Giglio v. United States

What Happened

John Giglio was prosecuted for passing forged money orders. The government’s case depended heavily on testimony from a coconspirator, Taliento, who linked Giglio to the scheme.

After trial, it emerged that an assistant United States attorney had promised Taliento he would not be prosecuted if he testified before the grand jury and at trial. The prosecutor who actually tried the case told the jury that no such promise had been made. The trial prosecutor said he did not know about the earlier commitment.

That set up the key question: when the government fails to disclose a promise that could be used to impeach its main witness, does due process require a new trial?

What the Court Decided

The Supreme Court said yes. A promise of leniency to a key witness is impeachment evidence, and impeachment evidence falls within the disclosure rule announced in Brady v. Maryland.

The Court also rejected the idea that the government could escape responsibility because the trial prosecutor lacked personal knowledge. The prosecution is treated as one office for disclosure purposes. If one prosecutor made the promise, the government had a duty to disclose it.

Because Taliento’s credibility was central and the promise could have affected the jury’s view of his testimony, the nondisclosure required a new trial.

What It Means in Practice

Giglio matters because many wrongful prosecutions turn less on hidden physical evidence and more on hidden deals, incentives, or credibility problems. A witness who expects leniency, cash, dismissal of charges, or some other benefit has an obvious motive the defense is entitled to expose.

For Section 1983 plaintiffs, Giglio helps explain why hidden impeachment material can be just as serious as hidden exculpatory evidence. A case can turn on whether the jury trusted a cooperating witness, jailhouse informant, or complaining witness who had undisclosed incentives.

How You Can Use It

• Use it when the hidden evidence is about credibility. Giglio confirms that impeachment material is part of Brady doctrine.
• Use it against compartmentalized excuses. The government cannot avoid disclosure by saying one prosecutor knew and another did not.
• Template: “The suppressed evidence was material impeachment evidence under Giglio v. United States, 405 U.S. 150 (1972), because it revealed a benefit promised to the government’s key witness.”

How It Can Be Used Against You

• The defense will argue the witness was not central. If multiple witnesses or strong physical evidence supported the case, defendants will say the undisclosed deal was cumulative.
• The defense will narrow the promise. They may argue there was no real agreement, only vague hope or informal conversation.
• Civil causation still matters. In a Section 1983 case, you still need to connect the nondisclosure to a constitutional injury and a proper defendant.

How to counter: Document the witness’s importance, identify exactly what benefit was offered, and show how disclosure would have changed cross-examination or the jury’s view of the case.