Theory: The costs imposed by income tax and other citizenship duties provide powerful incentives to free ride, yet the likelihood of getting caught-the primary self-interested deterrent to free-riding-is unlikely to be known with much accuracy by most citizens. Thus the duty heuristic provides not only a direct motivation to comply, but also influences behavior through cognitive processes that bias self-interested beliefs about the advantages of free-riding. Hypothesis: Citizens' beliefs about their duty to obey laws of the state bias their information processing and judgements about the fear of getting caught and punished for disobedience, so citizens reporting greater commitment to obey tax laws systematically overestimate the expected penalty for noncompliance. Methods: Regression analysis of survey and tax-return data for 445 taxpayers. Results: Subjective risk of getting caught is more closely related to duty than to objective risk factors. Objective audit probabilities affect only taxpayers with greater temptation to cheat, but duty influences tempted taxpayers as much as ordinary taxpayers.