An adversary system is a legal system in which cases brought before the court are presented by two opposing sides before a neutral panel of individuals which can include a judge and jury. Once both sides have argued their cases, the panel determines the facts of the case and any appropriate actions which need to be taken. This is in contrast with an inquisitorial system, in which cases brought to court are tried in front of a panel of people who act as questioners in addition to deciders. Both systems have advantages and disadvantages and there has been much debate about whether or not one is superior to the other.
Common law nations commonly use an adversary system, and the roots of this system are quite ancient. By contrast, nations which rely on civil law tend to be more likely to utilize an inquisitorial system, due to differing approaches in legal style, and this legal system is also quite venerable. Some nations utilize elements of both systems.
Adversarial systems, as they are also known, are sometimes criticized for setting up a system in which sides on a case are obliged to contest with each other. Some critics believe that the adversary system encourages deception and a variety of questionable legal tactics because the goal is to win at all costs, in contrast with an inquisitorial system which relies on a factfinding panel to determine the truth.
In an adversary system both sides are allowed to present evidence and witnesses to support their positions. The opposing side can cross examine witnesses, analyze the evidence independently, and challenge arguments made before the court. The goal of this process is to present all of the facts of the case for the benefit of the judge and jury, who can sift through the material to decide what happened and who, if anyone, should be held responsible.
The judge and jury are expected to remain impartial and are chosen in part using criteria which are designed to eliminate people who might have a bias in the case. The idea is that by presenting the contest to people without an interest in the outcome, people can receive fair trials because the facts will be evaluated objectively. In actuality, the situation in an adversary system can be much more complicated, and lawyers on both sides in the adversary system can use rhetorical and legal tricks to influence opinion, which may in turn have an impact on the outcome of the trial.