Argument from Expert Opinion as Legal Evidence: Critical Questions and Admissibility Criteria of Expert Testimony in the American Legal System*

While courts depend on expert opinions in reaching sound judgments, the role of the expert witness in legal proceedings is associated with a litany of problems. Perhaps most prevalent is the question of under what circumstances should testimony be admitted as expert opinion. We review the changing policies adopted by American courts in an attempt to ensure the reliability and usefulness of the scientific and technical information admitted as evidence. We argue that these admissibility criteria are best seen in a dialectical context as a set of critical questions of the kind commonly used in models of argumentation. There are four independent streams of research on expert opinion as evidence that are now at a point of convergence. The first is the longstanding concern in law with standards for the use of expert testimony in providing evidence in trials, and the changing rules for admissibility of this kind of evidence. The second is the growing literature in argumentation theory on argument from expert opinion as an argumentation scheme, or defeasible form of argument (Walton and Reed 2003). The third is the flowering of recent work in artificial intelligence and law (Schum 1994; Prakken and Sartor 1996, 2003), using argumentation schemes to represent forms of reasoning in law that are modeled in artificial intelligence (Walton 2005; Bex et al. 2003). In recent work of this sort, argument from expert * Research for this paper was made possible by separate research grants from the Social Science and Humanities Research Council of Canada held by each of the authors. The authors would like to thank the anonymous reviewers of Ratio Juris for their helpful comments. Ratio Juris. Vol. 19 No. 3 September 2006 (261–86) © 2006 The Authors. Journal compilation © 2006 Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden 02148, USA. opinion is the scheme most commonly taken as the leading case for studying how to identify, analyze and evaluate forms of argument used in law (Gordon 2005). The fourth is the new evidence scholarship (Anderson, Schum and Twining 2005) that applies technical tools and heuristics, like Wigmore diagrams, to the reconstruction of chains of evidential reasoning in trials, like the infamous trial of Sacco and Vanzetti. All four streams work with the same structure of rational argument, in which one party in a discussion uses the opinion of an expert to rationally convince another party to accept some proposition that the second party doubts. It is now long past time to bring these four streams together, so that researchers in each group can be informed about the parallel and overlapping work of researchers in the other three groups. The function of this paper is to carry out the interdisciplinary project of informing each of the four groups about how the work of the other groups relates to their own research initiatives. A large part of the problem for many scholars interested in expert opinion evidence is that Anglo-American law has undergone rapid and important changes in recent years, reflected in the changing criteria employed in the Federal Rules of Evidence (FRE). Thus a large part of the paper consists in summarizing and making sense of these developments in light of the concerns and methods of the other three streams of research. The goal is to put the problems currently faced in a clear perspective so that future interdisciplinary research on expert opinion evidence, of a kind that is much needed, will be made possible, and so that research in each field will be assisted. I. Expert Opinion in Argumentation and Law Argument from expert opinion (also called appeal to expert opinion) is a form of argument long held to be a fallacy in logic. However, recent work on argumentation has shown that it represents a form of reasoning that can sometimes be fallacious but that is often necessary, and can be reasonable under the right conditions of use (Walton 1997). The most obvious instance is the use of expert testimony in law, a form of argumentation that is so powerful and common in trials that it has now come to dominate as a form of evidence in Anglo-American law. American law, in particular, has struggled with the problem of devising criteria for dealing with expert opinion evidence, and it is the treatment of expert opinion in the American legal system that will be the focus of this paper. Many of us have heard 1 As Damaska 1997 has shown, it is mainly American law that approached the problem of expert evidence by providing a kind of criteria-based approach to admissibility. Continental law has relied on court-appointed experts and has tended to defer to them, while English law has approached the problem in a more gradualist manner (Damaska 1997, 151; Redmayne 2001). Indeed, especially following the Wolf reforms, the manner in which British courts have 262 David M. Godden and Douglas Walton © 2006 The Authors. Journal compilation © 2006 Blackwell Publishing Ltd. Ratio Juris, Vol. 19, No. 3 about landmark cases like Daubert and Kumho Tire, and such cases have been tangentially written about in argumentation theory. But those working in argumentation have not so far been well informed about the evolution of these legal criteria and their relationship to the work on the argumentation scheme for appeal to expert opinion. This is a great shame, for in our opinion expert evidence in law is an important subject in its own right, but is also one in which argumentation can play an important role. It is a subject that shows to students of argumentation why argument from expert opinion, although problematic, is by no means inherently fallacious. The argumentation scheme and its accompanying set of critical questions has become the tool of choice in argumentation studies for evaluating such arguments. We show how this scheme relates to the criteria, like the Daubert criteria, that have evolved in law to deal with expert testimony in trials. The main purpose of our exposition is to summarize the leading developments in the treatment of expert opinion evidence in law, from an argumentation perspective, and to show how current research on appeal to expert opinion as a form of fallible argumentation can help throw light on some serious problems with it. The ultimate goal of analyzing and evaluating such evidence as a species of rational argumentation by established criteria that have proved their worth is a collaborative research project for the future for argumentation researchers and legal professionals. II. The Dilemma of Expert Knowledge in Law The specialization of knowledge through scientific and technical advancement presents a unique dilemma to the legal system. On the one hand, we surely do not want the rulings and decisions of the court not to be founded on—or worse, to be at odds with—the best available technical or scientific knowledge (Black et al. 1994, 719). Yet, on the other hand, attempts to incorporate the technical and scientific elements of our knowledge into the legal procedure are associated with a litany of problems. handled scientific evidence in civil proceedings has moved away from the American approach. Similarly, as Burkhard Schafer observed to us, in continental legal systems, the combination of the principle of free evaluation of proof—freie Beweiswürdigung in German law—(see Art 261 StPo [Criminal Procedural Code] and Art 286 ZPO [Civil procedural code]) and the inquisitorial principle (in Germany, Art 155ff StPo) ensure that there are no equivalent formal rules that constrain the judge in the use of expert witness testimony and give him wide ranging discretion in interpreting, accepting, or rejecting such evidence. On the limited rules on expert evidence, see Art 72–93 stop—none of them however deal with the issues discussed in the cases analyzed here. 2 Daubert v Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579, 1993. 3 Kumho Tire Co. v Patrick Carmichael, 119 S. Ct. 1167, 1999. 4 For example, Black et al. cite a 1946 case (Berry v Chaplin, 169 P.2d 442 [Cal. Dist. Ct. App. 1946]) in which “a California court found that Charlie Chaplin had fathered a child, even though the child’s blood type made paternity impossible” (ibid.). 263 Expert Opinion as Legal Evidence © 2006 The Authors. Journal compilation © 2006 Blackwell Publishing Ltd. Ratio Juris, Vol. 19, No. 3 Perhaps the most general of these is that judges and juries are often required to make judgments in cases where they lack specific knowledge pertaining to the facts at issue in a case. The problem here is not so much one of the unavailability of evidence (which could also occur), as one of the fact-finder not having knowledge regarding the significance or probative weight of evidence that is available. Being ill-equipped to determine these matters for themselves, judges and juries must often rely on the knowledge of others in reaching their decisions. Indeed, with the increasing role of technology and other highly specialized fields of knowledge in our society, courts have become increasingly dependent on experts in those fields when deciding upon matters. Quite often, the courts are dependent on experts in their attempts to reach informed, scientifically sound verdicts. For example, Owen (2002) observes that knowledge of product design and manufacture is often required when determining a liability issue relating to possible product defectiveness. “[J]uries normally need the guidance of expert testimony to understand the technical aspects of both defectiveness and causation. Without such testimony, juries would be left to surmise, conjecture and speculation on these central elements of every case and cause” (Owen 2002, 347). Even in routine matters, courts have become dependent upon experts when conducting the day-to-day business of justice. So, whenever experts are involved with a court proceeding, it is in a situation where the fact-finders are dependent upon them in an epistemic sense. Because of this, the expert often appears as an authority that judges and juries are unfit to question or challenge, and to whom judges and juries will be tempted to defer in their judgments. Yet, if this happens the trial is in effect being decided by the experts rather than the courts. This situation is only aggravated by the fact that experts often have competing opinions, and trials can easily become a “battle of the experts” which the juries, by hypothesis, do not have the capacity to adjudicate (Hand 1901, 54–5; Walton 1997, 171–6). Moreover, with the increasing dependence on expert, scientific knowledge there appears also to have been a tremendous increase in the amount of “junk science” making its way into the courts through the testimony given in the expert witness box (Huber 1991; see also Chesebro 1993; McGarity 2004). These problems speak to the need to regulate expert knowledge in the courtroom. III. A Brief History of the Treatment of Expert Testimony