Expert Testimony Admissibility Standards

Expert psychological testimony, like any testimony, must meet certain criteria or standards for admissibility before it is allowed into court. Although the admissibility of expert evidence was initially governed by the general acceptance standard set in Frye v. United States (1923), more recent standards, including the Federal Rules of Evidence, have shifted focus to an evaluation of the reliability of the evidence. This research paper outlines the historical changes in admissibility standards, starting with the Frye decision and the Federal Rules of Evidence and progressing through a trio of recent Supreme Court decisions that address the admissibility of expert evidence. These decisions include Daubert v. Merrell Dow Pharmaceuticals (1993), which established whether the Federal Rules of Evidence superseded the Frye standard when judging the admissibility of scientific evidence, and General Electric Co. v. Joiner (1997) and Kumho Tire Company v. Carmichael (1999), which established the scope of the Daubert decision.

Frye v. United States

Before the Daubert decision, most courts insisted that testimony proffered by an expert be relevant to the triers of fact and based on generally accepted principals. Specifically, this meant that the area the expert was testifying to must result from theory and technique that have been generally accepted by a scientific community. This principle was adopted from a District of Columbia (DC) Court of Appeals ruling that occurred in 1923.

In Frye v. United States (1923), the DC Court of Appeals issued one of the first decisions governing the admissibility of expert evidence. When James T. Frye was on trial for murder in the first degree, the defense proffered an expert who would testify about a lie detection test that was based on changes in the examinee’s systolic blood pressure in response to questions. The trial court ruled that this expert testimony was inadmissible, and Frye was convicted. In an appeal of his conviction, Frye argued that the trial court’s failure to allow the expert testimony was improper and resulted in his conviction. The appellate court reexamined the case and upheld the original verdict, introducing what is now referred to as the “Frye test” for the admissibility of expert evidence. In its decision, the court wrote that the methods used by an expert must be generally accepted within the expert’s field. In the court’s opinion, the lie detection test Frye wished to introduce to trial had not gained recognition among other experts in the field, and this was grounds for its exclusion.

The court established the Frye test to ensure that unreliable expert testimony was not admitted at trial, in part because of concerns that jurors would be unable to differentiate between good and bad science. Rather than relying on trial court judges or on jurors to make determinations about the value or reliability of expert evidence, Frye leaves the responsibility of evaluating the reliability of novel scientific methods to the relevant scientific community. If there is a significant dispute in the relevant scientific community over the reliability of a theory or practice, then the court could choose to exclude the evidence. Essentially, the court placed the responsibility of ensuring that valid science entered the court on the practitioners in the field.

Over time, the Frye test became the primary standard that the courts used to evaluate the introduction of scientific evidence. However, dissatisfaction with the Frye standard began to grow as opponents argued that Frye was too vague and that it prevented novel scientific discoveries from being admitted in trial because there had not been sufficient time for the discovery to become generally accepted. As expert testimony became increasingly common in courts, it became apparent that more structured guidelines would have to be created. Finally, 50 years after the Frye standard had been established, new rules governing the admissibility of expert evidence were adopted in the Federal Rules of Evidence (FRE).

Federal Rules of Evidence

In the late 1960s and early 1970s, an advisory committee convened by then Chief Justice Earl Warren drafted new rules to govern the admissibility of evidence in all federal courts. These rules, known as the FRE, were eventually adopted by the Supreme Court and, with modification, enacted into legislation by Congress in 1975. Article VII of these rules addressed the admissibility of expert testimony specifically. Rule 702 states that “if scientific, technical or other specialized knowledge will assist the trier of fact” in weighing the evidence or determining facts, and the expert proffering such testimony has been qualified as an expert based on his or her “knowledge, skill, experience, training, or education,” then the jury should be allowed to hear the testimony. Thus, the rule states that experts do not necessarily have to come from a scientific background. Experts can come from groups that have a certain skill, such as bankers or plumbers, when that skill or experience allows them to form an opinion that assists the trier of fact in understanding the evidence. Moreover, expert testimony must be helpful to the trier of fact in determining facts at issue in the trial. Although these rules were designed to apply only in federal courts, many states modeled their own rules of evidence on the federal rules, including Rule 702.

Daubert v. Merrell Dow Pharmaceuticals

As concerns grew over the admissibility of expert evidence, which began to play a larger role in many cases, questions arose about which standard should be used to determine the admissibility. Some federal courts continued to rely on the Frye standard, whereas other federal courts relied on the FRE when making determinations about the admissibility of expert evidence. The decision in Daubert v. Merrell Dow Pharmaceuticals (1993) addressed these concerns by clarifying specifically what standards should be used when admitting expert testimony.

The petitioners in Daubert v. Merrell Dow Pharmaceuticals (1993) were two children, Jason Daubert and Eric Schuller, who were born with serious birth defects, and their parents, who were appealing a trial court’s ruling that excluded expert testimony supporting the position that the drug Bendectin could be a teratogen. Their initial suit alleged that the children’s birth defects had been caused by their mother’s use of an antinausea drug, Benedectin, manufactured by Merrell Dow, during her pregnancies. To support their position, the plaintiffs proffered eight experts who based their conclusions that Bendectin was a teratogen on both test-tube and live studies on animals that demonstrated a relationship between Bendectin and birth defects, research showing that Bendectin had a similar chemical structure to other drugs known to cause birth defects, and a meta-analysis of published epidemiological studies that examined the rate of birth defects among children born to women who had used Bendectin versus those who had not. Merrell Dow proffered a highly qualified expert who stated that he had reviewed all the literature surrounding Bendectin and human teratogens and found that there was no evidence to support its being responsible for the birth defects.

The District Court granted a summary judgment for Merrell Dow and dismissed the case, writing that the animal and chemical structure research on which the plaintiffs’ experts based their opinions was irrelevant and that the results of the meta-analysis had not been generally accepted within the field of epidemiology, because it had not been peer-reviewed or published. Thus, the District Court relied on the Frye standard for excluding the testimony proffered by the plaintiffs. The plaintiffs appealed the District Court’s decision, arguing that the court had improperly used the Frye standard when judging the admissibility of its expert evidence as Frye had been replaced by the FRE. The Supreme Court agreed to hear the case and ruled in favor of the petitioners, agreeing that the FRE had replaced Frye as the appropriate standard for judging the admissibility of expert testimony.

In its decision, the Supreme Court outlined a two-pronged test for the admissibility of expert testimony. One prong required that expert testimony must be relevant to an issue before the court to be admissible. The second prong required that the expert testimony be reliable. In essence, the Supreme Court ruled that judges must evaluate whether scientific evidence is based on reliable methodology rather than relying on general acceptance in the scientific community to determine whether the testimony is admissible. The court sought to help judges who lack the scientific training to make these determinations by offering suggestions for criteria that could be used to evaluate research. For one, the court suggested that judges examine whether the theory or hypothesis on which the research is based can be falsified or tested. Second, the court stated that another way to assess whether the proffered evidence is reliable is to determine if it has been peer-reviewed and published. Third, judges should evaluate whether the technique in question has a known or potential error rate. Finally, although no longer a necessary and sufficient characteristic for admissibility, the Court suggested that judges could still use general acceptance as a factor in determining whether or not to admit testimony.

The Daubert decision held that the admissibility of scientific evidence depends on its scientific validity. The guidelines for judging the admissibility of scientific evidence promulgated in Daubert shifted the focus of the admissibility decision from determining whether the evidence was accepted by other scientists to an examination of the methods of the research on which experts base their opinions. Essentially, the decision in Daubert transferred the role of gatekeeper from the relevant scientific community to the trial court judge.

General Electric Co. v. Joiner

Although the Daubert decision settled the controversy regarding the appropriate standards by which judges should evaluate the admissibility of scientific expert testimony, there was disagreement over the standard to be used when reviewing such decisions. The Supreme Court addressed this question in General Electric Co. v. Joiner (1997). In this case, Robert Joiner, the plaintiff, had brought suit alleging that his development of lung cancer was influenced by his exposure to dielectric fluid contaminated with poly-chlorinated biphenyls while on the job at General Electric. Joiner sought to introduce testimony from several experts that exposure to polychlorinated biphenyls, not his history as a smoker, caused the early onset of his lung cancer, but the District Court ruled that the experts’ evidence was mere speculation and, therefore, inadmissible. On appeal, the Eleventh Circuit Court of Appeals applied a stringent standard of review when assessing this ruling and decided that the District Court had been wrong to exclude the expert testimony. They believed that the FRE showed preference for the admission of expert testimony and based on this analysis concluded that the District Court should have allowed the expert testimony.

However, the United States Supreme Court reversed the decision of the Eleventh Circuit Court, ruling that the appellate court had applied the wrong standard of review. The Court determined that the proper standard of review for decisions regarding the admissibility of expert testimony should be whether the judge abused his or her discretion by excluding the expert testimony. The Court explained that although the FRE may provide for the admission of a greater variety of scientific testimony, the trial court judge still retains his or her role as gatekeeper and must evaluate the reliability of the proffered expert evidence to approve or deny its admissibility rather than abdicating this responsibility due to the perceived liberal thrust of the FRE. The Supreme Court then ruled that the District Court had reasonable grounds to question the reliability of the expert testimony proffered by Joiner and had not abused its discretion in excluding it.

Kumho Tire Company v. Carmichael

Although Daubert provided trial court judges with guidance regarding the factors to be considered when determining the admissibility of scientific expert evidence, it did not address the issue of nonscientific expert testimony. A later Supreme Court case, Kumho Tire Company v. Carmichael (1999), answered the question of whether the Daubert standards should apply to nonscientific testimony as well. In this case, a tire on Patrick Carmichael’s minivan failed, causing Carmichael to lose control of the car, resulting in a car accident, which resulted in serious injuries and one death. Carmichael then brought suit against Kumho Tire Co., alleging that the tire had been defective. Carmichael wished to introduce testimony from an expert in tire failure analysis to show that the blowout was due to a tire defect rather than age or overuse. The District Court refused to admit the expert testimony, citing that the testimony failed to meet the standards set forth in FRE 702 or the reliability factors put forth in Daubert.

The Eleventh Circuit Court reversed the decision, stating that the U.S. District Court was mistaken in its application of the Daubert reliability standard to non-scientific testimony. The Circuit Court ruled that the expert testimony at issue relied on experience-based rather than scientific knowledge, which was outside the scope of Daubert. Therefore, the Circuit Court ruled that the admissibility of the expert testimony had to be reconsidered under Rule 702 instead. However, the United States Supreme Court disagreed with the Circuit Court when it reviewed and reversed the decision. The Court decided that judges’ gate-keeping responsibility outlined in Rule 702 applied to specialized and technical knowledge, in addition to the previous application to scientific knowledge. Thus judges were responsible for determining the relevance and reliability of nonscientific expert testimony as well as scientific testimony. According to the Court, the FRE 702 and 703 did not distinguish between scientific and nonscientific expert testimony and allows experts testifying on technical and specialized topics, as they do to scientific experts, the same latitude in presenting their opinions—latitude that is not granted to nonexpert witnesses. The Court also recognized the difficulty in distinguishing scientific from technical or specialized knowledge, because experts often have more than one type of knowledge to offer.

The Court then considered how judges should assess the reliability of these other types of expert knowledge, ruling that the Daubert factors could be used to evaluate expert testimony based on specialized and technical knowledge just as they can be used to evaluate scientific expert testimony. However, the Court reemphasized the statement in Daubert that the four factors outlined in that case were not an exhaustive list of criteria and reaffirmed the power of judges to consider other factors when determining admissibility. The Court noted that not all types of expert testimony can be judged solely on the basis of those four factors, as they do not always apply, but also recognized that they can apply to non-scientific knowledge, such as experientially based knowledge, as well. Therefore, they do not need to be applied exclusively, or even considered at all, in every case. The Court noted that judges have the same broad freedom in determining how to assess reliability of expert testimony as they enjoy in appellate review of their admission decisions as determined by Joiner. Kumho again reaffirmed judges’ discretionary authority when evaluating expert testimony and deciding its admissibility.

Effects of Changing Standards on the Admissibility of Expert Evidence

Given the shift in standards, it was expected that rates of admissibility for scientific testimony might be affected. Some scholars argued that, under Daubert,it was now possible for novel scientific evidence that was based on reliable and valid methods, but had not yet had time to gain general acceptance, to be admitted. This change would result in an increase in the rates of admission for scientific testimony. Other scholars argued that some generally accepted findings were based on unreliable methods and could now be challenged, resulting in a decrease in the admissibility of some types of evidence. Research on admissibility decisions pre- and post-Daubert has shown that the overall rates of admission were not significantly affected by Daubert. In contrast, the shift in standards did have an impact on the criteria used by judges to evaluate the admissibility of scientific testimony. Judges were less likely to rely on general acceptance or the Frye standard after Daubert and more likely to rely on Daubert criteria (e.g., peer review, falsifiability, error rates) to justify their admissibility decisions than they were before Daubert. Despite this increase in the use of Daubert criteria to justify admissibility decisions, after the decision, the best predictor of whether judges ruled to admit expert testimony were issues related to the FRE rather than the Daubert criteria or general acceptance.

Research has also shown that there have been relatively few changes in the admission rates for expert testimony post-Joiner and Kumho. Admission rates remained the same for experts in both civil and criminal cases. However, research has shown that admission rates for scientific expert testimony have actually increased post-Kumho, which suggests that judicial review of scientific testimony became less stringent while preserving judges’ previous approach to determining the admissibility of other types of expert testimony. Other research has suggested that no effect for Kumho was seen because judges had already started judging nonscientific expert testimony on the basis of reliability before the decision was handed down. Therefore, Kumho affirmed the practice of gatekeeping for nonscientific expert testimony that many judges had already assumed, without a negative impact on the admission rates for expert testimony.


  1. Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993).
  2. Dixon, L., & Gill, B. (2002). Changes in the standards for admitting expert evidence in federal civil cases since the Daubert decision. Psychology, Public Policy, & Law, 8, 251-308.
  3. Faigman, D., & Monahan, J. (2005). Psychological evidence at the dawn of the law’s scientific age. Annual Reviews of Psychology, 56, 631-659.
  4. Frye v. United States, 293 F. 1013 (D.C. Cir. 1923).
  5. General Electric Co. v. Joiner, 522 U.S. 136, 118 S. Ct. 512 (1997).
  6. Groscup, J. L. (2004). Judicial decision-making about expert testimony in the aftermath of Daubert and Journal of Forensic Psychology Practice, 4, 57-66.
  7. Groscup, J. L., Penrod, S., Huss, M., Studebaker, C., & O’Neil, K. (2002). The effects of Daubert v. Merrell Dow Pharmaceuticals on the admissibility of expert testimony in state and federal criminal cases. Psychology, Public Policy, & Law, 8, 339-372.
  8. Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999).
  9. Youngstrom, E. A., & Busch, C. P. (2000). Expert testimony in psychology: Ramifications of Supreme Court decision in Kumho Tire Co., Ltd. v. Carmichael. Law and Ethics, 10, 185-193.

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