Mistrial

A mistrial is a court ruling that invalidates a trial before it reaches a verdict. The judge ends proceedings on the grounds that continuing would be unfair, futile, or legally improper. A mistrial does not acquit or convict the defendant; in most cases the prosecution may retry the case, though specific rules vary by jurisdiction.

Common grounds for declaring a mistrial include:

• Hung jury — the jury cannot reach the required verdict (unanimous in U.S. federal criminal cases under Federal Rule of Criminal Procedure 31; some U.S. states and other common-law systems permit majority verdicts).
• Prejudicial error — inadmissible evidence, improper remarks by counsel, or exposure of jurors to outside information.
• Juror misconduct or incapacity — bias, improper communication, illness, or insufficient alternates.
• Death, illness, or disqualification of judge or counsel.
• External events making fair continuation impossible.

A mistrial may be declared on the motion of either party or sua sponte by the judge. In the United States, the Double Jeopardy Clause of the Fifth Amendment generally permits retrial after a mistrial, but bars it where the mistrial was caused by prosecutorial misconduct intended to provoke the defense into requesting one — the standard articulated in Oregon v. Kennedy, 456 U.S. 667 (1982). Where the defense did not consent to the mistrial, retrial is permitted only if there was "manifest necessity," a standard tracing to United States v. Perez, 22 U.S. 579 (1824).

Civil-law systems use analogous mechanisms — for example, annulment of proceedings (nullité de la procédure) in French criminal practice — though the procedural triggers and re-prosecution rules differ.

For researchers, mistrials are a useful indicator of due-process strain in high-profile prosecutions and a recurring feature in transitional-justice and war-crimes proceedings, where evidentiary and witness-protection issues frequently force restarts.