By Geoffrey M. Pipoly. Full text here.
“Forensics” is a broad term used to describe many techniques utilized by law enforcement for the purpose of gathering evidence and solving crimes. Fingerprinting, firearm and toolmark analysis, and shoeprint analysis are all examples forensic techniques. Although many forensics practitioners describe their fields as “science” or as “rooted in science,” these techniques actually depart from traditional science in meaningful ways. Traditional science is based in the scientific method: asking objective questions about the world, formulating hypotheses based on those questions, and testing to attempt to disprove those hypotheses. By contrast, forensic techniques all rely, to varying degrees, on the subjective judgment and professional experience of the practitioner. Consequently, forensic techniques lack, to varying degrees, scientific reliability and validity.
Once on the witness stand at trial, forensics practitioners often describe the efficacy of their techniques in bold absolutes. For example, a firearm and toolmark analyst may testify that a bullet from a crime scene was fired by a suspect’s gun, “to the exclusion of all other firearms in the world,” or a fingerprint analyst may testify that the discipline of fingerprint analysis is more than 99 percent accurate in all cases. Given forensics’ scientific deficiencies, such testimony is disingenuous. Nonetheless, federal courts almost universally admit forensics testimony without limitation or nuance. This is problematic because, among other reasons, juries defer to experts, and an expert permitted to testify disingenuously may well prejudice the jury.
The legal standards governing the admissibility all expert testimony in federal court, including forensics testimony, originate from two Supreme Court cases: Daubert v. Merrell Dow Pharmaceuticals, Inc., and Kumho Tire Co. v. Carmichael. In Daubert, the Court held that trial judges are “gatekeepers” for expert testimony, and that the key to the admissibility of expert testimony was whether the testimony was relevant and reliable. The Court laid out five factors to aid courts in their admissibility determinations. In Kumho Tire, the Court held that the Daubert factors were non-exhaustive, and that courts may apply some, all, or none of the Daubert factors in individual cases, so long as the trial court determined that the testimony is relevant and reliable.
Forensics’ scientific deficiencies have been well documented, and many scholars argue that those deficiencies are a basis for excluding forensics testimony from evidence in criminal trials. The Note, however, argues that, under existing case law, those deficiencies cannot be a basis for exclusion; Kumho Tire eviscerated any distinction between scientific and non-scientific expert testimony for purposes of admissibility. However, forensics’ well-documented scientific deficiencies may well be a principled reason to limit the scope of testimony once a forensics practitioner takes the witness stand. The Note argues that once a district judge rules that forensic evidence is admissible, it should re-apply the Daubert factors to limit the scope of a practitioner’s testimony. Because the Daubert factors gauge, among other things, scientific reliability and validity, this approach will limit the scope of forensics testimony in a meaningful way, giving juries reliable information, and preventing the “bold, absolutist” testimony permitted under the status quo.