In the 1963 case Hardy v. United States, the Supreme Court examined whether a defendant's constitutional rights were violated when he was not allowed to cross-examine a government witness about his mental history. The defendant, Hardy, had been convicted of bank robbery and appealed on these grounds. The court ruled against him in an 8-1 decision stating that while defendants generally have broad rights to question witnesses' credibility, this right is not unlimited and does not extend to inquiries into areas which are only marginally relevant such as psychiatric history unless it directly relates to their ability or inability to tell the truth at the time of their testimony. Therefore, they held that there was no violation of Hardy's Sixth Amendment right.

In the dissenting opinion for Hardy v. United States, it was argued that the majority's decision to uphold Hardy's conviction under a federal narcotics law was incorrect due to issues with evidence and witness credibility. The dissenting justices believed that there were significant problems with the government’s case, including questionable testimony from an undercover agent who had previously been involved in illegal activities himself. They also pointed out inconsistencies in other witnesses' testimonies and raised concerns about whether certain pieces of evidence should have been admitted at all. Furthermore, they disagreed with how much weight was given to circumstantial evidence by the prosecution during trial proceedings. In their view, these factors created reasonable doubt about Hardy’s guilt which should have led to his acquittal rather than conviction.