Complex scientific testimony: How do jurors make decisions?

Critics of the civil jury system question whether jurors can adequately evaluate complex expert testimony. Based on current models of research in persuasion, we hypothesized that when expert testimony is complex, factors other than content will influence persuasion. Participants, serving as mock jurors, watched a videotaped trial in which two scientists provided evidence on whether PCBs could have caused a plaintiff's illness. The complexity of the expert's testimony and the strength of the expert's credentials were varied in a 2×2 factorial design. After watching the videotape, mock jurors rendered a verdict and completed a number of attitude measures related to the trial. Overall, consistent with our prediction, we found that jurors were more persuaded by a highly expert witness than by a less expert witness, but only when the testimony was highly complex. When the testimony was less complex, jurors relied primarily on the content of that testimony, and witness credentials had little impact on the persuasiveness of the message.     

Jurors'Evaluations of Expert Testimony: Judging the Messenger and the Message

Jurors are laypersons with no specific expert knowledge, yet they are routinely placed in situations in which they need to critically evaluate complex expert testimony. This paper examines jurors'reactions to experts who testify in civil trials and the factors jurors identify as important to expert credibility. Based on in-depth qualitative analyses of interviews with 55 jurors in 7 civil trials, we develop a comprehensive model of the key factors jurors incorporate into the process of evaluating expert witnesses and their testimony. Contrary to the frequent criticism that jurors primarily evaluate expert evidence in terms of its subjective characteristics, the results of our study indicate that jurors consider both the messenger and the message in the course of evaluating the expert's credibility.     

Educating Jurors about Forensic Evidence: Using an Expert Witness and Judicial Instructions to Mitigate the Impact of Invalid Forensic Science Testimony

Invalid expert witness testimony that overstated the precision and accuracy of forensic science procedures has been highlighted as a common factor in many wrongful conviction cases. This study assessed the ability of an opposing expert witness and judicial instructions to mitigate the impact of invalid forensic science testimony. Participants (N = 155) acted as mock jurors in a sexual assault trial that contained both invalid forensic testimony regarding hair comparison evidence, and countering testimony from either a defense expert witness or judicial instructions. Results showed that the defense expert witness was successful in educating jurors regarding limitations in the initial expert's conclusions, leading to a greater number of not-guilty verdicts. The judicial instructions were shown to have no impact on verdict decisions. These findings suggest that providing opposing expert witnesses may be an effective safeguard against invalid forensic testimony in criminal trials.     

Expert psychological testimony

Experimental psychologists increasingly are asked to give expert testimony in court, especially with regard to issues of eyewitness reliability. Whether or not experimental psychologists should give expert testimony on these matters is a controversial issue. The empirical literature suggests that potential jurors do not have a good understanding of the variables influencing eyewitness accuracy and that they cannot discriminate adequately between accurate and false eyewitness identification testimony. Experiments using expert testimony as a treatment variable, however, have not made a definitive case that expert testimony can benefit trial outcomes. The question of whether or not to give expert testimony must be broadened to consider not only the effects on verdicts but also the effects of expert testimony on the process by which verdicts are reached, the practices of police in subsequent investigations, the public's view of psychology, the practices of judges in subsequent cases, and the interaction between expert testimony and research activities.     

The Bottom Line: The Effect of Written Expert Witness Statements on Juror Verdicts and Information Processing

Mock jurors recruited from jury rolls were either not given written statements of expert witnesses' direct testimony or were provided with such statements before or after the presentation of that testimony. Presentation of the statements before the testimony and cross-examination provided jurors with a schema that allowed them to distinguish more effectively among the claims of four differentially worthy plaintiffs because they processed more probative evidence than other jurors. Jurors in receipt of written statements before the testimony found the evidence to be more comprehensible than other jurors. Jurors provided with written statements following testimony and cross-examination were able to differentiate between the most and least severely injured plaintiffs, whereas jurors not in receipt of any written statements were unable to differentiate among any of the differentially worthy claimants. The limitations of this case management technique and of the study are discussed.     

Impact of Defense-Only and Opposing Eyewitness Experts on Juror Judgments

Previous research shows that expert testimony on eyewitness memory influences mock-juror judgments. We examined the extent to which opposing expert testimony mitigates the impact of defense-only expert testimony. Participants (N = 497) viewed a video-taped trial involving an eyewitness identification and individually rendered verdicts and evaluated the evidence and the experts. We manipulated the Foils (unbiased vs. biased) and Instructions (unbiased vs. biased) of the lineup and Expert Testimony (no expert vs. defense-only expert vs. opposing experts). Expert testimony did not significantly influence juror judgments, but the opposing expert testimony diminished the credibility of the defense expert in the eyes of the jurors. Results point to the need for further research on conditions that qualify the impact of expert testimony.     

Expert Testimony Regarding Child Witnesses: Does It Sensitize Jurors to Forensic Interview Quality?

Does expert testimony on forensic interviews with children help adults distinguish between poorly conducted and well-conducted interviews? This study evaluates the effects of social framework expert testimony regarding child witnesses in a case involving allegations of child sexual abuse. A 2 (Expert Testimony: present or absent) × 3 (Child Forensic Interview Quality: poor, typical, or good) × 2 (Child’s Age: 4- or 10-year-old) factorial design was used to examine whether expert testimony is prejudicial or beneficial to jurors (N = 463). The results revealed that, without expert testimony, mock jurors did not consider the forensic interview quality when reaching a verdict. However, with expert testimony, mock jurors were more likely to render guilty verdicts if the interview quality was good versus poor. Further expert testimony increased mock jurors’ knowledge about child witnesses. These findings suggest that expert testimony related to the impact of interview techniques on the reliability of children’s reports may assist fact-finders in evaluating child abuse cases.     

Expert Testimony Pertaining to Battered Woman Syndrome: Its Impact on Jurors' Decisions

Participants (N = 200) were presented with a criminal homicide trial involving a battered woman who had killed her abuser. Within the trial, both the response history (passive, active) and presence of expert testimony pertaining to battered woman syndrome (present, absent) were systematically varied. As well, half of the participants in each of these conditions were provided with a nullification instruction informing them that they were free to disregard the law and acquit should a strict application of the law result in an unjust verdict. Results indicated that, compared to the passive response condition, the mock jurors were no less receptive to the expert testimony in the active response condition. The impact of the testimony on participants' verdicts, however, was moderated by the nullification instruction. That is, although the presence of the testimony did result in greater verdict leniency, this only occurred when the mock jurors had been released from a strict application of the law. The implications of these findings are discussed.     

The impact of general versus specific expert testimonyand eyewitness confidence upon mock juror judgment

Subjects (n=128) initially viewed an eyewitness of high or low confidence. Subsequently, participants viewed a psychologist who gave either espert testimony on the unreliability of eyewitness identification, specific expert testimony, or no expert (control) testimony. Subjects viewing expert testimony believed the eyewitness identified the gunman significantly less often, gave the defendant lower guilt ratings, estimated a lower general percentage of correct identifications under similar circumstances, estimated a lower percentage of general accurate eyewitness testimony, and gave significantly lower ratings to the belief that one can generally tell from eyewitness confidence whether an eyewitness is accurate than subjects in control conditions. Significant differences were also obtained between general and specific expert testimony. Participants viewing specific expert testimony estimated lower general percentages of correct identifications under the circumstances of the crime and reported relying more upon the psychologist's testimony than subjects viewing general expert testimony. Additionally, subjects viewing general expert testimony had significantly less confidence in their gunman vs. innocent person decision than subjects in specific testimony or control conditions. Subjects who viewed the high confidence eyewitness decided that the eyewitness correctly identified the gunman more often, gave the defendant higher guilt ratings, and estimated the general percentage of accurate eyewitness testimony to be significantly higher than jurors in low eyewitness confidence groups. The finding that jurors may continue to rely on eyewitness confidence to gauge the accuracy of the witness even after viewing expert testimony is discussed.     

The effects of complainant age and expert psychological testimony in a simulated child sexual abuse trial

Mock jurors viewed a videotape of a simulated child sexual abuse trial and then deliberated to a unanimous verdict. The complainant was described as either a 13- or 17-year-old female child. Jurors voted to convict more often when the younger complainant was seen, and the younger complainant was rated as more credible than the older complainant. Female jurors voted the defendant guilty more often and rated the complainant as being more credible than male jurors. Jurors voted to convict more often and rated the defendant as less credible when expert psychological testimony was specific to the case than when they were presented with either general expert testimony or no expert testimony. Jurors who saw a psychological expert testify became less accepting of child sexual abuse misconceptions than those in the no expert control condition. The implications of these findings are discussed.Millbrook Correctional Centre     

Considering forensic science: individual differences,opposing expert testimony and juror decision making

Two experimental studies examined the effect of opposing expert testimony on perceptions of the reliability of unvalidated forensic evidence (anthropometric facial comparison). In the first study argument skill and epistemological sophistication were included as measures of individual differences, whereas study two included scores on the Forensic Evidence Evaluation Bias Scale. In both studies participants were assigned to groups who heard: (1) no expert testimony, (2) prosecution expert testimony, or (3) prosecution and opposing expert testimony. Opposing expert testimony affected verdict choice, but this effect was mediated by perceptions of reliability of the initial forensic expert's method. There was no evidence for an effect on verdict or reliability ratings by argument skill or epistemology. In the second experiment, the same mediation effect was found, however scores on one subscale from the FEEBS and age also affected both verdict and methodological reliability. It was concluded that opposing expert testimony may inform jurors, but perceptions of the reliability of forensic evidence affect verdict, and age and bias towards forensic science influence perceptions of forensic evidence. Future research should investigate individual differences that may affect perception or bias towards forensic sciences under varying conditions of scientific reliability.     

Expert evidence and hearsay

In upholding the admission of expert evidence, some courts have held that hearsay information conveyed via an expert may be admitted as long as the jury is instructed to ignore the facts asserted in the hearsay statements and to use the information only for determining the weight to attribute to the expert's opinion. Results of a mock juror simulation indicated that although hearsay elements conveyed via an expert were perceived as less likely compared to a condition in which the information was independently admitted at trial, it was not completely ignored by the jurors. Further, the findings tended to suggest that the impact of the hearsay on verdict decisions operated primarily by influencing evaluations regarding the likelihood of the hearsay events as opposed to judgments regarding the expert testimony.     

Jurors' knowledge of battered women

An increasing number of psychologists with expertise in the area of battered women are participating in the legal system as expert witnesses and occasionally testify on behalf of a battered woman who has injured or killed her partner. Testimony about the battered woman syndrome has been offered to help the jury understand why the defendant reasonably perceived that she was in danger of harm. One of the requirements of expert testimony is that it be beyond the common understanding of the jury. Many commentators assume that jurors are uninformed or misinformed about battered women and, thus, that expert testimony is necessary to educate them. This study evaluated what jurors know about violent relationships. Approximately 300 jurors read scenarios about spousal violence and answered a questionnaire dealing with circumstances surrounding such abuse. Results suggest that on certain dimensions of spousal violence, jurors are aware of empirical research findings. On other dimensions, jurors are less well-informed and could potentially benefit from the testimony of an expert.     

Two conjectures about the issue of expert testimony

This article responds to concerns about expert testimony in experimental psychology by conjectur that disagreements about the propriety of the testimony are camouflaged arguments about the strength of psychological knowledge. Differences between proponents and opponents of expert testimony are about the state of psychological knowledge and certainty, rather than about the proper standard for psychologists to use when deciding whether to testify. A second conjecture is stimulated by the assumption that laypersons generally overvalue eyewitness testimony and that expert psychological testimony is a required corrective. The truth of this assumption rests on the debatable assertions that eyewitness identifications, without more, are potent sole determinants of trial outcome, and that lay juries need instruction from experimental psychologists about aspects of human behavior of which the jurors are definitive producers and consumers. One need not resolve these debates in order to understand that psychologists should not rely on the legal community to set the psychologists' standards for expert testimony. And psychologists, in considering their role as courtroom experts, should guard against a self-serving critique of the acumen of lay juries.     

Expert testimony in child sexual abuse cases

We presented participants with syndromal, witness credibility, or anatomically detailed doll evidence to determine (a) whether these different types of expert evidence exert differential influence on participants' judgments and (b) whether the influence of this evidence could be better explained by the relative scientific status or the probabilistic qualities of the research presented. Additionally, we investigated whether a strong or weak cross-examination of the expert would be more successful in discrediting the information provided in the expert's testimony. Findings suggest that participants are less influenced by expert testimony based on probability data (i.e., syndromal evidence) than by expert testimony based on case history data (i.e., credibility of anatomically detailed doll evidence). Participant responses did not differ as a function of the strength of the cross-examination of the expert. As expected, women were more likely to respond in a pro-prosecution direction than were men. Implications for the use of expert evidence in child sexual abuse cases are discussed.     

Expert evidence and medical manslaughter: vagueness in action

This article examines the reliance placed on expert evidence in prosecutions of health professionals for gross negligence manslaughter, where juries must decide whether conduct goes beyond civil negligence and constitutes the crime of involuntary manslaughter. It argues that the test for liability is vague and examines some of the consequences of this. Given the vagueness of the offence, jurors are likely to place great reliance on expert medical evidence. Little is known about how experts negotiate the legal process, empirically speaking: how they approach their task, how they view their role as expert witnesses, and the attitudes, biases, and beliefs that may underpin their testimony. Drawing on the experiences and perceptions often medical experts, this article explores how experts manage the vagueness inherent in the task of interpreting and applying gross negligence. Experts appear to go beyond offering purely medical opinion and enjoy engaging with law and the legal process.     

The Testimony of Forensic Identification Science: What Expert Witnesses Say and What Factfinders Hear

This research examined how variations in the presentation of forensic science information affect factfinders’ judgments in a trial. Participants read a summary of a murder case, the critical testimony being the results of a microscopic hair comparison given by a forensic expert. Across two experiments we manipulated how the expert expressed his results, whether he gave an explicit conclusion concerning identity of the hair, and whether the limitations of forensic science were expressed during trial. Qualitative testimony was more damaging to the defense than quantitative testimony, conclusion testimony increased the defendant’s culpability ratings when findings were presented quantitatively, and expressing limitations of forensic science had no appreciable effect. Results are discussed in terms of factfinders’ interpretation of forensic identification evidence.