The law of evidence governs the use of testimony (e.g., oral or written statements, such as an affidavit) and exhibits (e.g., physical objects) or other documentary material which is admissible (i.e., allowed to be considered by the trier of fact, such as jury) in a judicial or administrative proceeding (e.g., a court of law).
In every jurisdiction based on the English common law tradition, evidence must conform to a number of rules and restrictions to be admissible. Evidence must be relevant that is, it must have a tendency to make a fact at issue in the proceeding be more or less probable than it would be without the evidence.
However, the relevance of evidence is ordinarily a necessary condition but not a sufficient condition for the admissibility of evidence. For example, relevant evidence may be excluded if it is unfairly prejudicial, confusing, or cumulative. Furthermore, a variety of social policies operate to exclude relevant evidence. Thus, there are limitations on the use of evidence of liability insurance, subsequent remedial measures, settlement offers, and plea negotiations, mainly because it is thought that the use of such evidence discourages parties from carrying insurance, fixing hazardous conditions, offering to settle, and pleading guilty to crimes, respectively.
The question of how the relevance or irrelevance of evidence is to be determined has been the subject of a vast amount of discussion in the last 100-200 years. There is now a consensus among legal scholars and judges in the U.S. that the relevance or irrelevance of evidence cannot be determined by syllogistic reasoning if-then logic alone. There is also general agreement that assessment of relevance or irrelevance involves or requires judgments about probabilities or uncertainties. Beyond that, there is little agreement. Many legal scholars and judges agree that ordinary reasoning, or common sense reasoning, plays an important role. There is less agreement about whether or not judgments of relevance or irrelevance are defensible only if the reasoning that supports such judgments is made fully explicit. However, most trial judges would reject any such requirement and would say that some judgments can and must rest in part on unarticulated and unarticulable hunches and intuitions. However, there is general (though implicit) agreement that the relevance of at least some types of expert evidence particularly evidence from the hard sciences requires particularly rigorous, or in any event more arcane reasoning than is usually needed or expected. There is a general agreement that judgments of relevance are largely within the discretion of the trial court although relevance rulings that lead to the exclusion of evidence are more likely to be reversed on appeal than are relevance rulings that lead to the admission of evidence.
Under the Federal Rules of Evidence (FRE) Rule 401: "Relevant evidence" means evidence having any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence.
Federal Rule 403 allows relevant evidence to be excluded if its probative value is substantially outweighed by danger of unfair prejudice, confusing or misleading the jury or waste of the court's time. California Evidence Code §352 also allows for exclusion to avoid "substantial danger of undue prejudice."
Some legal experts, notably Stanford legal historian Lawrence Friedman, have argued that the complexity of American evidence law arises from two factors: (1) the right of American defendants to have findings of fact made by a jury in practically all criminal cases as well as many civil cases; and (2) the widespread consensus that tight limitations on the admissibility of evidence are necessary to prevent a jury of untrained laypersons from being swayed by irrelevant distractions. In Professor Friedman's words: "A trained judge would not need all these rules; and indeed, the law of evidence in systems that lack a jury is short, sweet, and clear.
However, some respected observers disagree with the commonplace thesis that the institution of trial by jury is the main reason for the existence of rules of evidence even in countries such as the United States and Australia; they argue that other variables are at work.
Evidence of a confession may be excluded because it was obtained by oppression or because the confession was made in consequence of anything said or done to the defendant that would be likely to make the confession unreliable. In these circumstances, it would be open to the trial judge to exclude the evidence of the confession under Section 78(1) of the Police and Criminal Evidence Act 1984 (PACE), or under Section 73 PACE, or under common law, although in practice the confession would be excluded under section 76 PACE.
Other admissible evidence may be excluded, at the discretion of the trial judge under 78 PACE, or at common law, if the judge can be persuaded that having regard to all the circumstances including how the evidence was obtained “admission of the evidence would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it.
Today all persons are presumed to be qualified to serve as witnesses in trials and other legal proceedings, and all persons are also presumed to have a legal obligation to serve as witnesses if their testimony is sought. However, legal rules sometimes exempt people from the obligation to give evidence and legal rules disqualify people from serving as witnesses under some circumstances.
Privilege rules give the holder of the privilege a right to prevent a witness from giving testimony. These privileges are ordinarily (but not always) designed to protect socially valued types of confidential communications. Some of the privileges that are often recognized are the marital secrets privilege, the adverse spousal testimony privilege, the attorney-client privilege, the doctor-patient privilege, the psychotherapist-patient and counselor-patient privilege, the state secrets privilege and the clergy-penitent privilege. A variety of additional privileges are recognized in different jurisdictions, but the list of recognized privileges varies from jurisdiction to jurisdiction; for example, some jurisdictions recognize a social worker-client privilege and other jurisdictions do not.
Witness competence rules are legal rules that specify circumstances under which persons are ineligible to serve as witnesses. For example, neither a judge nor a juror is competent to testify in a trial in which they are serving in that capacity; and in jurisdictions with a dead man statute, a person is deemed not competent to testify as to statements of or transactions with a deceased opposing party.
Some people believe that all evidence is circumstantial because some observers think (and some thoughtful judges agree) no evidence ever directly proves a fact.
One special category of information in this area includes things of which the court may take judicial notice. This category covers matters that are so well known that the court may deem them proven without the introduction of any evidence. For example, if a defendant is alleged to have illegally transported goods across a state line by driving them from Boston to Los Angeles, the court may take judicial notice of the fact that it is impossible to drive from Boston to Los Angeles without crossing a number of state lines. In a civil case, where the court takes judicial notice of the fact, that fact is deemed conclusively proven. In a criminal case, however, the defense may always submit evidence to rebut a point for which judicial notice has been taken.
Nevertheless, because of its importance to the practice of law, all American law schools offer a course in evidence, and most require the subject either as a first year class, or as an upper-level class, or as a prerequisite to later courses. Furthermore, evidence is heavily tested on the Multistate Bar Examination ("MBE") - of the 200 multiple choice questions asked in that test, approximately one sixth will be in the area of evidence. The MBE predominantly tests evidence under the Federal Rules of Evidence, giving little attention to matters for which state law is likely to be inconsistent.