Expert evidence comes to court in a variety of forms and, in particular, at a couple of levels of generality. It often involves general research findings that, although relevant to a particular case, also transcend that case. General research findings will sometimes be used in the establishment of applicable law and decided by judges but, more typically, will be considered a component of the fact-finding duties of triers of fact. A large part of expert evidence, however, involves factual issues that are specific to particular cases, though they may be claimed instances of more general phenomena. Courts and scholars have proposed various models to account for the several ways in which expert evidence is manifested in court. The three most influential of these models are considered here.
To be admitted, expert opinion must be relevant to a material fact in dispute. This basic requirement ties proffered expertise to the substantive law of the case. Hence, for example, if applicable law requires that a substance be proven to have caused the plaintiff’s injury, expert proof regarding causation will be relevant; and if this proof is reliable and valid, it will usually be admitted. But proving simple causation of this sort can be a complicated matter. The plaintiff must first prove that the substance in question sometimes does cause the injury and, moreover, that it did cause the injury in this case. As this example illustrates, the facts in dispute in a legal case can appear at a couple of levels of generality. This fact has great relevance both to how and whether experts testify and the procedural legal response to proffers of different forms of expert opinion.
The recognition that facts arrive in court in different forms has spawned several scholarly attempts to impose some schematic theme on them. Three frameworks, in particular, have received considerable attention and are discussed below. These are (1) Kenneth Culp Davis’s distinction between legislative facts and adjudicative facts; (2) John T. Monahan and Laurens Walker’s three-part division of social authority, social frameworks, and social facts; and (3) post-Daubert courts’ differentiation between general causation and specific causation.
The Davis Model
In 1942, Kenneth Culp Davis identified two basic kinds of facts having evidentiary significance—what he termed legislative facts and adjudicative facts. According to Davis, legislative facts are those facts that transcend the particular dispute and have relevance to legal reasoning and the fashioning of legal rules. Although judges are responsible for deciding questions of legislative facts, they very often are the subject of expert testimony. For instance, the question of whether juveniles are as cognitively competent as adults for purposes of evaluating the constitutionality of imposing the death penalty for offenses committed before adulthood is a legislative fact. Such legislative facts are decided by judges, typically, at the trial stage and reviewed de novo on appeal. At the appellate level, legislative facts are sometimes referred to as mixed questions of law and fact.
Adjudicative facts, on the other hand, are those facts particular to the dispute. In ordinary litigation, these are the facts that drive the dispute. Examples of such facts include whether the traffic light was red or green, the presence or absence of a stop sign, or the kind of weapon allegedly brandished by the defendant. Adjudicative facts are within the province of the trier of fact (the jury or, if there is no jury, the judge) to decide. On appeal, in nonconstitutional cases, review is deferential under the abuse-of-discretion standard. The appellate standard of review for constitutional adjudicative facts is considerably more complicated, but in most instances, they are reviewed de novo.
The Monahan-Walker Model
Although the Davis dichotomy has been extremely influential in providing a nomenclature that is regularly employed by the courts, it fails to capture the complex interrelationship between different kinds of facts in actual courtroom practice. Monahan and Walker refined Davis’s dichotomy in the context of social science research, though their three-part taxonomy could be applied to most forms of expert evidence. A basic difference between the Davis and the Monahan and Walker approaches concerns the respective focus each takes to the subject. Davis primarily focused on the nature of the legal question involved— legislative-like decisions or case-specific adjudications. In contrast, Monahan and Walker focused on the kinds of social science available to answer the various kinds of legal issues in dispute. In this way, the Monahan-Walker model is more finely tuned to the relevant issues raised by expert evidence. In particular, their model takes into account both the science available on the subject and the level of fact in dispute under applicable law.
Monahan and Walker identified three levels of evidentiary convergence between social science and law: social authority, social frameworks, and social facts. Social authority refers to social science research relevant to the determination of legislative facts and thus the formation of legal rules. According to their proposal, social authority is analogous to legal authority and should be consulted similarly. Hence, judges would consider social science “precedent” (i.e., available research) as presented through briefs, through arguments, and sua sponte. The information found to be relevant would then be incorporated into the judge’s conclusions of law. Alternatively, in the Monahan-Walker model, social science research might be relevant to social facts (largely equivalent to Davis’s adjudicative facts), in which case, after being deemed admissible, it would be presented to the trier of fact through expert testimony. Finally, social science research might have relevance as a combination of social authority and social fact. Monahan and Walker label this concept “social frameworks.” In social frameworks, some issue in the particular dispute is claimed to be an instance of a social scientific finding or theory of general import. According to the model, the judge would consider and instruct the jury on the verity of the general claim, but the jury would also hear expert testimony on how the theory applies in the case before it.
Although the Monahan and Walker model has generally received positive recognition, by far their identification of the concept of social frameworks has had the greatest influence. This impact has occurred largely because of the fact that most social science comes into the courtroom in this two-stage form. This is so even when the two levels of the framework are not explicitly set forth, as occurs perhaps too often in practice. To illustrate the power of the social framework concept, consider three examples: the battered woman syndrome (BWS), eyewitness identification, and predictions of violence.
BWS illustrates social frameworks in their conventional sense, in that courts have always expected that there is a general fact that had to be proved and an issue as to whether the case at hand was an instance of that general fact. In the traditional context in which BWS is offered as proof, the defendant is a woman who has killed her abuser under circumstances that traditionally would not have qualified as self-defense. This may be because she killed when the victim was sleeping or otherwise not attacking her and when, seemingly, she was not in “imminent harm” of “serious physical injury or death.” According to proponents of BWS, however, a person who experiences chronic uncontrolled domestic violence develops certain psychological reactions to this violence. Courts have generally concluded that the psychological reactions attending BWS are relevant to the question of whether a defendant killed with the reasonable belief that she was in imminent harm. As the Monahan and Walker social frameworks concept makes clear, there are two separate social science issues that are relevant here. The first is the framework itself and concerns the question of whether social science research supports the claim that particular—legally relevant— psychological reactions develop as a result of chronic battering. If, and only if, the answer to this initial framework question is yes, a second question arises in these cases. Specifically, the social fact part of the framework requires proof that the particular defendant in the case suffers from BWS.
A second example of a social framework is the research on the unreliability of eyewitness identifications. This work nicely demonstrates how some framework research might be demonstrable at the general level but is left to the trier of fact to apply at the social fact level. Indeed, it is interesting to note that the reluctance to apply eyewitness research to particular cases appears largely to reside with the experts in this area, rather than with any specific limitations historically placed on this evidence by the courts. It appears that findings that have largely been generated by research scientists are used in court at the framework level but not applied in any case-specific way by the experts themselves. This is a use specifically contemplated by the Federal Rules of Evidence. The Advisory Committee’s Note, for example, states that Rule 702 “recognizes that an expert on the stand may give a dissertation or exposition of scientific or other principles relevant to the case, leaving the trier of fact to apply them to the facts.”
A third example of social frameworks is clinical predictions of violence—a form of expertise that is often offered as a social fact but without support of any social framework. Although there is a robust research literature on actuarial predictions of violence, which effectively would provide a social framework for this kind of evidence, many clinicians continue to testify based on clinical judgment alone. In effect, therefore, this evidence is being offered as social fact evidence, without any supporting research framework in which to set it. Courts have been particularly clueless as regards this sort of evidence, which should be excluded systematically. As a general matter, no applied science exists only at the social fact level. Whether explicit, implicit, or ignored, every scientific opinion regarding a particular case depends on the existence of some general theory or framework of which a particular case is a claimed instance. Courts display their scientific ignorance, and clinicians display their legerdemain, when they pretend that this is not so.
The Courts’ Model of Causation: General and Specific
In many contexts, courts have seemingly begun to develop a basic sophistication regarding scientific foundations in considering the different kinds of expert opinion that might be offered. Beyond the taxonomies of Davis and Monahan and Walker, working courts have been forced to recognize the basic difference between what most scientists study and what most legal disputes are about. Scientists typically study variables at the population level, and most of their methodological and statistical tools are designed for this kind of work. The trial process, in contrast, usually concerns whether a particular case is an instance of the general phenomenon. There has been little systematic work done on the problem of reasoning from general research findings to making specific statements about individual cases.
Courts have increasingly noted the different levels of abstraction at which science comes to the law. Science comes to courts as an amalgam of general principles or theories and specific applications of those principles. Courts have recognized these two levels of abstraction most clearly in medical causation cases, in which they routinely distinguish between general causation and specific causation.
General and specific causation are merely subinstances of the inherent division between the general and the specific in applied science. Indeed, these concepts closely parallel the Monahan and Walker social frameworks idea, though the courts have primarily developed the concept outside of the context of social science. General causation refers to the proposition that one factor (or more) can produce certain results, and thus the finding transcends any one case. Specific causation considers whether those factors caused those results in the particular case at bar. Consider, for example, the complaint in Daubert v. Merrell Dow Pharmaceuticals (1993) itself. The plaintiffs claimed that Jason Daubert’s mother’s ingestion of Bendectin during pregnancy caused or contributed to his birth defects. This claim had both general and specific components. As a matter of general causation, the plaintiffs were obligated to show that Bendectin sometimes causes birth defects. This hypothesis transcends the particular dispute and is as true in California as it is in New York. In addition, the plaintiffs had to show that Jason’s birth defects were attributable to his mother’s ingestion of Bendectin. This proof might involve showing that she took the drug during the relevant period of gestation and that other factors did not cause the defects. This is specific causation.
Virtually all scientific evidence shares this basic dichotomy between the general and the specific. Science provides methods by which relationships or associations can be identified and, typically, quantified. Scientific theories often involve induction from the particular to the general. The law, however, needs to apply these general lessons to specific cases. Although still little appreciated among psychologists, this division between what scientists do and what the law needs is one of the areas needing the greatest attention by those interested in the psychology-and-law connection.
- Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993).
- Davis, K. C. (1942). An approach to problems of evidence in the administrative process. Harvard Law Review, 55, 364—425.
- Monahan, J., & Walker, L. (1986). Social authority: Obtaining, evaluating, and establishing social science in law. University of Pennsylvania Law Review, 134, 477-517.
- Monahan, J., & Walker, L. (2002). Social science in law: Cases and materials (5th ed.). Westbury, NY: Foundation Press.
- Walker, L., & Monahan, J. (1987). Social frameworks: A new use of social science in law. Virginia Law Review, 73, 559-598.
- Walker, L., & Monahan, J. (1988). Social facts: Scientific methodology as legal precedent. California Law Review, 76, 877-896.
- Expert Psychological Testimony
- Expert Psychological Testimony Admissibility Standards
- Expert Testimony and Qualifications of Experts