In Pennsylvania, a jury in a criminal trial consists of 12 citizens drawn from the county where the charges are filed, unless it is a high profile case, in which case the jurors may be brought in from another county. These 12 jurors must reach a unanimous decision when deciding whether a defendant is guilty or not guilty of a particular offense. If the jury cannot reach a unanimous decision, and is "hopelessly deadlocked," then the jury is said to be "hung." At that point, the Court of Common Pleas judge declares a mistrial.
Judges will do everything in their power to avoid a hung jury. When a case goes to trial, the lawyers on each side have spent numerous hours preparing their cases. The county has also gone to great expense to pay for the trial. Thus, when the jury foreman tells the judge's tipstaff that the jury is hopelessly deadlocked, the judge will read them some standard jury instructions, designed to encourage them to continue to deliberate until they have reached a unanimous decision. In my experience, in the vast majority of cases where it initially appears that the jury may be deadlocked, the jury ultimately comes around with unanimous verdicts. Often, this is achieved through compromises. For example, some jurors may want convictions for all counts, while other jurors want acquittals on all counts. They may compromise by finding the defendant not guilty of the most serious charges, but guilty of less serious charges. When mixed verdicts are logically inconsistent based upon the facts of a case, then it is quite obvious that they were compromised verdicts.
In the rare cases where a judge declares a mistrial based upon a hung jury, the prosecutor must decide how to proceed. He could opt to retry the case with a new jury, or he could simply decide to nolle pross (drop) the charges. Often, the prosecutor's decision is based upon the breakdown of the jury's votes. For example, if the jury was 11-1 in favor of conviction, the prosecutor will feel confident that he can achieve a conviction at the next trial. On the flip side, if the jury was 11-1 in favor of acquittal, a prudent prosecutor will do whatever he can to make the case go away.
I only have had one hung jury in my long trial career, and it was in a "theft of cable services" case, of all things. The jury was 11-1 in favor of acquittal, but unfortunately, the jury had one obstinate curmudgeon, who refused to vote for acquittal, despite the fact that a not guilty verdict was the only logical conclusion to draw from the evidence, in my biased opinion. Fortunately, we had an experienced prosecutor on this case who could see the biog picture. He offered to drop the charges if my client paid $40 in restitution to the cable company. She did not even have to pay court costs. We took the deal and the charges were nolle prossed (dropped).
Matt McClenahen is a defense lawyer based in State College, Pennsylvania, with extensive trial experience. He limits his practice to criminal law. http://www.mattmlaw.com/Criminal-Defense-Overview/