What Type of Evidence Do Expert Witnesses Give?
Expert witnesses are called to present testimony when a disputed issue involves analysis beyond the knowledge of the average juror. Generally, expert witness testimony is subject to the Federal Rules of Evidence. States have their own rules of evidence; however, these rules differ only slightly from the Federal Rules of Evidence where expert testimony is concerned.
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One issue connected to expert witness testimony concerns determining what topics are appropriate. Because there's a risk of jurors giving great deference to a witness with significant professional qualifications, parties are only permitted to proffer expert testimony if a court is persuaded that the testimony will be helpful to jurors. Rule 702 of the Federal Rules of Evidence explains that a person with "specialized knowledge" may testify in "the form of an opinion" if the witness qualifies as an expert and if the testimony will help jurors understand the evidence in a particular case.
Types of Evidence
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Expert testimony on rape trauma syndrome is typically admissible. An expert might testify that the behavior of an alleged rape victim is consistent with behavior of other rape victims. In criminal trials, medical examiners and forensic experts may testify about cause of death and blood castoff patterns. Courts may allow experts to offer testimony describing a character profile. However, an expert witness usually can't say whether the defendant matches a particular profile; rather, jurors are to decide for themselves whether the defendant matches the profile.
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Types of Data
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Rule 703 requires expert testimony to be based on "sufficient facts or data." Expert witnesses may testify as to data that other experts in the same field reasonably rely upon. For example, a medical expert might base her testimony on a particular type of X-ray if other medical experts generally consider the same type of X-ray to be reliable. Expert witnesses may also give testimony on DNA and fingerprint evidence, as both types of data are frequently relied upon by medical experts.
Reliability
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Expert testimony based on medical or scientific principles must pass what's called the Frye test. The Frye test stands for the proposition that theories and methodology must be generally accepted and well-established in the scientific community. Experimental practices or practices considered to be on the fringe of scientific reliability typically don't pass the Frye test. Examples of experimental practices that are generally inadmissible include results of polygraph tests and ear print identification.
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