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|Part of the common law series|
|Types of evidence|
|Hearsay and exceptions|
|Other common law areas|
A witness is someone who has, who claims to have, or is thought, by someone with authority to compel testimony, to have knowledge relevant to an event or other matter of interest. In law a witness is someone who, either voluntarily or under compulsion, provides testimonial evidence, either oral or written, of what he or she knows or claims to know about the matter before some official authorized to take such testimony.
A percipient witness or eyewitness is one who testifies what they perceived through his or her senses (e.g. seeing, hearing, smelling, touching). That perception might be either with the unaided human sense or with the aid of an instrument, e.g., microscope or stethoscope, or by other scientific means, e.g.,a chemical reagent which changes color in the presence of a particular substance.
A hearsay witness is one who testifies what someone else said or wrote. In most court proceedings there are many limitations on when hearsay evidence is admissible. Such limitations do not apply to grand jury investigations, many administrative proceedings, and may not apply to declarations used in support of an arrest or search warrant. Also some types of statements are not deemed to be hearsay and are not subject to such limitations.
An expert witness is one who allegedly has specialized knowledge relevant to the matter of interest, which knowledge purportedly helps to either make sense of other evidence, including other testimony, documentary evidence or physical evidence (e.g., a fingerprint). An expert witness may or may not also be a percipient witness, as in a doctor or may or may not have treated the victim of an accident or crime.
A reputation witness is one who testifies about the reputation of a person or business entity, when reputation is material to the dispute at issue.
In law a witness might be compelled to provide testimony in court, before a grand jury, before an administrative tribunal, before a deposition officer, or in a variety of other proceedings (e.g., judgment debtor examination). Sometimes the testimony is provided in public or in a confidential setting (e.g., grand jury or closed court proceeding).
Although informally a witness includes whoever perceived the event, in law, a witness is different from an informant. A confidential informant is someone who claimed to have witnessed an event or have hearsay information, but whose identity is being withheld from at least one party (typically the criminal defendant). The information from the confidential informant may have been used by a police officer or other official acting as a hearsay witness to obtain a search warrant.
A subpoena commands a person to appear. It is used to compel the testimony of a witness in a trial. Usually, it can be issued by a judge or by the lawyer representing the plaintiff or the defendant in a civil trial or by the prosecutor or the defense attorney in a criminal proceeding. In many jurisdictions, it is compulsory to comply, to take an oath, and to tell the truth, under penalty of perjury.
Court procedure 
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Calling a witness 
In a court proceeding, a witness may be called (requested to testify) by either the prosecution or the defense. The side that calls the witness first asks questions, in what is called direct examination. The opposing side then may ask their own questions in what is called cross-examination. In some cases, redirect examination may then be used by the side that called the witness, but usually only to contradict specific testimony from the cross-examination.
Recalling a witness means calling a witness, who has already given testimony in a proceeding, to give further testimony. A court may only give leave to a party to recall a witness to give evidence about a matter adduced by another party if the second party's testimony contradicts evidence given by the original witness on direct examination.
Witness are usually only permitted to testify to what they experienced first hand. In most cases, they may not testify about something they were told (hearsay). This restriction does not apply to expert witnesses. Expert witnesses, however, may only testify in the area of their expertise.
Eyewitness testimony is generally presumed to be more reliable than circumstantial evidence. Studies have shown, however, that individual, separate witness testimony is often flawed, and parts of it can be meaningless. This can occur because of flaws in Eyewitness identification (such as faulty observation and recollection, or bias), or because a witness is lying. If several people witness a crime, it is probative to look for similarities in their collective descriptions to substantiate the facts of an event, keeping in mind the contrasts between individual descriptions.
One study involved an experiment, in which subjects acted as jurors in a criminal case. Jurors heard a description of a robbery-murder, then a prosecution argument, and then an argument for the defense. Some jurors heard only circumstantial evidence; others heard from a clerk who claimed to identify the defendant. In the first case, 18% percent found the defendant guilty, but in the second, 72% found the defendant guilty (Loftus 1988).
Police lineups, where the eyewitness picks out a suspect from a group of people in the police station, are often grossly suggestive, and give the false impression that the witness remembered the suspect. In another study, students watched a staged crime. An hour later they looked through photos. A week later they were asked to pick the suspect out of lineups. 8% of the people in the lineups were mistakenly identified as criminals. 20% of the innocent people whose photographs were included were mistakenly identified (University of Nebraska 1977).
Another study looked at sixty-five cases of "erroneous criminal convictions of innocent people." In 45% of the cases, eyewitness mistakes were responsible (Borchard p. 367).
The formal study of eyewitness memory is usually undertaken within the broader category of cognitive processes — the different ways in which we make sense of the world around us. We do this by employing the mental skills at our disposal such as thinking, perception, memory, awareness, reasoning and judgment. Although cognitive processes can only be inferred and cannot be seen directly, they all have very important practical implications within a legal context.
If one were to accept that the way we think, perceive, reason and judge is not always perfect, then it becomes easier to understand why cognitive processes and the factors influencing these processes are studied by psychologists in matters of law; not least because of the grave implications that this imperfection can have within the criminal justice system.
The study of witness memory has dominated this realm of investigation. As Huff and Rattner[who?] note: the single most important factor contributing to wrongful conviction is eyewitness misidentification.
Credibility of a witness 
Several factors affect witnesses' credibility. Generally, a witness is deemed to be credible if they are recognized (or can be recognized) as a source of reliable information about someone, an event, or a phenomenon. As an example, the 2009 arrest of an illegal immigrant from El Salvador in the murder of federal intern Chandra Levy saw many questions arise surrounding the credibility of various witnesses. Contesting the credibility of so-called "expert" witnesses rose into more common practice in the 1860s and 1870s.
See also 
- Courthouse dog
- Eyewitness identification
- Eyewitness memory
- Martyr (the word originally meant witness)
- Material witness
- United States Marshals Service
- Witness protection
- Garraghan, Gilbert J. (1946). A Guide to Historical Method. New York: Fordham University Press. ISBN 0-8371-7132-6.
- Gottschalk, Louis (1950). Understanding History: A Primer of Historical Method. New York: Alfred A. Knopf. ISBN 0-394-30215-X.
- Johnson, M. K. (2001). False Memories, Psychology of. IN: Smelser, N. J. & Baltes, P. B. (eds.) International Encyclopedia of the Social and Behavioral Sciences. Amsterdam: Elsevier. (pp. 5254–5259).
- Lakatos, I. (1970). Falsification and the methodology of scientific research programmes. In: Lakatos, I. & Musgrave, A. E. (eds.), Criticism and the Growth of Knowledge. Cambridge, UK: Cambridge University Press: 59-89.
- Loftus, Elizabeth F. (1996). Eyewitness Testimony. Revised edition. Cambridge, MA: Harward University Press. (Original edition: 1979).
- Read, J. D. (2001). Eyewitness Memory: Psychological Aspects. IN: Smelser, N. J. & Baltes, P. B. (eds.) International Encyclopedia of the Social and Behavioral Sciences. Amsterdam: Elsevier. (pp. 5217–5221).
- Roediger III, H. L. (2001). Reconstructive Memory, Psychology of. IN: Smelser, N. J. & Baltes, P. B. (eds.) International Encyclopedia of the Social and Behavioral Sciences. Amsterdam: Elsevier. 12844-12849.
- Ross D F, Read J D, Toglia M P (1994) Adult Eyewitness Testimony: Current Trends and Developments. New York: Cambridge University Press.
- Shepherd J W, Ellis H D, Davies G M (1982). Identification Evidence: A Psychological Evaluation. Aberdeen University Press, Aberdeen, UK
- Thompson C P, Herrmann D, Read J D, Bruce D, Payne D G, Toglia, M P (1998). Eyewitness Memory: Theoretical and Applied Perspective. Mahwah, NJ: Erlbaum.
- "Law and Police". Otago Daily Times. 18 September 1865. Retrieved 30 October 2011. "A strong effort was made to impeach her credibility as a witness... it is competent to prove that the witness is an expert and not a mere pretender."
- "Last Day of the Scandal Trial". Ithaca Democrat. 8 July 1875. Retrieved 30 October 2011. "There was an irreconcilable difference of opinion as to the credibility of witnesses on each side."
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