Many criminal and civil investigations turn to expert witnesses in court to help resolve cases where the facts are unclear or in need of some explanation. An expert witness is a person who can provide information and opinion drawn from the body of knowledge that makes up their own area of expertise. An expert witness does not need to be legally qualified. Commonly the witness will be a forensic scientist or forensic pathologist providing information outside the realm of common knowledge about the circumstances of a crime. For instance, a scientist with knowledge of paint analysis or soil can help interpret trace evidence . A pathologist can help with the difficult questions such as the cause of death when a body is recovered from water or what bloodstain evidence might really mean in terms of how someone was killed.
Being an expert witness is not a profession in its own right. The expert witness is created and recognized as such by the judge and the court. There are many databases of people willing to act as expert witnesses in various specialties, and they also have their own professional organizations to represent their interests. The expert witness will have been vetted for suitability; they will also have undergone training in court procedures so that evidence may be given to the best of their ability to help the judge and jury come to a decision. Although many expert witnesses come from a forensic science or medical discipline, they can be drawn from any area of expertise, depending on the circumstances and background of the case. Document examiners and structural engineers may be called in to advise and give evidence, as may those from non-scientific disciplines as diverse as art history, mountaineering, or martial arts.
Either prosecution or the defense may call in an expert witness. He or she is expected to look at the evidence relevant to their discipline and put it in the context of the whole case. They will produce a report that can be taken up into the witness stand. First of all, the party who engaged the expert witness will ask questions that prove identity, experience, and background to the court. Then they will ask questions that generally take the court through the expert witness' report.
The expert witness can expect to be cross-examined by the opposing counsel. That is, an expert witness for the prosecution will be questioned by lawyers for the defense, and vice-versa. The other side may also engage their own expert witness. The purpose of cross-examination is to find flaws in the evidence and conclusions presented by the expert witness. Many people are experts in their own subjects, but it takes special skill and training to defend one's findings in public while still remaining objective and impartial.
The expert witness has a large responsibility, as the manner of giving evidence can influence the judge and jury—the decision makers in the case. Expert testimony is especially important when other evidence is insufficient to help come to a clear verdict. At the same time, in a stalemate situation, the expert witness must remain completely unbiased. Expert witnesses have a duty to give concise, detailed, and clear answers to all the questions put to them. After all, the jury often consists of lay people who may have had little or no experience in medicine , building construction, or the many other subjects that are often relevant in a crime. The judge is, naturally, more knowledgeable about crime and its circumstances, from his or her experience. However, there will be cases in which the judge also needs more information about what may have occurred and how. The expert witness performs a vital role in helping judge and jury come to the right conclusions about what really happened at a crime scene.
The expert witness is the only one in court who is allowed to give opinion as well as facts. This is because the court has confidence in the facts and knowledge on which the opinion is based. Thus, the forensic psychiatrist is allowed to say, "I believe this man to be capable of this murder , with this degree of violence, based upon my assessment of his mental state." With other evidence, that opinion may be what is required for judge and jury to make up their minds in the case.
see also Evidence.