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A well known game-theoretic model of tax compliance developed by Graetz et al.(1986) predicts the optimal equilibrium strategies of taxpayers and tax auditors when a common knowledge assumption of individual rationality is applied to game participants. Previous experimental evidence in tax compliance game setting, however, shows that decision makers face cognitive limitations. That is, tax auditors and taxpayers adjust their strategy based on a belief about the opponent’s response to parametric change, but do not correctly recognize that their opponent does likewise(Kim 2011). The hypotheses in this study assume that the auditor with limited rationality is sensitive to both the proportion of unethical taxpayers and uncertainty about the proportion. To represent limited rationality, this study proposes that the auditor makes the audit strategy choice through an anchoring and adjustment process. The anchor is a function of the factors that directly affect the auditor, such as the audit cost. The adjustment is a function of the auditor’s expectation about the taxpayer’s reporting strategy. The experiment in this study operationalizes the game-theoretic model of Graetz, et al.(1986) to examine how the auditor with limited rationality responds to the variation in the proportion of unethical taxpayers and uncertainty about the proportion of unethical taxpayers. The experimental results show a significantly positive relationship between the level of unethical taxpayers and the audit probability. The results show that the auditors used the level of unethical taxpayers as a cue for setting an anchor for the audit rate. The auditors who know the level of unethical taxpayers increase the audit probability as the level increases. When the level of unethical taxpayers is common knowledge, the auditors respond to normatively irrelevant variation in the level of unethical taxpayers, reflecting non-strategic or simple strategic thinking. The experiment also shows how to eliminate the auditors’ strategic decision bias by suppressing normatively irrelevant information, given the previous experimental results showing that humans normally cannot work through the higher levels of strategic thinking as predicted by game-theoretic models. When the auditors know the level of unethical taxpayers in each round, they commit the bias of under-auditing when the level is low. However, when the auditors know only the uniform distribution over possible levels of unethical taxpayers, the tendency to under-audit disappears. A counterintuitive policy implication for auditors is to ignore variation in the proportion of unethical taxpayers unless the variation is a private information. A key element in the hypothesis development and experimental design is the players’ formation of a belief about their opponent’s strategy and incorporation of this belief into their own strategy choice. Future research might further examine the belief formation process and its impact on strategy choice, in two ways. One way is to elaborate the subjects’ task. A future experiment additionally might collect the subjects’ estimates of their opponent’s estimate of their own strategy. The extra data would clarify how the subjects make belief-based adjustments in their strategy choice. The other way is to simplify the taxpayers’ reasoning process by informing the audit probability before they state their under-reporting probability.

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