This study investigates experimentally how mutual monitoring affects effort when employees are compensated via rank-order tournaments. Theory and anecdotal evidence suggest that mutual monitoring may either decrease effort by facilitating collusion or increase effort by stimulating competition. In our first experiment, we find that mutual monitoring increases effort, because participants do not attempt to collude but rather behave competitively. This result leads us to expand our theory and develop hypotheses to predict that the effect of mutual monitoring depends on whether employees have the inclination to collude or compete. Specifically, we predict that mutual monitoring decreases effort when employees are inclined to collude and increases effort when employees are inclined to compete; that is, mutual monitoring will not change the basic inclination created by the workplace setting, but will “turn up the volume” on the effect that such inclination has on effort. Consistent with our predictions, our second experiment finds that mutual monitoring leads to lower effort when participants have a collusive inclination and (eventually) higher effort when they have a competitive inclination. Overall, the results from these two experiments suggest that allowing employees to observe each other's productive effort in tournament incentive settings may have positive or negative consequences for the firm, depending on whether environmental factors predispose employees to collude or compete.