Excepting cases that may be decided by applying legal rules to undisputed facts, the determination of disputed factual propositions must be central to adjudicating the rights and liabilities of litigants. As an initial matter, a society might adopt an "inquisitorial" system, under which a public official investigates and decides the facts. In the Anglo-American legal tradition, however, we structure the litigation process so that every dispute has at least two parties, each charged with the primary responsibility for proving its factual propositions and therefore discovering and presenting the evidence to support its version of the facts before an impartial arbiter.
In criminal cases, this adversary system is reinforced by rules that place the BURDEN OF PROOF on the prosecution, presuming that the defendant is innocent, and that grant the defendant a RIGHT AGAINST SELF-INCRIMINATION?thus shielding him from being forced to be a witness against himself, and depriving the prosecution of an obvious source of evidence. The structuring of criminal litigation as a contest between the state as prosecutor and the defendant?with the judge as arbiter?has two major consequences. First, this procedure gives greater weight to the autonomy of the individual litigant. Second, placing responsibility on each party to advance its own cause will, in general, result in the production of more evidence for the finder of fact than would be produced by disinterested?and perhaps bored and overworked?public officials. Though our prototypical case is the criminal case, we use similar procedures and rules in civil cases.
In both civil and criminal cases, TRIAL BY JURY means that a group of laymen decides issues of disputed fact. A great many of the intricacies of our laws of evidence result from two specific worries about the jury. The first is that the jury may systematically overvalue or undervalue some kinds of evidence, such as HEARSAY. The second is that the ad hoc nature of the jury, which is empaneled to decide a particular case, will produce a verdict at odds with the values of a legal system handling many cases over a long period of time. Often a rule of evidence will keep out testimony not so much because a jury might overweigh it but simply because other policies of the law are entitled to equal weight along with the proper resolution of factual issues. In this category fall the exclusion of reliable evidence because it has been unconstitutionally seized; because it has been obtained in violation of the MIRANDA RULES; because it is a coerced confession (which, though typically unreliable, may in a particular case be thoroughly corroborated); or because its exclusion is necessary to enforce a privilege, such as that protecting confidential communications between the attorney and the client.
Nor is the exclusion of evidence confined to cases where we choose this means of vindicating the rights of the individual. Though it is by no means clear that the rule is of constitutional dimension, every Anglo-American JURISDICTION in civil and (until the passage of California's "Victims' Bill of Rights" initiative) in criminal cases kept from the jury certain evidence of the prior character of the accused?not so much because the jury might over-value it as out of fear that the jury might succumb to the temptation to be lawless and decide that the defendant was either so bad a person that he should be punished regardless of his fault in the particular case at issue. That kind of jury behavior might appeal to common sense, but it would be at odds with our principles requiring a particular act as a precondition of guilt and requiring fair NOTICE of the charge made against a defendant.
Despite the huge body of statutory and COMMON LAW evidence law, the Constitution nowhere states flatly a rule as to admissibility of evidence...