Admitting Expert Testimony in Criminal Cases: What You Need to Know

Anjelica Cappellino, J.D.

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— Updated on August 23, 2021

Admitting Expert Testimony in Criminal Cases: What You Need to Know

Expert Evidence in Criminal Cases

Over the years, expert witnesses have become as ubiquitous at criminal trials as the judge and the jury. Experts specializing in DNA evidence, fingerprints, hair follicle analysis, and other areas of forensic identification have become part and parcel when trying violent crimes. Likewise, psychologists and criminologists that testify to a defendant’s mental state can often make or break a conviction. Law enforcement experts who can testify to the structure and inner workings of criminal organizations are often utilized as well. The list of potential experts in a criminal case, like civil litigation, can go on and on, especially as certain scientific fields develop and advance. However, criminal cases have their own particular set of rules – some de jure and some de facto – that make the admission of an expert’s testimony different from its civil counterparts.

Expert Disclosure Obligations: Civil v. Criminal Trials

 It is important to always be mindful of the rules surrounding expert disclosures, as it can affect case preparation, trial strategy, and even attorney-expert communications. In federal courts (and state courts that have adopted such rules), the procedure for disclosing an expert differs in civil and criminal proceedings.

Under Rule 26 of the Federal Rules of Civil Procedure, expert witnesses must disclose a written report previewing the expert’s proposed testimony to the opposing party. The report must contain “all opinions the witness will express and the basis and reasons for them.” Rule 26(a)(2)(B) also mandates disclosure of facts or data considered by the expert, any exhibits the expert intends to use, the expert’s qualifications, a list of other cases in which the expert testified, and a statement regarding compensation. In light of the Rule 26 disclosures, nearly any written communication between the expert and attorney may be discoverable, and as such, nothing should be committed to writing before thoroughly reviewing the communication.

In contrast, under Rule 16(a)(1)(G) of the Federal Rules of Criminal Procedure, an expert is not required to draft and disclose a report prior to testifying. A party in a criminal proceeding must only disclose “a written summary of any testimony” but, similar to Rule 26 of the Federal Rules of Civil Procedure, must also “describe the witness’s opinions, the bases and reasons for those opinions, and the witness’s qualifications.” Fed. R. Crim. P. 16(b)(1)(C). However, except for scientific or medical reports, Rule 16 does not permit the discovery of reports, memoranda, or other documents made by the defendant or attorney during the case’s investigation or defense, including statements made by the defendant and witnesses. In addition, under the civil rules, an expert may be deposed while depositions are only available to preserve testimony in criminal proceedings. The rules are different in criminal proceedings largely due to the constitutional protections against self-incrimination that are afforded to defendants.

Daubert in Criminal Proceedings: The Likelihood of Admissibility

The seminal Supreme Court case, Daubert v. Merrell Dow Pharmaceuticals, Inc. and its progeny established the governing standard for the admissibility of expert witness testimony in federal courts. The Daubert decision enumerates a non-exhaustive list of factors that a judge may consider when admitting expert testimony. Codified in Rule 702 of the Federal Rules of Evidence, Daubert focuses on whether an expert’s scientific, technical or other specialized knowledge will help the trier of fact understand the evidence. It also questions whether the expert testimony is based on sufficient facts or data and is the product of reliable principles and methods that were reliably applied to the facts of the case. The holding in Daubert is generally considered to be more flexible than the older standard set forth in Frye v. United States, which held that an expert’s testimony must be generally accepted in the scientific community in order to be admissible. At the same time, Daubert stresses the importance of a judge’s “gatekeeper function” in ensuring expert evidence is relevant and reliable.

In criminal cases, however, the admissibility of expert witnesses has proven to be more arduous than one would assume under Daubert, specifically for defense counsel, while the prosecution’s experts maintain a standard of admissibility that is considered more flexible. One study found that out of 67 criminal appellate cases challenging a prosecution’s expert witness, the prosecution prevailed in 61 times. Meanwhile, out of the 54 appeals made by defendants arguing improper exclusion of their witnesses, the defendants won only ten times. In contrast, the study found that 90% of civil appeals related to Daubert challenges were made by the defendants challenging the plaintiffs’ experts, and the defendants prevailed two-thirds of the time.

On the other side of the spectrum, studies have shown that the admissibility of the prosecution’s expert witnesses is oftentimes less scrutinized then the defense’s witnesses. In fact, it has been held that admitting an expert witness for the prosecution is less stringent than the general admissibility of any expert in civil proceedings.

 Generally, in both civil and criminal trials, there will always be a type of expert that is considered particularly reliable, whether it is due to the practice area or the science behind the testimony. But particularly in criminal cases, there are certain experts that are automatically given more or less credence than others, sometimes depending which side is proffering the testimony.

Studies have indicated that there is a greater disparity of admissibility between criminal and civil cases. In criminal cases, prosecution experts were found to be rarely excluded. However, “[t]he situation appears to be very different in civil cases. Plaintiffs and defendants, equally, are more likely to have access to expert witnesses in civil cases, while prosecutors usually have an advantage over most defendants in offering expert testimony in criminal cases.”

The court often excludes expert testimony regarding the reliability of eyewitness identification, which is common for the defense to try to admit. Although in cases where the prosecution also attempts to offer such evidence, it has been found that both sides are equally treated. Experts in psychology are also typically met with skepticism. In one study, it was found that psychologists were only admitted 49.7% of the time (in comparison to the 69.1% rate of admissibility for all criminal experts).

On the other hand, there are certain experts that are routinely looked upon favorably by the court and consistently admitted. Law enforcement expert testimony is frequently admitted, with the above study on psychology experts finding that police officers were admitted 85.7% of the time. The vast majority of police officers and law enforcement agents testify about the behavior of drug dealers, providing information about the structure and hierarchy of drug organizations. Courts have likened such testimony to that of anthropologists, as the agents are using their educational and practical experience to testify about the practices of a particular group. That being said, courts have also recognized that such testimony, which is usually proffered by officers personally involved in the case, must be limited to avoid abuse. In U.S. v. Mejia, the Second Circuit Court of Appeals vacated the convictions of two gang members and held that the testimony of a law enforcement agent who had handled the investigation testified to matters outside his expertise, such as factual evidence. As the Court held, “[a]n increasingly thinning line separates the legitimate use of an officer expert to translate esoteric terminology or explicate an organization’s hierarchical structure from the illegitimate and impermissible substitution of expert opinion for factual evidence.”

The differences between expert admissibility in civil and criminal proceedings are vast and multi-faceted. For attorneys and experts accustomed to practicing in civil proceedings, it is critical to familiarize oneself with the nuances of admitting testimony in criminal trials.

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