Theory of mind (ToM) is required when reasoning about mental states such as knowledge, beliefs, desires, and intentions. Many complex reasoning tasks require domain-general cognitive resources such as planning, resistance to interference, and working memory. In this paper we present a study of the additional cognitive costs of reasoning about mental states. We presented participants with sequential games in which they have to reason about another player. In the so-called player condition, the other player is reasoning about the participant, whereas in the so-called balance condition, the other player is reasoning about a balance scale. Both types of games require the same comparisons, but only differ in the required depth of ToM reasoning. Games in the player condition require one additional switch between perspectives. The results show that participants make different types of mistakes in the player condition as compared to the balance condition. This finding implies a different reasoning process when reasoning about mental states. The results also show faster decreasing reaction times in the balance condition than in the player condition. Based on these findings, we argue that reasoning about mental states requires unique cognitive resources.