Figure 2. Results.

The frequency of choosing Prospect 1 (the prospect with higher reward probability) is plotted as a function of the EV-ratio for (graph A) all subjects, (graph B) subjects in Version 1, in which probability is represented numerically – see Figure 1A, and (graph C) subjects in Version 2 in which probability is represented pictorially – see Figure 1B. EV-ratio>0 indicates that Prospect 1 with the higher reward probability also has the higher EV-ratio; EV-ratio<0 indicates that Prospect 2 with the larger reward is the better choice. The point of equivalence (when frequency of choosing higher probability is equal to frequency of choosing larger reward, i.e. the dotted horizontal line showing frequency = 0.5) should occur when the EV-ratio = 0 (dotted vertical line) in a rational decision-maker, but this is shifted in our subjects to the left, indicating greater tendency to choose Prospect 1, which has the higher reward probability. This is more so in Version 2 than Version 1 (graph A vs. B), indicating that there is an additional bias in favour of the property depicted pictorially. In (A), the solid grey lines indicate thresholds for 25% and 75% frequency of choice: half the distance on the x-axis covered by the fitted curve between these two frequencies is taken as our discriminative threshold. Error bars are one standard error.