Also known as: skilled witnesses · expert witness
Written by attorneys · grounded in primary & secondary sources — see below
A witness who possesses specialized knowledge, skill, experience, training, or education that qualifies the witness to offer opinion testimony on matters beyond the understanding of ordinary jurors. The testimony must rest on sufficient facts or data, employ reliable principles and methods, and reflect a reliable application of those principles and methods to the facts of the case.
In a federal prosecution for wire fraud, the government calls a forensic accountant to analyze ledgers showing fund transfers. The accountant describes the timing and amounts of the transfers but stops short of stating whether the defendant possessed the intent to defraud. The court admits the factual analysis while excluding any opinion on the defendant's mental state.
A state forensic analyst testifies that a DNA profile generated by an outside laboratory matches a profile produced from the defendant's blood sample. The analyst explains that the outside report served only as the foundation for her independent comparison and was not offered to prove the truth of its contents. The court permits the testimony over a confrontation objection.
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During cross-examination of a retained toxicologist, defense counsel directs the witness to a passage in a peer-reviewed journal article on chemical exposure thresholds. The toxicologist acknowledges the article as a reliable authority and confirms that the passage contradicts part of her direct testimony. The court allows the passage to be read into the record.
A criminal defendant offers a psychologist to testify that a novel lie-detection technique establishes the defendant's truthfulness. The prosecution objects that the technique has not gained general acceptance in the scientific community. The court excludes the testimony after finding the method lacks the requisite acceptance among experts in the field.
Property owners retain an economist to calculate the economic impact of a rent-control ordinance on their apartment buildings. The economist compares pre- and post-ordinance cash flows and concludes that the regulation eliminates all reasonable investment-backed expectations. The court admits the calculations as evidence bearing on the takings claim.
An indigent defendant charged with capital murder requests funds to retain a psychiatrist to evaluate his mental condition at the time of the offense. The trial court denies the request. On appeal the Supreme Court holds that the denial violated due process because the expert assistance was necessary to prepare an effective defense.
The witness must demonstrate knowledge, skill, experience, training, or education that will help the trier of fact understand the evidence or determine a fact in issue. The proponent must also show that the testimony rests on sufficient facts or data, employs reliable principles and methods, and reflects a reliable application of those methods to the case facts.
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No. In a criminal prosecution an expert may not state an opinion on whether the defendant possessed a mental state or condition that constitutes an element of the crime or a defense. Such determinations remain exclusively for the jury.
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On a party's motion or on its own initiative, a court may order the parties to show cause why a neutral expert should not be appointed. The court may select any expert who consents to serve and who the parties agree upon or whom the court chooses.
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Not necessarily. An expert may rely on a DNA profile produced by an outside laboratory when the profile is used only to explain the basis of the expert's independent opinion and is not offered for its truth, provided the report was not prepared primarily to accuse a targeted individual.
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…that the steering mechanism was broken. He said that the steering post was broken and that the steering gear was broken. An expert witness testified for the plaintiffs that he examined the car after the accident and found that the steering gear was broken. He said that the break was due to a defect in the manufacture of the…