Fig. 1

The experimental design of the mini UG. a 7 conditions of the proposer’s choices. The blue lines represent the proposer’s own shares and the red lines represent the offers for the responder. The solid lines represent the options chosen by the proposer and the dash lines represent the alternative options. Specifically, the 7 conditions are: 1, hyper-unfair offer (8 vs. 2 / 10 vs. 0); 2, hyper-fair alternative (8 vs. 2 / 2 vs. 8); 3, fair alternative (8 vs. 2 / 5 vs. 5); 4, no alternative (8 vs. 2 / 8 vs. 2); 5, hyper-unfair alternative (8 vs. 2 / 10 vs. 0); 6, fair offer (8 vs. 2 / 5 vs. 5); 7, hyper-fair offer (8 vs. 2 / 2 vs. 8). b Diagram illustrating the structure of a single round of the mini Ultimatum Game (mini UG). Each trial started with a 1500 ms fixation interval. In the second stage, picture (left) representing the responder, options (here 8 vs. 2 / 5 vs. 5) for the proposer and choices for the responder (YES vs. NO) first appeared, and lasted for 2000 ms. Then the picture (right) representing the proposer appeared, and 1000 ms later, the chosen option was encircled in white. Participants as responders had at most 5000 ms to make a choice. The feedback of their decision would last for 1500 ms. Here is an example of a trial in the human proposer condition in the mini UG