In Blackstone's day, a verdict was required for the double jeopardy principle to operate, but this was probably because outcomes short of a verdict were virtually unknown. The current standard for deciding when a mistrial is equivalent to a verdict, drawn from the 1824 case of United States v. Perez, is whether the first trial ended because of "manifest necessity." If so, the first trial does not erect a double jeopardy bar to a second trial. If there was no "manifest necessity" to end the first trial, then a second one is a forbidden second jeopardy.
Three general principles can be drawn from the "manifest necessity" cases. First, if the defendant requests the mistrial and the judge grants it, this will almost always constitute manifest necessity for ending the first trial. This defendant can be retried. Second, if the judge decides that the jurors have been unfairly prejudiced—for example, by hearing something they should not have heard—the judge's decision to terminate the trial will almost always constitute manifest necessity. This defendant, too, can be retried. Third, if the first trial ends because the jury might have acquitted—such as when the prosecution's chief witness did not appear—there is no manifest necessity. If the state fails to produce enough evidence at trial, the defendant is entitled to an acquittal. This defendant cannot be retried.
Other kinds of reasons can lead to a mistrial—for example, one judge granted a mistrial because his mother-in-law died unexpectedly. In these miscellaneous cases, courts balance the reason for the mistrial, including how carefully the judge considered other alternatives, against the unfairness of asking a defendant once again to defend the criminal charge. In the case where the trial judge's mother-in-law died suddenly, the appeals court held that there was no manifest necessity for the mistrial, in part because the judge did not consider asking another judge to take his place. The double jeopardy clause thus barred a second trial.
Law Library - American Law and Legal InformationCrime and Criminal LawDouble Jeopardy - Mistrials, Multiple Punishment, Second Prosecution After Conviction, Second Prosecution After Acquittal, Appeals, Lower Courts