Scientific expert testimony and intellectual due process.

• Author: Brewer, Scott

Suppose that two groups of expert mathematicians disagree about a complex mathematical question--say, whether Princeton mathematician Andrew Wiles really did solve "Fermat's Last Theorem," which no mathematician had been able to prove since Louis Fermat first propounded it about 360 years ago.(1) These experts have had an opportunity to hear one another's reasons for their competing conclusions about Wiles's proof, and neither group is convinced by the other. How might we decide which of the two groups is making the correct mathematical judgment? Here's a suggestion: Convene a group of twelve or so nonmathematicians, give them an opportunity to hear from representatives of each of the competing groups of mathematicians, and have the nonmathematicians decide whether Wiles's proof really succeeded. If the truth of the matter was among one's chief concerns, would this decision procedure seem sound? There is serious reason to doubt it. The most obvious problem with such a procedure is that it seems to turn the decision about this disputed, highly complex question in the science of mathematics over to those who are least competent to answer it.

Many legal systems, including the state and federal systems of the United States, use decision procedures that are disturbingly close to the one just imagined, procedures in which nonexpert judges and juries are called upon and authorized to evaluate expert scientific testimony. This Article's goal is to offer a sustained critical analysis of the legal rules and doctrines that create and administer this procedure. Expert scientific information is relevant to, even decisively important in, a rapidly growing percentage of decisions throughout civil and criminal law. Most judges and juries, however, are not sufficiently familiar with relevant scientific fields to be able independently and reliably to bring scientific information to bear on their decisions. Instead, they must solicit and defer to the judgments of expert scientific witnesses.

Moreover, almost inevitably in litigated cases in which expert scientific evidence is offered, nonexpert judges and juries are presented, not with one authoritative "voice" of scientific truth, but instead with competing scientific expert witnesses who testify to contrary or even contradictory scientific propositions. Lacking the information necessary to make cogent independent judgments about which of the competing scientific experts to believe, nonexpert legal decisionmakers choose among the experts by relying on such indicia of expertise as credentials, reputation, and demeanor. Thus, even the act of soliciting and deferring to expert scientific judgment requires nonexperts to use a reasoning process--the process of selecting the experts, deciding which expert to believe when the experts compete, and, finally, deciding how to use the believed expert's information in resolving the central dispute being litigated.

Drawing on work in jurisprudence, epistemology, philosophy of science, and theories of practical reasoning, as well as on doctrines and leading cases on scientific expert evidence, this Article carefully models the reasoning process by which nonexpert legal reasoners defer to scientific experts in the course of applying a law to individual litigants. Drawing on this model, I argue for four central conclusions. Taken together, these conclusions have far-reaching consequences for virtually all legal systems in which nonexpert legal decisionmakers confront expert scientific testimony.

First, the Article argues that in order to avoid making an epistemically arbitrary choice about which of the competing scientific experts ought to be believed, a person must understand (in a special sense discussed in the text) the cognitive aims and methods of science. But nonexpert judges and juries lack just that kind of understanding, which is why they rely instead on other indicia of expertise, such as credentials, reputation, and demeanor. Second, nonexpert judges' and juries' lack of understanding of the cognitive aims and methods of science and their reliance on such indicia of expertise as credentials, reputation, and demeanor to choose between competing scientific experts thus yield only epistemically arbitrary judgments. Third, the conclusions that nonexpert judges and juries ultimately reach by relying significantly on expert scientific testimony are often also epistemically arbitrary and are therefore not justified from a legal point of view. Fourth, I identify--and begin the process of explicating--an emerging norm that belongs in the family of rule-of-law values: the norm I call intellectual due process. Showing that this norm is immanent in values to which many legal systems--including those in the United States--are already committed, I explain the way in which this norm places important epistemic constraints on the reasoning process by which legal decisionmakers apply laws to individual litigants. This norm requires, among other things, that the decisionmaking process not be arbitrary from an epistemic point of view. In other words, nonexpert judges and juries often fail to satisfy the demands of intellectual due process when they solicit and rely upon scientific expert testimony.

In sum, I argue that values to which legal systems are and ought to be committed actually condemn one of the most firmly entrenched evidentiary methods currently in place. As scientific theories continue to become more specialized, complex, and relevant to a widening range of cases, this incoherence between normative aspiration and actual doctrinal and institutional procedure will increasingly threaten the legitimacy of nonexperts' legal decisions. I conclude the Article with some brief observations about the kinds of doctrinal and institutional reforms that might better enable legal systems to meet the high aspirations of intellectual due process, thereby restoring, or at least improving, the coherence of value and institutional practice.

1. THE DOCTRINAL AND INSTITUTIONAL FRAMEWORK OF EPISTEMIC DEFERENCE

   A. Evidence, Procedure, and the "Law's Epistemology"

   In the course of making legal decisions, judges, juries, lawyers, and other legal reasoners must constantly repair to factual judgments about the world.(2) Indeed, if one treats judgments about what legal authorities have decided as being within the scope of "the world," one might even conclude that every legal decision involves some judgment about the way the world is.(3) One of the most important overall decisions legal systems must make is how, if at all, to regulate the descriptive claims about the world--claims, that is, about how the world is, was, or will be--that enter into the legal system. In most civil and criminal cases, the principal doctrinal and institutional mechanisms for this kind of regulation are rules of evidence and procedure. In virtually every area of civil and criminal law, these are the rules that regulate judgments by legal officials (principally judges, but also legislative officials) about which conclusions of fact are both relevant to given legal judgments and adequately justified. In regulating these judgments, rules of evidence and procedure comprise what we may call the "law's epistemology"--the set of rules and institutions that determine what, from a legal point of view, can be believed with sufficient justification for the purposes of the legal system.

   Ultimately, my argument is a philosophical one: When a certain condition ("the underdetermination condition") is satisfied by the aggregate testimony of scientific witnesses in a particular case, there is compelling reason to doubt that a nonexpert legal reasoner can acquire expert information from a scientific expert in a manner that is both epistemically and legally justified. I set up this philosophical analysis with a detailed and focused examination of legal rules and institutions--chiefly those in U.S. federal law. There is an important heuristic relation between the highly abstract philosophical analysis presented in this Article and the rather concrete analysis of doctrine: Each keeps the other intellectually honest. Philosophical analysis without detailed facts is blind; recitation of detailed facts without philosophical analysis is ignorant. Philosophical inquiry into the practices of an institution proceeds by examining the structures of the concepts, arguments, and other intellectual procedures that comprise those practices.(4) My specific philosophical inquiry is whether and under what conditions it is possible for scientific experts to convey justified beliefs to nonexpert judges and juries.(5) To be successful, such inquiry must be deeply informed by at least some actual practices of the type that are examined and explained. In this area of "legal epistemology," an understanding of "what is actual" will materially inform the analysis of "what is possible," just as detailed analysis of scientific theories and methods deeply6 informs the philosophical analysis of science.(6) Thus, an adequate philosophical investigation of whether and how a legal system might achieve justified epistemic deference by nonexperts to experts requires attending to the ways in which current legal systems seek to achieve it. In this way, the detailed doctrinal discussion in Part II will inform the philosophical inquiry that this Article pursues throughout.

   In the American federal system (to which, for the most part, I confine my discussion of doctrine(7)), the rules most directly concerned with the shepherding of experts' beliefs from their minds into those of legal decisionmakers are the rules of evidence that pertain to expert testimony,(8) relevance,(9) and judicial notice.(10) Close examination of two well-known U.S. Supreme Court decisions will serve to introduce this basic rule framework and to call attention to the nature and importance of nonexpert deference to experts in the American legal system. One of...