The law of evidence, also known as the rules of evidence, encompasses the rules and legal principles that govern the proof of facts in a legal proceeding. These rules determine what evidence must or must not be considered by the trier of fact in reaching its decision. The trier of fact is a judge in bench trials, or the jury in any cases involving a jury.[1] The law of evidence is also concerned with the quantum (amount), quality, and type of proof needed to prevail in litigation. The rules vary depending upon whether the venue is a criminal court, civil court, or family court, and they vary by jurisdiction.
The quantum of evidence is the amount of evidence needed; the quality of proof is how reliable such evidence should be considered. Important rules that govern admissibility concern hearsay, authentication, relevance, privilege, witnesses, opinions, expert testimony, identification and rules of physical evidence. There are various standards of evidence, standards showing how strong the evidence must be to meet the legal burden of proof in a given situation, ranging from reasonable suspicion to preponderance of the evidence, clear and convincing evidence, or beyond a reasonable doubt.
There are several types of evidence, depending on the form or source. Evidence governs the use of testimony (e.g., oral or written statements, such as an affidavit), exhibits (e.g., physical objects), documentary material, or demonstrative evidence, which are 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).
When a dispute, whether relating to a civil or criminal matter, reaches the court there will always be a number of issues which one party will have to prove in order to persuade the court to find in their favour. The law must ensure certain guidelines are set out in order to ensure that evidence presented to the court can be regarded as trustworthy.
The Old Testament demanded at least two witnesses for conviction of a crime.[2][3]
Ancient Roman law allowed freedom to judges to evaluate evidence, but insisted that "proof is incumbent on the party who affirms a fact, not on him who denies it" and "no-one should be convicted on suspicion".[4] Medieval Roman law developed an elaborate grading of degrees of evidence. Building on the Biblical two-witness rule, it concluded that a single witness, or private documents, could constitute half-proof, which though insufficient for conviction might justify torture to extract further evidence.[4]: 26–7, 59 Because evidence in the continental (civil law) system was evaluated by judges rather than juries, that system did not develop exclusionary rules of evidence in the way English law did.
A distinct feature of English common law historically was the role of the jury as a finder of fact, as opposed to the role of the judge as finder of law.[5] The creation of modern jury trials in the 16th and 17th centuries necessitated rules of evidence to regulate what testimony and other evidence could be put before the jury.[6] While much of the early common law evidence rules came from judicial decisions, the English Parliament also played a role. In 1677, Parliament and the Crown enacted the Statute of Frauds and Perjuries, prohibiting plaintiffs from alleging certain contractual breaches to the jury unless accompanied by a signed, written instrument. Another early evidence rule was the prohibition on hearsay, the admission of an out-of-court statement to prove the truth of what is asserted. In the early 19th Century, Chief Justice Lord Mansfield of the Court of Common Pleas stated:
"In Scotland and most of the continental states, the judges determine upon the facts in dispute as well as upon the law; and they think there is no danger in their listening to evidence of hearsay, because, when they come to consider their judgment on the merits of the case, they can trust themselves entirely to disregard the hearsay evidence, or to give it any little weight which it may seem to deserve. But in England, where the jury are the sole judges of the fact, hearsay evidence is properly excluded, because no man can tell what effect it might have upon their minds."[7]
Hearsay rules have subsequently been updated numerous times. Most recently in England and Wales, the Civil Evidence Act 1995, section 1, specifically allows for admission of 'hearsay' evidence; legislation also allows for 'hearsay' evidence to be used in criminal proceedings, which makes it possible for the accuser to induce friends or family to give false evidence in support of their accusations because, normally, it would be rejected by the presiding authority or judge. There are several examples where presiding authorities are not bound by the rules of evidence. These include the military tribunals in the United States and tribunals used in Australia to try health professionals.
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 be directed at proving or disproving a legal element.
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 the relevance or irrelevance of evidence cannot be determined by logical analysis. There is also general agreement that assessment of relevance or irrelevance involves or requires judgements 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 judgements of relevance or irrelevance are defensible only if the reasoning that supports such judgements is made fully explicit. However, most trial judges would reject any such requirement and would say that some judgements can and must rest partly 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.
According to Rule 401 of the Federal Rules of Evidence (FRE), evidence is relevant if it has the "tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable th