In the 1990 case of John L. Cheek v. United States, the Supreme Court ruled in favor of Cheek, overturning his conviction for willfully failing to file a federal income tax return and attempting to evade taxes. The court held that "willfulness" involved in these crimes required proof that the law imposed a duty on the defendant, he knew about this duty and intentionally violated it. Cheek had argued that he truly believed due to misinformation that he was not violating any laws by not paying taxes; hence his actions were not 'willful'. The government contended such an argument should be irrelevant as ignorance is no excuse for breaking laws. However, Justice Byron White wrote for majority opinion stating if a person genuinely believes they are not breaching tax law (even if this belief is irrational or unreasonable), then technically they are not doing so ‘wilfully’. This ruling clarified what constitutes 'willfulness' in federal criminal tax cases.

In the dissenting opinion for Cheek v. United States, Justice Blackmun argued that the majority's decision undermined tax law enforcement by allowing defendants to avoid conviction simply by claiming they sincerely believed their actions were legal, regardless of how unreasonable this belief might be. He contended that willfulness in tax evasion cases should not require proof that the defendant knew his conduct was unlawful; rather it should only necessitate demonstrating a voluntary and intentional violation of known legal duties. By requiring evidence of knowledge about illegality, he feared it would encourage individuals to deliberately remain ignorant about tax laws or embrace fringe interpretations in order to evade taxes without punishment. The dissent also expressed concern over potential difficulties juries may face when determining whether a defendant truly held such beliefs or merely claimed them as an excuse after being caught.