<p>Previous studies have indicated that two-person judgments are more accurate than individual judgments in perceptual decision-making. Most studies have indicated that the improvement of performance accuracy reflects changes in visual sensitivity by using a bias-free paradigm to eliminate the effects of decision criteria. However, biases in judgment are natural during daily decision-making. Therefore, we should carefully consider the effect of response bias on two-person decision-making. Thus, this study investigated changes in two-person decision-making using signal detection theory. We conducted two experiments. The results of the two experiments revealed that not only does two-person judgment improve perceptual sensitivity but also shifts the decision criterion. This study provides new insights into response bias as a factor for better understanding two-person interactions.</p>