(A) Experimental set-up. The monkeys sat side-by-side, facing a screen. On each trial, they covertly chose, in succession, to cooperate (orange hexagon) or defect (blue triangle). Following delay, both choices were revealed on screen and reward was delivered.
(B) Payoff matrix. Reward outcome for all possible choice combinations. Cooperation and defection were defined operationally by whether mutual benefit or loss is incurred.
(C) Trial timeline. The order in which the monkeys made their selections was randomized on each trial.