Figure 1.

Illustration of the testing procedure using the 5/3;2 game as an example. (a) A paper strip and 10 grape halves are placed on a sliding tray by the experimenter. (b) Pulling the paper results in three of the grape halves that would have gone to the responder being added to the proposer's portion. (c) The proposer pulls the rope (which can be done without pulling the paper), bringing the rod within the responder's reach. (d) The responder can pull the rod, making the food accessible to both subjects. Failure to pull the rod within 1 min results in all the food being removed by the experimenter.