The Purpose of a Jury – Law 200 Level Course
The purpose of trial by jury, as the Supreme Court itself has noted, is to prevent “oppression by the government.” To perform that role, jurors must act independently and conscientiously, and they must be prepared to “just say no” if they believe that a conviction would be unjust. Nothing else satisfies the purpose of trial by jury, or provides the protection to liberty that the Founders intended to provide in our Bill of Rights. http://www.cato.org/dailys/12-09-98.html
The jury are also to judge whether the laws are rightly expounded to them by the court. Unless they judge on this point, they do nothing to protect their liberties against the oppressions that are capable of being practiced under cover of a corrupt exposition of the laws. If the judiciary can authoritatively dictate to a jury any exposition of the law, they can dictate to them the law itself, and such laws as they please; because laws are, in practice, one thing or another, according as they are expounded.
They must also judge whether there really be any such law, (be it good or bad,) as the accused is charged with having transgressed. Unless they judge on this point, the people are liable to have their liberties taken from them by brute force, without any law at all.
The jury must also judge of the laws of evidence. If the government can dictate to the jury the laws of evidence, it can not only shut out any evidence it pleases, tending to vindicate the accused, but it can require that any evidence whatever, that it pleases to offer, be held as conclusive proof of any offense whatever which the government chooses to allege.
Criminal and civil juries reflect the differences between the criminal and civil law, and most cases in the United States are settled before they go to trial. In cases that go to trial, a person does not have the right to a jury trial in every instance. Finally, an accused person has the right to a trial by jury but may waive that right in favor of a bench trial before a judge; in civil trials, both parties can agree to a bench trial. Thus a jury trial generally takes place only in those instances where one side in a civil case, or the accused, in a criminal case, believes it is in their best interest. The right to trial by jury, however, influences even the resolution of cases that never go to trial, “like the visible cap of an iceberg, expos[ing] but a fraction of its true volume” (Kalven and Zeisel, 32). http://www.crfc.org/americanjury/introduction.html