Expert Evidence: The (Unfulfilled) Promise of Daubert

Expert witnesses and the evidence they provide occupy a unique and privileged position in the U.S. justice system. Put simply, expert witnesses can do things that other witnesses cannot do. Whereas lay or fact witnesses are typically limited to testifying about what they saw or heard, expert witnesses in most jurisdictions and in most legal contexts can offer opinions, including opinions on the ultimate legal issue, and they can rely on inadmissible evidence in reaching their opinions. Given the credentials required to be an expert witness and the unique nature of their testimony, expert witnesses have the potential to wield tremendous power in influencing judges and juries. The extraordinary role of expert witnesses and expert evidence had led courts and legislatures to formulate various frameworks for determining who should be recognized as an expert and what type of expert evidence should be admitted. Several influential court decisions have established admissibility criteria for expert evidence in U.S. courts. In Frye v. United States (1923), the United States Court of Appeals for the District of Columbia Circuit held that proffered expert evidence must be based on generally accepted scientific methods. Specifically, the federal appellate court held that “the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs” (p. 1014). This “general acceptance” test was the predominant admissibility standard for expert evidence in U.S. courts, and it remained largely unchallenged for more than half a century. Seventy years after Frye, in Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993), the Supreme Court of the United States held that Frye had been superseded by the Federal Rules of Evidence (FRE); the FRE was adopted in 1975 and therefore did not exist when Frye was decided in 1923. Specifically, the Supreme Court held that FRE 702, not Frye, governed the admissibility of expert testimony. In their gatekeeping function after Daubert, trial court judges were tasked with determining whether proffered expert evidence is scientific knowledge that will assist the trier of fact. To assist trial court judges in this new role, the Supreme Court in Daubert articulated four criteria that courts can consider when determining admissibility under FRE 702, including whether the proffered evidence (a) was derived from methodology that has or can be tested empirically, (b) has been subjected to peer review and publication, (c) has a known or documented potential rate of error, and (d) has achieved general acceptance in its relevant scientific community. The Supreme Court believed that its interpretation of FRE 702 was consistent with the liberal thrust of the FRE. A later decision from the Supreme Court held that Daubert applies to all forms of expert evidence, not just the scientific expert evidence that was at issue in Daubert (see Kumho Tire Co. v. Carmichael, 1999). Daubert is the admissibility standard in all federal courts, and most states have adopted Daubert or a close derivative (see Slobogin, Hafemeister, Mossman, & Reisner, 2014). Whether trial court judges are functioning as effective gatekeepers for expert testimony is an empirical question that calls for well-conducted research. Given the rate at which expert testimony is being offered by psychologists (see Melton et al., 2018), examining whether the assessment methods used by psychologists satisfy the Daubert criteria has clear importance. Neal, Slobogin, Saks, Faigman, and Geisinger (2019) answered the call in their ambitious, rigorous, and timely study of the admissibility of psychological assessment tools. In their article, Neal et al. (2019) report on the results of their two-part investigation of psychological assessment tools in legal contexts. Using the results from 23 previous surveys of forensic mental health professionals, they first conducted a systematic review of the 364 psychological assessment tools that psychologists have reported using in legal contexts. In the second part of their article, they provide a detailed analysis of admissibility challenges 894336 PPIXXX10.1177/1529100619894336DeMatteo et al.Commentary research-article2020