Yes, co-defendants can testify against each other at trial. What no one can do is force them to. The Fifth Amendment protects every defendant from being compelled to incriminate themselves, so a co-defendant who takes the stand against you is doing it voluntarily, almost always in exchange for a plea deal or under a grant of immunity. That single distinction, between voluntary and compelled testimony, shapes nearly every rule that follows: the deals prosecutors offer, the way confessions are handled when the co-defendant refuses to testify, whether the trial gets split in two, and how the jury is told to weigh what they hear.
Why a Co-Defendant Cannot Be Forced to Testify
Each co-defendant has a separate legal identity and a personal right against self-incrimination. The Fifth Amendment guarantees that no person “shall be compelled in any criminal case to be a witness against himself,” and courts read that protection broadly. It covers not just outright confessions but any answer that could “furnish a link in the chain of evidence” needed to prosecute the witness.1Constitution Annotated. Amdt5.4.3 General Protections Against Self-Incrimination Doctrine and Practice
The privilege belongs to the individual, though, and individuals can give it up. Prosecutors can invite, incentivize, and negotiate for a co-defendant’s testimony. What they cannot do is drag someone to the stand and make them talk over a valid claim of privilege.
Plea Deals and Immunity: Why They Testify Anyway
Most testimony from one co-defendant against another exists because the prosecution offered something for it. The two main vehicles are plea agreements and immunity grants, and the difference matters.
In a plea deal, the cooperating co-defendant pleads guilty, usually to reduced charges or with a recommendation for a lighter sentence, in exchange for truthful testimony. They still take a conviction, but they avoid the worst outcome. Prosecutors like this arrangement because the cooperator has a personal stake in being believed.
Immunity is different. When a witness invokes the Fifth Amendment and refuses to testify, a court can compel their testimony by granting immunity. The federal statute provides what is known as “use and derivative use” immunity: the compelled testimony, and any evidence derived from it, cannot be used against the witness in a later prosecution.2Office of the Law Revision Counsel. 18 USC 6002 – Immunity Generally The Supreme Court upheld that approach in Kastigar v. United States, holding that use and derivative use immunity leaves the prosecution in essentially the same position as if the witness had stayed silent, which is enough to satisfy the Fifth Amendment.3Justia. Kastigar v United States, 406 US 441 (1972)
The federal system does not offer transactional immunity, which would shield a witness from prosecution for the entire transaction they testified about regardless of independent evidence. Some states still do. Under use immunity, prosecutors can still charge the immunized witness if they can prove their evidence came from sources completely independent of the compelled testimony.3Justia. Kastigar v United States, 406 US 441 (1972) Under transactional immunity, they cannot bring related charges at all.
What Happens Once a Co-Defendant Takes the Stand
A co-defendant who begins testifying on direct examination waives the Fifth Amendment privilege for topics within the scope of that testimony. The Supreme Court held in Brown v. United States that a defendant who takes the stand can be compelled to answer relevant questions on cross-examination, and the breadth of the waiver is set by the scope of the direct. The waiver does not extend to unrelated matters raised only to attack credibility.
In practice, a co-defendant who testifies about their role in the charged crime can be cross-examined about the details of that crime, but opposing counsel cannot use the opening to fish for information about wholly separate conduct. That limited waiver creates a real tactical calculation on both sides. Once direct opens a door, cross walks through it, and the cooperator cannot selectively invoke the Fifth to dodge hard questions inside the scope of what they already discussed.
When the Co-Defendant Won’t Testify but Their Confession Exists
One of the trickiest situations in a joint trial is when a co-defendant gave an out-of-court confession that names or implicates you, then refuses to take the stand. The Sixth Amendment guarantees every accused person the right “to be confronted with the witnesses against him.”4Constitution Annotated. Right to Confront Witnesses Face-to-Face If the person who confessed never testifies, you have no chance to cross-examine them about it.
The Supreme Court addressed this in Bruton v. United States, holding that introducing a non-testifying co-defendant’s confession at a joint trial violates the other defendant’s confrontation rights, even when the judge tells the jury to consider it only against the person who confessed.5Justia. Bruton v United States, 391 US 123 (1968) The Court recognized what trial lawyers already knew: telling a jury to ignore a powerful confession sitting right in front of them is asking the impossible.
The Bruton rule does not bar every co-defendant confession from a joint trial. In Richardson v. Marsh, the Court held that a confession redacted to remove all references to the other defendant can be admitted with a limiting instruction, because the jury would then need to connect the dots using other evidence rather than seeing a direct accusation.6Justia. Richardson v Marsh, 481 US 200 (1987) When redaction is not feasible and the confession directly names you, the court’s main options are severing the trial or excluding the confession.
Statements That Come In Without Anyone Testifying
Even without formal testimony, things a co-defendant said during the alleged crime can come in as evidence against you. Under Federal Rule of Evidence 801, a statement made by a co-conspirator during and in furtherance of the conspiracy is not hearsay and can be admitted against the other defendants.7Legal Information Institute. Federal Rules of Evidence Rule 801 – Definitions That Apply to This Article
Three conditions have to be met. There must have been a conspiracy involving both the speaker and the defendant the statement is used against. The statement must have been made while the conspiracy was still active, not after it fell apart or someone got arrested. And it must have been made to advance the conspiracy’s goals, not as idle talk about it. A co-conspirator bragging about the crime afterward would not qualify. The Supreme Court held in Bourjaily v. United States that the judge decides whether these conditions are met and can consider the statements themselves in making that call.8Legal Information Institute. Bourjaily v United States, 483 US 171 (1987)
This rule matters because it lets the prosecution introduce damaging statements without your co-defendant ever taking the stand. Text messages planning the crime can come in against you through this exception even if your co-defendant invokes the Fifth Amendment at trial.
Asking the Court to Split the Trial
When a co-defendant’s testimony or confession creates a serious risk of unfairness, either side can ask the court to sever the case into separate trials. Federal Rule of Criminal Procedure 14 gives judges the authority to sever when joinder “appears to prejudice a defendant or the government.”9Legal Information Institute. Federal Rules of Criminal Procedure Rule 14 – Relief from Prejudicial Joinder
Getting severance is not easy. The Supreme Court held in Zafiro v. United States that severance should be granted “only if there is a serious risk that a joint trial would compromise a specific trial right” or would keep the jury from reliably judging each defendant’s guilt separately.10Justia. Zafiro v United States, 506 US 534 (1993) Mutually antagonistic defenses alone do not automatically require separate trials. Judges tend to manage prejudice through jury instructions and evidence redaction rather than run two full trials.
What the Prosecution Must Tell the Defense About the Deal
When the prosecution uses a cooperating co-defendant, it has to hand over certain information. The Supreme Court held in Brady v. Maryland that suppressing evidence favorable to the accused violates due process when the evidence is material to guilt or punishment. Under Giglio v. United States, that obligation extends to impeachment evidence, including any promises, plea agreements, or benefits given to a witness, because those go directly to credibility. The Department of Justice’s own policy incorporates this duty.11Department of Justice. Justice Manual 9-5.000 – Issues Related to Discovery, Trials, and Other Proceedings
You are entitled to know exactly what your co-defendant was offered in exchange for cooperating. If the prosecution fails to disclose a deal, or a side understanding that sweetened it, that can be grounds for overturning a conviction. Cases sometimes unravel years later when undisclosed agreements surface.
How Juries Are Told to Weigh Cooperator Testimony
Courts recognize that a co-defendant who testifies in exchange for a deal has an obvious motive to shade the truth. Federal courts routinely give cautionary jury instructions when cooperating witnesses take the stand. The Sixth Circuit’s pattern instructions are typical: jurors are told to consider an accomplice’s testimony “with more caution than the testimony of other witnesses” and warned not to convict based on that testimony alone unless they believe it beyond a reasonable doubt.12United States Court of Appeals for the Sixth Circuit. Pattern Jury Instructions, Chapter 7 – Testimony of an Accomplice Separate instructions apply to paid informants and to witnesses testifying under immunity, each flagging the specific incentive that might color their account.
Defense attorneys lean into this dynamic on cross-examination. The standard approach is to nail down exactly what the cooperating witness received, probe for inconsistencies between the current story and earlier statements, and show how the deal gives them every reason to tell the prosecution what it wants to hear. Cooperator testimony is some of the most powerful evidence a prosecutor can offer and also some of the most suspect.
What Happens if the Cooperator Lies
A co-defendant who lies on the stand faces real consequences beyond losing their deal. Perjury is a separate federal offense, and under the federal sentencing guidelines, a defendant found to have committed perjury or provided materially false information receives a two-level increase to their offense level under the obstruction of justice enhancement.13United States Sentencing Commission. Amendment 347 – Section 3C1.1 Obstructing or Impeding the Administration of Justice “Material” here means information that would tend to influence the outcome if believed.
The guidelines draw a clear line: simply denying guilt or refusing to cooperate is not obstruction. The enhancement targets affirmative lies and deceptive conduct, not the exercise of constitutional rights.13United States Sentencing Commission. Amendment 347 – Section 3C1.1 Obstructing or Impeding the Administration of Justice Plea agreements with cooperating witnesses almost always require “truthful testimony,” and prosecutors will pull the deal if they catch the cooperator lying. Fabricating on the stand risks losing the plea benefit and picking up new charges.
If the Co-Defendants Are Married
Marriage adds a layer of protection that most co-defendant pairs do not have. Federal courts recognize two spousal privileges under common law, preserved by Federal Rule of Evidence 501.14Legal Information Institute. Federal Rules of Evidence Rule 501 – Privilege in General
The first is testimonial privilege. In federal criminal cases, the witness-spouse alone holds the privilege to refuse to testify against their husband or wife. The Supreme Court established in Trammel v. United States that the witness-spouse “may be neither compelled to testify nor foreclosed from testifying,” meaning the defendant-spouse cannot block willing testimony.15Legal Information Institute. Trammel v United States, 445 US 40 (1980) The privilege covers events before and during the marriage but generally applies only while the marriage is intact. A minority of jurisdictions give the privilege to the defendant-spouse instead, letting them prevent their partner from testifying regardless of willingness.
The second is the confidential marital communications privilege, which protects private conversations between spouses made during the marriage. It can survive divorce or the death of a spouse, and in most jurisdictions either spouse can assert it. Both must consent before the communication can be disclosed. For married co-defendants, private conversations about the alleged crime may be off-limits as evidence, but only if they were genuinely intended to be confidential rather than made in front of others.