Character Evidence

The basic rule limiting character evidence is quite sensible. Personality traits predict less than most people (including jurors) realize; situations, and person-by-situation interactions, are more potent forces. As the law suspects, people tend to perceive the behavior of others through lenses of propensity; consequently, they over-attribute and over-predict consistency between character and conduct. In fashioning the character evidence rules, common law judges correctly diagnosed a problem and took steps to temper those attributional tendencies to avoid inaccurate and unfair verdicts. The rules allow numerous exceptions, admitting some character evidence out of fairness or to permit helpful evidence while barring its most misleading variants. For example, defendants in criminal cases are permitted to offer evidence of their own character or the character of a victim. Other exceptions are made to assist factfinders to evaluate witness credibility. A special class of that rule deals with witnesses’ criminal records: a maze of sub-rules governs admissibility of prior crimes. Research finds that people tend to rely on prior crime evidence for its improper propensity purpose, contrary to judicial instructions about the limited use to which it may be put. A relatively new set of rules permits prior criminal sexual conduct to be admitted, allowing factfinders to draw inferences about “any matter to which it is relevant.” These rules are controversial because they invite jurors to engage in the very propensity thinking that centuries of evidence doctrine prohibited. Moreover, behavioral data do not support the theory behind a special rule for prior criminal sexual conduct.

Judges Versus Juries

The rules of evidence that have evolved prevent lawyers from using the most powerful, yet the most informationally empty, techniques of persuasion. The rules compel litigators to fight their battles by presenting juries with information. Studies conducted on jury decision-making indicate that evidence—factual information about the events in dispute—is the most potent force driving the verdicts of trials. Studies show that judges and jurors would reach the same verdicts in four-fifths of trials; that similarity is because they are responding to the same information. Studies of differences among jurors in demographics, attitudes, personalities, and knowledge have found that in the great majority of cases such differences matter very little to the outcomes of cases. Variation in the strength of evidence influences decisions far more than who is hearing the evidence. That is good news if we want trials to produce rational decisions based on evidence. The focus on evidence makes a juror’s job a demanding one, presenting challenges to understanding, remembering, evaluating, drawing inferences, and using evidence (in conjunction with the law) to reach conclusions about a disputed matter. Working as a group helps. Groups have advantages over individuals: they possess more cognitive and social resources such as wider background knowledge and experience, the ability of multiple minds to remember, to correct each other’s errors, to think about the proper meaning of the evidence, and so on.

Introduction

Rules of evidence are designed to facilitate trials by controlling what evidence may be presented at trial. Those rules came into being, and evolved over time, due to changes in trial process and structure – especially following the rise of adversarial procedure, whereby control over the marshaling and presentation of facts shifted from judges to lawyers. Refinements and reforms continue to be undertaken to try to improve the scope and clarity of the rules. Trial judges must not only apply the rules, they also have the discretion to make rulings in light of their expectations of the impact they think the evidence will have on jurors. This task involves metacognition: one human trying to estimate the thought processes of others. Thus, evidence rulemakers have been and are, effectively, applied psychologists. The rules of evidence reflect their understanding of the psychological processes affecting, and capabilities and limitations of witnesses, lawyers and jurors. Psychological research and methods provide an additional source of insight and assistance in that endeavor. Better psychological understanding should lead to more effective rules. Psychologists typically employ the scientific method: empirically testing hypotheses in an effort to discover which are valid understandings of how people perceive, store, and retrieve information. To evaluate evidence rules, one could conduct experiments directly on a rule, or borrow from existing knowledge developed in basic research and see whether those understandings support existing or proposed evidence rules.

Witness the Witness

The major source of information in a trial’s search for factual truth is the oral testimony of witnesses. The main worry about those witnesses is that they will be in error or will intentionally lie or mislead. Consequently, the law provides tools to help assess the veracity and accuracy of witnesses. However, the effects of some of those tools (e.g., oath) are unknown, and we know that people in general are bad at using certain other tools (e.g., demeanor cues in assessing veracity). Cross-examination often is praised as the law’s greatest method for getting at both veracity and accuracy, but surprisingly little psychology research exists on it. One worry about cross-examination is the same worry that psychologists have about leading questions: source confusion (jurors sometimes don’t remember which witness said what, and so don’t remember which testimony was discredited during cross-examination). Like the problem of disregarding inadmissible evidence, it can be difficult or impossible to erase what has already been learned. Historically, evidence law precluded people from testifying when the law assumed they would be likely to lie or make mistakes. The rules then opened the courtroom doors wider, allowing nearly everyone to testify, with the assumption that lawyers could expose or jurors could recognize when witnesses were lying. Interestingly, in the realm of privileges, which prevent some potential witnesses from testifying for various psychological and policy reasons, end up precluding many of the potential witnesses who would have the most incentive to lie on the witness stand.

Balancing Acts

Many rules of evidence employ balancing tests. The best known is Rule 403, requiring a judge to compare the probative value of evidence against (for example) its prejudicial effect. The problem of metacognition arises when a trial judge is making a decision in a specific case. The required balancing involves more than knowing a rule and the import of the evidence: it engages the judge’s beliefs about how jurors’ cognitive and emotional processes work, predicting how they will respond to the evidence, and comparing that to how the judge thinks the evidence ought best to be responded to. For some categories of evidence, the rulemakers have performed the metacognitive balancing for the judge, deciding that because jurors likely will over-value the evidence, it is best to prevent the jury from hearing those types of evidence. Sometimes the rules aim to encourage socially desirable behavior. For example, to promote repairs of dangerous situations, the rules bar using the fact that repairs were made to prove negligence. A well-established body of research on the “hindsight bias” supports the law’s suspicion that jurors will use evidence of the accident and the repair to answer the question of whether the risk should have been recognized ex ante, and conclude that the failure to correct the situation earlier was negligent. Balancing tests make good psychological sense in theory. Much remains to be learned, however, about whether they “work” (that is, are efficient, lead to good results, encourage socially-desirable behavior) in practice.

Hearsay and Exceptions

Witnesses typically may testify only about things they have personal knowledge of. But exceptions to that rule permit witnesses to testify to what they have learned from others, outside the courtroom, and that testimony to be offered to prove the truth of matters witnesses have no first-hand knowledge of. Courts and commentators have noted that hearsay testimony comes with various infirmities. Because the person with first-hand knowledge is not testifying, truth-seeking tools (oath, demeanor, impeachment, cross-examination) cannot be employed to evaluate the declarant's sincerity, memory, perception, or communication. Because of those worries, and concern that jurors don't appropriately discount hearsay evidence, hearsay exceptions are predicated on necessity and trustworthiness. Research has found, however, that in some circumstances people are appropriately skeptical of hearsay. They more readily disregard hearsay than other testimony when so instructed, discount hearsay suffering from specific infirmities, and decrease belief in hearsay the further away it is in a hearsay-within-hearsay chain. But researchers have not specifically studied many exceptions, in part because we do not actually know which exceptions are especially trustworthy. Basic principles of psychology, however, suggest that the courts’ and legislatures’ justifications for the trustworthiness of some exceptions are not well founded. But if, as suggested above, jurors can give the evidence appropriate weight, then admitting it creates no risk. Further research should investigate whether jurors can appropriately weight both current exceptions to the rule and other types of hearsay evidence that have long been considered too dangerous to admit.

Scientific and Other Expert Evidence

Although most evidence that comes to court is non-technical—observations of everyday events by witnesses, records of business and other routine activities—an important and growing minority of evidence is scientific or technical. Such evidence has great potential for helping to resolve what might otherwise remain unfathomable. But expert evidence demands much of judges and juries. Judges must serve as gatekeepers, deciding whether the evidence meets standards for admissibility (under Daubert), but judges are poorly equipped to evaluate the claims of various disciplines. In their turn, jurors are expected to understand and rationally use expert testimony that is admitted. So we have a paradox. By its definition, expert evidence occupies intellectual territory beyond the competence of judges and jurors. Moreover, it typically comes to court skewed, exaggerated, and cherry-picked. Further, it will almost certainly be contested. How are judges and juries to intelligently sift for helpful truths under such conditions? Human minds did not evolve to intuit statistical (and especially not probability) data, but instead to be persuaded by stories, by conclusions asserted by proclaimed authorities, and by superficial characteristics of speakers (witnesses). The acceptance of “social truth” and the use of System 1 thinking predominate. Research psychologists have been seeking ways to present scientific evidence that are more consistent with how the human mind functions. In addition, researchers have been exploring rapid teaching techniques for bringing laypeople to higher levels of numeracy and inferential competence.

Instructions to Disregard and to Limit Use

The problem of exposure to information that shouldn’t be considered when making a decision comes up in many guises for both jurors and judges. In balancing justice and expedience, the law sometimes asks jurors and judges to perform impossible feats of cognitive gymnastics: to disregard certain information or to use information for one purpose but not another. Substantial research shows that jurors often do not follow disregard or limiting instructions, though there is no single theory why the failure to disregard or limit use occurs. Perhaps jurors are unable to follow instructions to disregard information on command; perhaps they cannot program their minds to use evidence for one purpose but not another; perhaps they do not want to disregard or limit because the information that has been declared forbidden is nonetheless useful to solving the mystery of the trial. This problem also is encountered by judges who, when trying cases without juries, hear inadmissible evidence in their role as gatekeepers. If they cannot properly disregard it or limit its use to a proper purpose (and the research suggests they cannot), the inadmissible evidence will affect their verdict. The inability to disregard is also likely to affect appellate judges who rule on harmless error issues. More investment is needed in devising other ways to accomplish what the law seeks to accomplish. Some psychological research has discovered procedures that increase the likelihood that juries will disregard evidence when instructed to do so. More work might find more, and more effective, procedures.

Conclusion

The central lesson of our exploration of the psychological foundations of evidence law is that evidence rulemakers—be they common law judges or more modern rules committees—unavoidably operate as applied psychologists. At nearly every turn they find themselves having to devise rules based on what they think they know about the perceptions, memories, and motivations of witnesses; about the effects of trial processes on testimony and other evidence; and about the capabilities and limitations of human beings to comprehend and evaluate evidence, both as individuals (judges and jurors) and as members of groups (juries)....