This research aims at identifying a mechanism of leadership in teams. Specifically, we consider such a possibility that leadership is voluntarily taken by a team member who personally believes that effort is worth taking in his team. First, we formulated a game model of team production in which each member spends effort for his team given a judgment he personally holds about team productivity. We proved that leadership is realized in equilibrium if these judgments are independent across members, whereas it is not if they are correlated. Then, we tested this theory by conducting laboratory experiments. The result is that we observed emergence of leadership in teams in which subjects’ judgments about team productivity were stochastically independent, which coincides with the theoretical prediction. However, we also observed emergence of leadership in teams in which subjects’ judgments about team productivity were correlated, which contradicts with the theory.