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

Abstract

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 . The third is the flowering of recent work in artificial intelligence and law (Schum 1994; 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.

The Appeal to Expert Opinion: Quantitative Support for a Bayesian Network Approach

“…Our theoretical treatment of the appeal to expert opinion is also consistent with applications of Bayesian Networks in the legal domain (e.g., Kadane & Schum, ), specifically with recent demonstrations that complex chains of inference in legal contexts might be broken down into simple components that recur repeatedly (Fenton, Neil, & Lagnado, ; Lagnado, ; Lagnado et al., ). A similarly generic model of the appeal to expert opinion is important here, because such appeals often play a hugely prominent role within the law (e.g., Godden & Walton, ; Walton, , Chapter 6, ).…”

A normative framework for argument quality: argumentation schemes with a Bayesian foundation

“…This is not to deny that there may be contexts, such as the law, in which distinguishing between being 'in a position to know' and 'being expert' might be meaningful (see e.g.,Godden and Walton 2006). However, in order to justify different argument schemes there must minimally be some consequential difference to either the basic inference or the critical questions.…”

An Argumentation Interface for Expert Opinion Evidence

“…An extensive discussion of how the standard of reliability is to be met has been given in Godden and Walton (, 273–6). Note that this rule applies to the admissibility of scientific evidence, but in this paper we are also concerned with the evaluation of scientific evidence.…”

“…This observation encapsulates the essential theoretical and epistemological nature of the problem, which is the transfer of evidence from one context to a different one, where the methods and standards for evaluating evidence are not the same. Godden and Walton (, 269–70) expressed the nature of this transference as a shift from one type of investigative procedure to another, where each procedure has its distinctive standards of evidence and burdens of proof. Godden and Walton (ibid., 270) offered an explanation of this transfer of evidence by diagnosing it as a shift from one context of investigation to another, “a shift from a scientific inquiry dialogue to a persuasion dialogue of the kind that takes place in a trial.” The difference here between the two evidential settings is that the goal of a scientific inquiry is to prove a hypothesis, or alternatively to disprove it, if that is what the evidence indicates.…”