Figure 1:

A. Experimental model testing Prisoner’s dilemma in pairs of rats responding for food reward in an operant chamber bisected by a metal screen. In each pair, one rat is designated as the Subject. His partner is the Stooge, whose responses are controlled by computer. During a 2-second lever presentation, Subjects choose to cooperate (c, withhold a response) or defect (d, respond on the lever), resulting in outcomes of mutual cooperation (reward, R), mutual defection (punishment, P) or unilateral cooperation (sucker, S/temptation, T). B. Payoff matrix for Prisoner’s Dilemma (numbers in parentheses represent sugar pellets to each partner). C. Payoff matrix for the Stag Hunt game. D. Tit-for-tat (TFT) and win-stay/lose-shift (WSLS) response strategies according to transition vectors r, t, s and p, which reflect the probability of cooperation when the previous trial resulted in outcomes of R, T, S or P, respectively.