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The History of the Expert Witness in The United States

December 18, 2023
Woman giving expert testimony in court

The need for experts who could explain complicated details to a lay audience first became obvious centuries ago. As with much of the American legal landscape, we imported the initial idea of the expert witness from England.

Experts in English Common Law and Beyond

Forensic experts have been part of the law since the days of the Roman Empire, which recognized handwriting experts, land surveyors, and midwives as legal experts. Experts appeared in English courts as early as the 16th century. Artisans and merchants were presented as neutral advisors, explaining the unique standards of their trades, and physicians and surgeons helped explain evidence in criminal cases.

Early on, English judges themselves directly examined the parties and witnesses. Over time, the role of both prosecutors and defense attorneys increased. Though defense counsel could not initially address the jury, they were able to examine witnesses (developing the first techniques of cross-examination) and influence the evidence given to the jury. As the adversarial roles of prosecutor and defense continued to evolve, both began summoning their own, potentially partisan, experts.

By the late eighteenth century, some standards for witnesses had emerged. The “opinion doctrine” required witnesses to speak factually, rather than inferentially, where possible. The “hearsay doctrine” attempted to limit testimony to that which was personally observed. What we would now call expert witnesses ended up as the exceptions to these rules… They could pronounce an opinion in court without observing the facts of the case personally, though they were still expected to rely exclusively on direct observational data.

The Foundations of Rules Governing Expert Evidence

By the end of the eighteenth century, courts were faced with an important new kind of expert, the “scientists” (though that word wasn’t invented until 1833) who were fueling Britain’s industrial revolution by employing philosophical reasoning and mathematics and trying to uncover the previously hidden laws of nature.

Folkes v. Chadd is widely recognized as the case that established several pieces of groundwork for admitting expert opinion evidence in English common law, including carving out a specific place for scientific thought in the courtroom.

This case arose from the silting up of Wells Harbor, an important tidal waterfront in northern England. After several attempts to rehabilitate the harbor failed, the harbor’s board of commissions took legal action, alleging that the reclamation of land by a large local landowner (Folkes) had caused the harbor failure, and that Folkes’ embankment should be removed.

In a 1781 trial, the commissioners presented a variety of traditional maritime experts to describe their observations of the harbor’s rapid deterioration after the construction of the Folkes embankment. The landlord’s lawyers’ sole expert, Robert Mylne, a famed London-based engineer, explained how the harbor had silted up because of logical, natural causes rather than the introduction of the embankment.

When the jury found for Folkes based on Mylne’s persuasive testimony, the commissioners’ lawyers moved for a new trial alleging that Mylne’s explanations were too theoretical to be consistent with the opinion doctrine and shouldn’t have been permitted. A new trial was set for the following summer. To avoid any further surprises, the judges asked the parties to exchange the opinions of any experts on whom they intended to rely in court.

In the second trial, the harbor commissioners hired several well-known engineers and surveyors who measured and mapped the lengths, depths, winds and tides of the great Wells estuary. Folkes’ team added one more expert: John Smeaton, considered England’s foremost expert on harbors. Smeaton’s report included a theoretical explanation of the principles underlying the creation and decay of tidal harbors. At trial, the harbor commission’s attorney claimed that Smeaton’s testimony should be excluded since the laws of nature were a “matter of opinion, which could be no foundation for the verdict of the jury, which was to be built entirely on facts, and not on opinions.”

The Chief Justice of the Royal Court of Common Pleas, Justice Gould, accepted that argument. Smeaton was not allowed to testify, and the harbor commission emerged victorious from the second trial. Citing their star witness’ exclusion, Folkes’ lawyers appealed for yet another trial.

Where Justice Gould had viewed Smeaton’s testimony as too speculative, the eminent Lord Mansfield, Chief Justice of the King’s Bench, to whom the third trial came, was unwilling to accept an approach which denied the court the opinion of the country’s foremost harbor expert: “I cannot believe that where the question is, whether a defect arises from a natural or an artificial cause, the opinions of men of science are not to be received…”

Lord Mansfield argued that even the commissioners in the original case were proposing opinions (for instance, “The embankment was preventing the backwater.”). If so, Lord Mansfield wanted to lean on experts to help the court weigh the possible explanations. To Lord Mansfield, if the proposed witness was known as an expert on the matter at hand, his opinion was proper evidence. In retrospect, Lord Mansfield’s decision in the Folkes v. Chadd case made the opinions of scientists related to facts of the case an exception to the opinion doctrine and significantly expanded the potential purview of experts in the courtrooms of England and eventually America.

With Smeaton’s testimony that the silting of the harbor was caused by activities upriver from Wells, Folkes’ team won the day in court.

Expert Battles: Who Can You Trust?

To Lord Mansfield, renown in the field sufficed to recommend an expert. Late 18th century judges counted upon gentlemen (almost all the first wave of scientists were gentlemen) to be truthful and honorably unbiased. If a person was qualified in a relevant field and their opinion was helpful to evaluating the facts of the case, they could be heard.

As legal use of scientific expertise grew, frustration with the many and varied opinions of scientific experts grew. Expert battles arose, as both sides of cases lined up eminent scientists putting forth contradictory experimental evidence, overwhelming the ability of judge and jury to understand whose science to trust. What did it mean if “science” couldn’t agree on how to interpret evidence?

Neither lawyers nor experts relished the situation. A popular joke in legal circles of the day was “There are three kinds of liars — the common liar, the damned liar, and the scientific expert.” Legal historian James Fitzjames Stephen’s 1863 A General View of the Criminal Law of England, paints a picture of experts as hired guns: “No one expects an expert, except in the rarest possible cases, to be quite candid. Most of them – are all but avowedly advocates, and speak for the side which calls them.”

The reform of expert testimony was a recurrent hot topic in both legal and scientific circles. A U.K. parliamentary bill passed in 1875 gave trial judges discretion to order a trial without a jury in any civil matter requiring scientific evidence that a jury might possibly not understand, but in the U.S., juries were largely left to their own devices to sort out valuable experts from charlatans. In 1905, Michigan passed a reform suggestion allowing the court to provide its own experts, but the Michigan Supreme Court soon held the statute unconstitutional.

What expert evidence should juries believe? An increasingly partisan legal system required more specific guidance. There was some agreement that professional codes of training and practice, increasingly common for the various sorts of experts who might appear in a courtroom, might be part of the solution, but how that would work was unclear. In 1922, there was still no better guidance for the admissibility of scientific evidence.

The Frye Standard (1923)

That year, a young man named James Frye was accused of murdering a prominent doctor. As part of Frye’s defense, his attorney put forward the results of a systolic blood pressure “lie detector” test and expert William Marston, the test’s inventor, as evidence of his innocence. But Walter Irvin McCoy, the trial judge, refused to admit Marston and his test into evidence. When Frye was found guilty, his attorney appealed, arguing that a scientific expert and evidence had been improperly excluded.

On hearing Frye’s case, the District of Columbia Court of Appeals reframed the discussion in an innovative and important way. Rather than focusing only the expert’s credentials, it suggested that courts should evaluate on the particular scientific theories being propounded – how well-established were they? This became the Frye Standard: An expert opinion is admissible if the scientific technique on which the opinion is based is “generally accepted” as reliable in the relevant scientific community.

The “General Acceptance” test in Frye v. United States was not widely adopted right away – the case was not even cited in another case for a decade following the decision – but it eventually became the broad and influential standard for expert admissibility in both criminal and civil cases in the U.S., helping courts discriminate between theoretical or experimental ideas and well-established scientific knowledge.

Expert witness reviewing documents for a report

The Federal Rules of Evidence (1975)

Congress muddled things up by enacting the Federal Rules of Evidence in 1975. Its Rule 702 outlined the admissibility of experts, saying:

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.

Despite appearing a half century after Frye, The Federal Rules of Evidence were in some ways a return to a previous wild west of expert admissibility. The Federal Rules of Evidence notably failed to explain which kinds of scientific, technical, or other specialized experts – if any – should not be allowed to appear in the courtroom.

Rule 702 opened the door for a broader variety of expert usage, but since it didn’t explicitly tell courts to abandon the Frye rule, many federal and state courts continued to evaluate the admissibility of scientific evidence using Frye’s “General Acceptance” criteria.

The Daubert Standard (1993)

Despite its inconsistency with Rule 702, Frye’s “General Acceptance” test continued to gain momentum. But some believed there ought to be more specific guidance.

Daubert v. Merrell Dow Pharmaceuticals (1993) set a new standard for expert witness admissibility.When Jason Daubert and Eric Schuller were born with severe birth defects, their parents took legal action against Merrell Dow Pharmaceuticals Inc., a subsidiary of Dow Chemical Company. The lawsuit asserted that the drug Bendectin was responsible for causing the birth defects.

Merrell Dow sought summary judgment. Their expert showed that no published scientific study had established a connection between Bendectin and human birth defects.

However, the plaintiffs submitted evidence based on in vitro and in vivo animal studies, pharmacological studies, and other methodologies that showed otherwise. The plaintiff’s evidence had yet to gain acceptance within the general scientific community, but the Supreme Court ruling in Daubert allowed it, setting the court up to be the gatekeeper for admitting expert testimony based on a broader list of criteria to consider, such as:

  • Can the expert’s technique or theory be tested and assessed for reliability?
  • Has the technique or theory been subjected to peer review and publication?
  • What is its known or potential error rate?
  • Are there standards for this technique?
  • Has the technique or theory been generally accepted in the scientific community?

Expert Witness Standards Continue to Change

General Electric Co. v. Joiner(1997) broadened trial judges’ gatekeeping role around expert evidence. Following Joiner, a trial court can now not only evaluate an expert’s methodology, but also question the conclusions of the experts when determining admissibility: A judge may also review the research data and studies on which an expert relies and evaluate whether he or she believes those studies support the asserted opinion.

In Kumho Tire Co. v. Carmichael (1999), the Supreme Court significantly expanded Daubert in U.S. Courts, holding that the Daubert standard applies to technical and other specialized expert testimony which is not purely scientific. As a result, the scope of expert testimony in complex legal proceedings is broader than ever, allowing courts to benefit from expert opinion in more fields.

Many legal experts prefer the greater authority Daubert gives judges to evaluate and accept or reject expert testimony. The federal court system and the majority of state courts now use Daubert, but a number of states, including California, Illinois, Maryland, Minnesota, New Jersey, New York, Pennsylvania, and Washington still follow the Frye standard.

Standards surrounding expert witnesses continue to evolve over time (for instance, the Supreme Court sent proposed changes to rule 702 to Congress in April 2023 which are anticipated to take effect in December 2023), and various courts differ in the rules followed and interpretations. As expert witnesses continue to exert strong influence over judgements, courts will continue to hold a microscope over them and study their effects.

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