Figure 1.

The schematic on the left (A) illustrates the testing setup in conditions in which the actor had access to both the cup on the right and the cup on the left (i.e., selfish training sessions and selfish control sessions). The adjacent chamber was always empty. The schematic on the right (B) illustrates the testing setup in conditions in which the actor had access to only the cup on the left. The adjacent chamber was either empty (i.e., empty training sessions and empty control sessions) or contained the recipient monkey (i.e., prosocial test sessions).