An expert witness is a witness, who by virtue of education, or profession, or experience, is believed to have special knowledge of his subject beyond that of the average person, sufficient that others may officially (and legally) rely his opinion.
A simplified example
You are the owner of a very successful car factory. You are proud with the quality of your cars. One day you find yourself in a courtroom because hundreds of your cars are found with faulty brakes. What do you do? You hire a good mechanic to tell the jury that your brakes are actually good. That mechanic is your expert witness.
Experts in the real world
Typically, experts are relied on by both sides to a dispute for opinions on severity of injury, degree of insanity, cause of failure in a machine or other device, and the like.
The tribunal itself, or the judge, can in some systems call upon experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge on the fact/action it is judging. The expertise has the legal value of an acquisition of data. The results of these experts are then compared to those by the experts of the parties.
The expert has heavy responsibility, especially in penal trials, and false witness by an expert is a severely punished crime in most countries. The use of expert witnesses is sometimes criticized in the United States because in civil trials, they are often used by both sides to advocate differing positions, and it is left up to a jury of laymen to decide which expert witness to believe. Sometimes one side has utilized an expert witness to provide fraudulant or junk science testimony in order to convince a jury.
In most systems, the trial (or the procedure) can be suspended in order to allow the experts to study the case and produce their results.
The earliest known use of an expert witness in English law came in the 1782, when a court that was hearing litigation relating to the silting-up of Wells harbour in Norfolk accepted evidence from a leading civil engineer, John Smeaton. This decision by the court to accept Smeaton's evidence is widely cited as the root of modern rules on expert evidence.
In the U.S., a party can hire experts to help him/her evaluate the case. For example, a car maker may hire an experienced mechanic to decide if its cars were built to specification. This kind of expert opinion will be protected from discovery. If the expert finds something that's against its client, the opposite party will not know it. Therefore, the expert will not be forced to lie. This privlege is similar to the work product protected by the attorney/client privilege.
If the witness needs to testify in the court, the privilege is no longer protected. The expert witness's identity and nearly all documents used to prepare the testament will become discoverable. Usually an experienced lawyer will advise the expert from taking notes on documents otherwise all of the notes will be available to the other party.
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