This article in the Wall Street Journal discusses the importance of unanimity in criminal trials.
Why Juries Must Agree
Typically, judges declare mistrials when a jury is unable to reach unanimous agreement on conviction or acquittal.
In the event of a mistrial, the government has a choice: It can abandon the prosecution entirely or try for a retrial. For instance, in the case of former Cendant Corp. chairman Walter A. Forbes, the government in 2006 won a fraud conviction on its third try, after juries deadlocked in two previous trials.
The rule that a criminal defendant can be convicted only if all jurors vote guilty was affirmed in a 1972 Supreme Court ruling, which found the Sixth Amendment to the Constitution requires unanimity in federal courts.
That ruling didn't extend to state courts. As a result, Oregon and Louisiana allow most felony convictions on juror votes of 10-2 or 11-1. Courts in the other 48 states require unanimity for felony convictions.
The requirement puts a serious line of defense between the accused and the government, with its vast resources, said Thaddeus Hoffmeister, a law professor at the University of Dayton. "You want to stack the deck a little against the government," he said.
The difficulty in achieving a unanimous verdict also serves a purpose. "There's evidence that those juries deliberate longer, lead to discussions that might not otherwise take place," said Sherry Colb, a law professor at Cornell.
But there are limits to how long a judge will make a deadlocked jury sit before a mistrial is called. "Judges have discretion," said Ms. Colb, "but that discretion can be abused. An overly aggressive judge can turn a deliberation into a hostage situation."