The reciprocity model used in the present experiments.

<p>Rounded rectangles on the left side represent the types of partners presented in the test (cooperators, cheaters, and new faces). Rectangles on the right represent the participants’ decisions (“I cooperate”, or “I do not cooperate”). The letters along the links represent the probabilities of different strategies (<i>R</i>₊: positive reciprocity; <i>R</i>_–: negative reciprocity; <i>A</i>: cooperation bias).