<p>A contingency under which responses by an individual yield a reinforcer delivered to another individual, and vice versa, sometimes has been called mutual reinforcement. The present experiments examined whether such so-called mutual reinforcement increases response rates in humans. Pressing one of two keys (S⁺ key) by one undergraduate (participant) provided another participant with points exchangeable for money, and vice versa. In a minimal social situation, in which participants were neither told to work together nor shown stimuli their partners could see, the results provided no evidence that mutual reinforcement increased response rates (Experiment 1). By contrast, in a somewhat social situation where the participants were told to work together and shown stimuli their partners could see, the results for 7 of the 16 participants demonstrated that mutual reinforcement increased the response rates. That is, for these seven participants, (1) response rates on the S⁺ key were higher with mutual reinforcement than with no reinforcement, and (2) discrimination ratios (the number of responses to the S⁺ key to the total number of responses) were higher with mutual reinforcement than with a variable-time schedule (Experiment 2). The implications of these findings were discussed in the context of the experimental analysis of human behavior and an evolutionary perspective.</p>