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Expert testimony is presented in legal proceedings when a judge or jury needs assistance evaluating a material fact in a court proceeding. Qualified by education, skill, or technical expertise, an expert provides fact finders (i.e., judges or juries) with specialized information to educate them about caserelevant issues that are not within the common knowledge of fact finders. In this research paper, expert testimony in common law and civil law systems is reviewed and the empirical research on the effects of expert testimony on fact finders’ decisions are examined.
1. Expert Testimony In Common Law Systems
In common law systems, one or both parties may proffer experts in a legal proceeding. Judges then make decisions about whether the proffered expert testimony will be admitted into evidence. Although each country has its own unique law regarding the admissibility of expert testimony, there are several striking similarities across several countries, including the USA, Australia, England, and Canada (Gatowski et al. 1996).
In each of these countries, judges may only admit expert evidence that is relevant to a factual issue in contention. Stated another way, expert evidence must be probative (i.e., make some fact in contention more or less probable). In addition, judges must weigh the probative value vs. the prejudicial value of the expert evidence. Even if the evidence is probative, it may be inadmissible if the value of this relevant information is outweighed by the risk that the evidence may confuse, mislead, or unfairly bias the jury. To be admitted at trial, the expert evidence must also be helpful; that is, the evidence must assist the trier of fact in making a decision. If the expert evidence is within the common knowledge of the fact finder, then the evidence will not be helpful and should not be admitted.
A recent trend in case law on the admissibility of expert evidence is increased attention to the reliability or scientific validity of the information that experts are providing in court. Recent decisions in both the USA and Canada have addressed concerns about the proliferation of unreliable or junk science in court. These decisions have delineated specific criteria that judges must use to assess the reliability of expert evidence, such as potential error rates, the use of appropriate methodology, and the general acceptance of the evidence by other experts in the field. Courts in England and Australia have not provided specific criteria for evaluating the reliability of expert evidence, however, they have ruled that expert evidence must be based on legitimate science or expertise (Gatowski et al. 1996).
2. Expert Testimony In Civil Law Systems
Although there are often provisions for judges in common law systems to find their own court appointed experts, this option is rarely exercised (Gross 1991). In civil law systems, however, experts are usually appointed by the judge presiding in a case, in which the issues to be resolved are beyond the understanding of the court. Moreover, the laws regarding expert testimony in these systems focus on the procedures used to appoint an expert, the definition of the scope of the expert’s duties, and whether the expert’s testimony or opinion is binding upon the presiding judge. For example, French courts maintain lists of experts that meet certain qualifications, although judges are free to appoint an expert that is not on the list (Taylor 1996). In French, German, and Korean courts, experts are charged with a specific task or tasks and their findings are presented to the court in a written report rather than in oral testimony (Bastuck and Gopfert 1994, Lee 1997, Taylor 1996). In Korea and France, the findings of the expert are not binding upon the judge (Lee 1997, Taylor 1996).
3. Empirical Research On Expert Testimony
Psychological researchers have primarily used trial simulation techniques to examine the effects of expert testimony on fact-finder judgments. These trial simulations involve presenting mock jurors (who may be college undergraduates, members of the community, or jurors who have reported to jury duty) with written, audiotaped, or videotaped trial stimuli. The trial stimuli vary the presence or absence of expert testimony or some characteristic of the expert testimony.
3.1 Empirical Research On Adversarial Expert Testimony
Research on the effects of expert testimony on jury decisions became common in the 1980s. Most of this research has examined whether the testimony of an expert called by one of the parties (an adversarial expert) affected juror or jury decisions. For example, early research examined the question of whether expert testimony influenced juror judgments of guilt. Trial simulation studies demonstrated that when jurors heard expert testimony presented by the defense they were more likely to return defense verdicts than were jurors who did not hear expert testimony (e.g., Schuller and Cripps 1998). In contrast, jurors who heard expert testimony presented by the prosecution were more likely to convict the defendant than were jurors who did not hear prosecution expert testimony (e.g., Brekke and Borgida 1988).
After establishing that expert testimony influenced juror decisions, researchers began to explore whether certain variables moderate the effects of expert testimony on juror judgments. For example, expert testimony has greater influence on jury decisions when it appears early rather than late in the trial (Brekke and Borgida 1988, Schuller and Cripps 1998). There is some speculation that early presentation is advantageous because it allows the expert evidence to serve as a framework for understanding other trial evidence. Several studies have also demonstrated that expert testimony is more influential if the expert explicitly links the scientific research to the facts of the case under consideration (Brekke and Borgida 1988, Schuller 1992).
Eventually researchers began to study whether expert testimony improves the quality of juror decisions. Cutler and colleagues (Cutler et al. 1989) presented jurors with a trial simulation in which they manipulated factors affecting the reliability of eyewitness identifications (e.g., presence of a weapon, disguised perpetrator, retention interval). They also manipulated the presence of expert testimony describing how these factors influence the accuracy of identifications (e.g., people are less likely to recognize a perpetrator who wore a disguise). In the absence of expert testimony, jurors’ decisions were unaffected by factors known to decrease the reliability of eyewitness identifications. After hearing expert testimony on the topic, jurors became sensitive to these factors and used this information in making decisions. Expert testimony’s sensitization effect is more likely to occur when the expert explicitly links the scientific research and the facts of the case (Kovera et al. 1997).
3.2 Empirical Research On Nonadversarial Expert Testimony
There may be important differences between the effects of adversarial expert testimony and the testimony provided by a nonadversarial or court-appointed expert. Some have argued that because a nonadversarial expert is proffered by the court rather than one of the parties in a case, jurors may be less likely to critically evaluate the evidence provided by that expert. Indeed testimony from a court-appointed expert does not have the same sensitizing effect that adversarial expert testimony has. In one study, nonadversarial expert testimony on the factors influencing the reliability of eyewitness identifications caused jurors to become skeptical of the reliability of all eyewitness identification rather than just those identifications made under unreliable conditions (Cutler et al. 1990). This pattern of effects suggests that jurors are not systematically processing the expert evidence presented by the court-appointed expert, therefore, they do not understand the subtleties of that testimony (e.g., that only some eyewitness identifications are unreliable, not all).
Another concern raised about the use of courtappointed or nonadversarial experts is that their testimony will be prejudicial because jurors may accord it undue weight. Trial simulation research comparing juror evaluations of nonadversarial and adversarial expert testimony demonstrated that jurors find both types of experts equally credible and that their testimony is given similar weight by jurors when making trial judgments (Brekke et al. 1991). Moreover, jurors seem to remember evidence presented by the adversarial expert better than they remembered evidence presented by the nonadversarial expert, providing additional evidence that jurors do not scrutinize nonadversarial expert testimony as closely as they scrutinize adversarial testimony.
3.3 Can Fact-finders Distinguish Between Valid And Flawed Expert Evidence?
Because most of the social scientific research on expert evidence has relied on trial simulations in which mock jurors make decisions, little experimental evidence addresses how judges evaluate expert testimony. This void in the research literature is particularly problematic because many countries that allow expert testimony have judges, not juries, serving as factfinders during trials. Moreover, most research in this area has assumed that it is desirable for expert testimony to influence the decisions of fact finders. This assumption neglects the possibility that flawed expert evidence may be presented to the court, requiring that judges or jurors be able to differentiate between flawed and reliable expert evidence.
A recent trend in the empirical research is to question whether fact finders have the ability to make competent judgments about the quality of expert evidence. In one study, researchers asked judges to evaluate the admissibility of expert testimony that varied in its scientific validity after reading a description of a psychological experiment that was fundamentally flawed (e.g., was missing a control group) or was scientifically valid (Kovera and McAuliff 2000). Judges’ decisions to admit the expert evidence at trial and their judgments of evidentiary reliability were unaffected by the methodological quality of the expert evidence. Trial simulation studies also show that jurors’ trial judgments are unaffected by variations in methodological quality (Kovera et al. 1999) and that a scientifically informed cross-examination that highlights the methodological flaws does not sensitize jurors to the problems with flawed studies (Kovera et al. 1999). Further research is needed to determine whether attorneys’ pretrial motions objecting to the admissibility of expert testimony will help judges and whether judicial instruction or opposing experts will help jurors make more informed decisions about the quality of expert testimony.
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