Figure 1.

Experimental apparatus. The apparatus was positioned in the area in front of the indoor compartment (where the subject was located) and the experimenter sat on the other side of the apparatus (facing the subject). In each trial, the subject faced a choice between different quantities of tokens A and B and it could choose one of the two offers by pulling one of the two sliding trays where the tokens were placed. The subject could reach the trays through two openings in the wire mesh (8.5 cm×3.8 cm each), each positioned in correspondence with one of the trays. After the subject performed its choice, the experimenter blocked the other tray to prevent the subject from pulling both trays, and the subject could exchange the selected token(s) with the experimenter to obtain the corresponding reward. The drawing depicts an adult male who, after choosing between one token B (on the left, worth three pieces of peanut) and two tokens A (on the right, worth one piece of peanut each), exchanges the token B with the experimenter.