The Role of Expert Witnesses in the American Criminal Justice System

By Musa Kamara (PO’22)

In a typical court proceeding, several different types of witnesses can be called upon to testify on behalf of the defense, plaintiff, or prosecution. When these witnesses are called to the stand to testify, their testimony can be regarded as evidence, defined as anything that can help a judge or jury better ascertain the truth in a certain case and thus the outcome. Perhaps the most recognizable type of witness is an expert witness, who is allowed to speak upon their opinions based on their own analysis in their particular field of expertise. These witnesses are an integral part of the American legal system; they can help make sense of otherwise difficult-to-understand evidence and convey that evidence to the jury and judge in a way that they can understand. The qualification process for an expert witness has been established and re-established numerous times in American legal history in an attempt to ensure reliability. However, once experts are qualified, what they can say in terms of their testimony is largely unrestricted. As such, they can give testimony that may be erroneous in its methods or conclusions by, for instance, misinterpreting or creating data in a manner that can gravely affect a case. Ultimately, improper testimony has serious implications for the perpetuation of the phenomenon of wrongful convictions in the U.S.

All evidence in a United States court of law and its admissibility is governed by a set of rules called the Federal Rules of Evidence (FRE). These rules, which were submitted to Congress by an order of the Supreme Court in 1972 and signed into law in 1975, apply in all federal proceedings and have been adopted in many states with few or no variations. The FRE are broken down into 11 articles, each addressing a specific area of evidence law. Of those 11, two specific articles address the role of witnesses in the courtroom: Article VII applies to expert witness and Article VI deals with all other types of witnesses.

Though the articles are separate, a relationship can be drawn between the two to analyze how the role of an expert witness differs from that of other types of witness. Generally speaking, under FRE 602, witnesses are prohibited from testifying to their own opinions and analysis. This restriction is not extended to expert witnesses, as long as these experts are testifying within their field of expertise. Nevertheless, expert witnesses are not allowed to go on a witness stand and state their opinions free of any restrictions; they must first be qualified as an expert and then also must comply with Article VII of the FRE, specifically FRE 702.

Experts are typically qualified with two methods: a voir dire during trial or a Daubert hearing before trial. Whether or not an expert is qualified by a voir dire is largely based on individual circumstances, specifically whether or not attorneys from the opposite side raise a challenge before trial or not.

If no challenge is raised before trial, then the witness will go through a process called a voir dire during trial. During a voir dire (which translates roughly from French to “to speak the truth), an attorney from the side for which the expert is testifying will ask the expert questions concerning their background in the field, their education, and other relevant facts. Following the voir dire, the judge will determine whether or not the witness can testify as an expert.

However, if an expert witness’ credibility is challenged before trial, the witness must go through a process called a pre-trial Daubert hearing, named after the Supreme Court case that established the hearing itself, Daubert v. Merrell Dow Pharmaceuticals (1993). In a Daubert hearing, a judge will determine the credibility of the expert witness and that expert’s methods, based upon FRE 702 which states that expert witnesses may testify in the form of an opinion if:

(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;

(b) the testimony is based on sufficient facts or data

(c) the testimony is the product of reliable principles and methods; and

(d) the expert has reliably applied the principles and methods to the facts of the case.

Although FRE 702 does, in theory, restrict the opinions that experts can testify to and who can be qualified as an expert, the rule also contains subjective terms like “sufficient” and “reliable” which can largely be left to interpretation. Since judges are tasked with determining what is sufficient and reliable, considerable variation can exist in determining if facts and data were sufficient and if methods were applied reliably.  Explain how terms like sufficient and reliable can be interpreted differently. A series of cases called the Daubert trilogy, which consists of the aforementioned 1993 Supreme Court case and two others, attempt to mitigate this subjectivity and its resulting inconsistency.

The original case, Daubert v. Merrell Dow Pharmaceuticals, effectively builds upon a previous Supreme Court ruling, Frye v. United States (1923), which ruled that expert testimony was inadmissible unless the methods it used had gained “general acceptance in the field.” So, if an expert made a new discovery in their analysis based on sound methods, any testimony including this discovery would be ruled inadmissible based on the “Frye Standard.” Daubert further specified and established that the following standards could be used by a trial judge in determining the reasonableness and admissibility of testimony using a certain method.

(1) whether the theory or technique in question can be and has been tested

(2) whether it has been subjected to peer review and publication

(3) its known or potential error rate

(4) the existence and maintenance of standards controlling its operation

(5) whether it has attracted widespread acceptance within a relevant scientific community

The next case in the Daubert trilogy, General Electric Co. v. Joiner (1997), established that too far an “analytical gap” could not exist between an expert’s conclusions and the data that the expert used. In other words, experts are not allowed to so heavily extrapolate in their testimony that their conclusions are only barely supported by their analysis. The final case in the trilogy, Kumho Tire Co. v. Carmichael (1999), extended the expert standards of Daubert and Joiner to experts that do not use scientific methods but “skill or experience-based observation.”

While FRE 702 works in conjunction with the Daubert trilogy to qualify experts and restrict what they say, the issue of experts contributing to wrongful convictions by providing erroneous testimony still remains. According to a report released by the National Registry of Exonerations, 151 men and women were exonerated in 2018 alone for crimes they did not commit. The Registry, a project run by the University of California Irvine’s Newkirk Center for Science and Society, the University of Michigan Law School, and the Michigan State University College of Law, reported that a significant number of the convictions occurred as a result of misleading forensic evidence. Namely, experts have been reported to entirely invent probabilities based on DNA, bite mark, or hair evidence which ultimately contributed to wrongful convictions of defendants. While some of these overstatements are discovered, one can’t help but wonder how many are not discovered.

Though expert witnesses surely play an important role in the American legal system and rules governing their role do exist, National Registry of Exoneration’s report shows that experts are perhaps not as limited in what they can say as established law may suggest. One possible solution to the issue of improper expert testimony may exist in the form of greater accountability for expert witnesses. Previous case law, established in Briscoe v. Lahue and a 1989 Washington Supreme Court case, ruled that witnesses—including expert witnesses—were immune from liability following testimony. However, to date, only West Virginia and New Jersey have touched upon the issue of holding adverse expert witnesses liable, in order to encourage experts to refine their methods and conclusions before testifying.  Seeing as improved accountability serves as a potential solution to the issue of improper expert testimony and thus the issue of an unjust penal system, perhaps it is exactly where those interested in expert witness reform should focus their efforts.


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