Figure 1

Task structure and timeline. From left to right the order and duration of the phases of a typical trial are shown; first, a fixation point was shown, then participants could make a promise regarding their payment frequency (Promise phase). After another fixation point, the subjects waited for 6 s their investor's decision, who could be the computer, the stranger, or the friend (Anticipation phase). In order for the participants to experience distrust of the investor, the decisions of the three trustors were programmed to randomly invest in 6 out of 8 trials in the participant, and in 2 out of 8 not to invest. During the decision phase, the participants decided whether to pay or keep the money. In the feedback decision, the payments for the current trial were shown. If the participant reciprocated, payments were $5 and $5, while if he did not, the payments were $10 and $0. In half of the trials, during the promise phase, a 9-s message was displayed indicating that it was possible to play without a promise, which provided the opportunity to contrast the participants' decisions when they made a promise and when they did not. Also, during 25% of the trials the trustors decided not to invest, in this case, during the decision phase, a message was shown saying that the partner had not invested, and in the feedback phase, it was indicated that the payments were $2 for the partner and $0 for the participant (Supplementary Table 1).