for the inherent confound between reward magnitude and the visual impact of more icons. Data are presented as proportion (circles) ± s.d. of the proportion (error bars; generally smaller than circle) from N = 9637 or 27,952 trials from monkeys N or K (trial counts evenly distributed across offer sizes). Logistic function was fit to aggregated data for display purposes only (black curve); all statistical tests in Supplementary Fig. 1 were performed on individual sessions. c Hazard rate (colored curves) ± s.d. (shading) of fixation breaks are shown for non-singleton offers as function of time from onset of the offer period (double arrows). Early, mid-trial, and late phases of rejections referred to in main text are indicated by dark, middle, and light gray tapered bars, respectively.