In a court of law, standard witnesses are permitted to testify only on matters of fact: what they observed or otherwise know to be true. An expert testimony is given by an individual who has subject matter knowledge beyond that which is typically possessed by the general population. Individuals who give expert testimony are called expert witnesses. Expert witnesses are permitted to offer their professional opinion to the court in the area of expertise of which they have special knowledge, if doing so will assist the jury in its decision-making process.
Although experts in many different fields can be called upon to testify in legal matters, those used most frequently are mental or behavioral health and medical professionals. They typically offer expertise in criminal, civil rights, personal injury, or medical malpractice cases. Attorneys for each side and the judge have access to the reports and credentials of the expert witness before the trial begins, so they can verify the expert's level of expertise and lack of bias in advance of court proceedings.
See also Psychiatry.
Committee on the Judiciary, House of Representatives. Federal Rules of Evidence. Washington, DC: U.S. Government Printing Office, 2010.
Cummings, William K., and Martin J. Finkelstein. Scholars in the Changing American Academy. New York: Springer Science & Business Media, 2012.
Johnson, Molly Treadway, et al. Expert Testimony in Federal Civil Trials: A Preliminary Analysis. Washington, DC: Federal Judicial Center, 2000.
Rix, K.J.B. “Expert evidence and the courts: 1. The history of expert evidence.” Advances in Psychiatric Treatment 5 (1999): 71–77. Available online at http://apt.rcpsych.org/content/aptrcpsych/5/1/71.full.pdf (accessed July 15, 2015).
Cornell University Law School: Legal Information Institute. “Expert Witness.” http://www.law.cornell.edu/wex/expert_witness (accessed July 15, 2015).
American Psychiatric Association, 1000 Wilson Blvd., Ste. 1825, Arlington, VA, 22209, (703) 907-7300, (888) 3577924), firstname.lastname@example.org, http://www.psychiatry.org .