|
The law of evidence governs the use of testimony (eg. oral or written statements, such as an affidavit) and exhibits (eg. physical objects) or other documentary material which is admissible (ie. allowed to be considered by the trier of fact, such as jury) in a judicial or administrative proceeding (eg. a court of law).
Relevance and related social policy concerns
In every jurisdiction, evidence must conform to a number of rules and restrictions in order to be admissible. Evidence must be relevant - that is, it must have a propensity to show that a fact at issue in the proceeding is more or less likely. In common law jurisdictions, relevant evidence may still be excluded if it is unfairly prejudicial, confusing, or cumulative. Social policies also operate to exclude relevant evidence. For example, many jurisdictions prohibit the introduction of evidence of liability insurance, subsequent remedial measures, settlement offers in civil trials, or plea negotiations in criminal trials, because courts do not wish to discourage parties from carrying insurance, fixing hazardous conditions, offering to settle, or pleading guilty to crimes.
Authentication
Certain kinds of evidence, such as documentary evidence, may be subject to further restrictions such as the best evidence rule, which requires certain documents to be produced unless they can shown to be unavailable.
Witnesses
In common law systems, all evidence must be introduced by or through a witness, who has been sworn under oath. The law of evidence governs the direct examination and cross-examination of witnesses, as well as the various issues of privileges and competence.
A privilege, in the law of evidence, refers to a situation where persons can refuse to testify based on the confidential relationship in which the communication took place. The most commonly invoked are the spousal privilege, attorney-client privilege, doctor-patient privilege, and priest-penitent privilege, although various jurisdictions recognize other such privileges.
Competence covers situations where persons can be prohibited from testifying based on their status, irrespective of any expectation of confidentiality within the relationship. For example, a lay person is not competent to testify on matters that require an expert witness; neither a judge nor a juror is competent to testify in a trial in which they have served in that capacity; and in jurisdictions with a dead man statute, a person is deemed not competent to testify as to statements of a deceased opposing party.
Hearsay
- Main article: Hearsay
Hearsay is one of the largest and most complex areas of the law of evidence. The underlying rule can be stated briefly - that a statement made by a declarant out of court, and therefore not under oath or subject to cross-examination, should not be admissible as evidence of the truth of that statement. However, under both the Federal Rules of Evidence and the laws of various jurisdictions, there are dozens of exclusions and exceptions to the hearsay rule, some of which hinge on the availability of the declarant.
Burdens of proof
- Main article: Burden of proof
Different types of proceedings require parties to meet different burdens of proof, the typical examples being reasonable doubt, clear and convincing, and preponderance of the evidence. Many jurisdictions have burden-shifting provisions, which require that if one party produces evidence tending to prove a certain point, the burden shifts to the other party to produce superior evidence tending to disprove it.
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.
Evidentiary rules stemming from other areas of law
Some rules that affect the admissibility of evidence are nonetheless considered to belong to other areas of law. These include the exclusionary rule of criminal procedure, which prohibits the admission in a criminal trial of evidence gained by unconstitutional means, and the parol evidence rule of contract law, which prohibits the admission of extrinsic evidence of the contents of a written contract.
Evidence as an area of study
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 - of the 200 multiple choice questions asked in that test, approximately one sixth will be in the area of evidence.
Among Evidence scholars, the study of evidence in recent decades has become broadly interdisciplinary, incorporating insights from psychology, feminism, economics, and probability theory.
One area of particular interest and controversy has been Bayes' Theorem. Bayes’ Theorem is an elementary proposition of probability theory. It provides a way of updating, in light of new information, one’s prior estimate of the probability that a proposition is true. Evidence scholars been interested in its application to their field, either to study the value of rules of evidence or to help determine facts at trial.
Suppose, for example, that the proposition to be proven is that defendant was the source of a hair found at the crime scene. Before learning that the hair was a genetic match for the defendant’s hair, the factfinder believes that the odds are 2 to 1 that the defendant was the source of the hair. If she used Bayes’ Theorem, she could multiply those prior odds by a “likelihood ratio” in order to update her odds after learning that the hair matched the defendant’s hair. The likelihood ratio is a statistic derived by comparing the odds that the evidence (expert testimony of a match) would be found if the defendant was the source with the odds that it would be found if defendant was not the source. If it is ten times more likely that the testimony of a match would occur if the defendant was the source than if not, then the factfinder should multiply her prior odds by ten, giving posterior odds of 20 to one.
Bayesian skeptics have objected to this use of Bayes’ Theorem in litigation on a variety of grounds. These run from jury confusion and computational complexity to the assertion that standard probability theory is not a normatively satisfactory basis for adjudication of rights.
Bayesian enthusiasts have replied on two fronts. First, they have said that whatever its value in litigation, Bayes’ Theorem is valuable in studying evidence rules. For example, it can be used to model relevance. It teaches that the relevance of evidence that a proposition is true depends on how much the evidence changes the prior odds, and that how much it changes the prior odds depends on how likely the evidence would be found (or not) if the proposition were true. These basic insights are also useful in studying individual evidence rules, such as the rule allowing witnesses to be impeached with prior convictions.
Second, they have said that it is practical to use Bayes' theorem in a limited set of circumstances in litigation (such as integrating genetic match evidence with other evidence), and that assertions that probability theory is inappropriate for judicial determinations are nonsensical or inconsistent.
In recent years, the debate has become stagnant. The parties have been talking past each other, and not much is happening at the high-theory level. The most interesting work is in the empirical study of the efficacy of instructions on Bayes’ Theorem in improving jury accuracy.
See also
| 
