Expert Psychological Testimony on Eyewitness Identification

Psychologists occasionally testify about the factors that influence eyewitness identification accuracy in criminal cases in which eyewitness identification is a pivotal issue. Considerable research has addressed the need for this testimony and its impact on the jury. Typically, the expert witness is trained in cognitive or social psychology and has published scholarly work about eyewitness identification accuracy. Most often, the expert witness is offered by the attorney for the defendant. In such cases, the expert undergoes “direct examination” by the defense attorney and “cross-examination” by the prosecuting attorney.

The purpose of this form of expert testimony is to educate the jury about the factors that are known from research to influence the likelihood of false identification. The content of the expert testimony typically focuses on issues such as factors affecting the likelihood of mistaken identification, the suggestiveness of lineup and photoarray procedures, and the relation between eyewitness confidence and identification accuracy. For example, with respect to witnessing factors, an expert might testify that witnesses are more likely to make false identifications when identifying a perpetrator of another race than when identifying a perpetrator of their race. Extreme stress associated with a violent crime inhibits a witness’s ability to accurately encode information. The presence of a weapon creates a “weapon focus” effect and increases the likelihood of false identification. With respect to lineups, an expert witness might testify that false identifications are more likely with suggestive lineup instructions and when lineup members are presented simultaneously rather than sequentially. An expert witness might also testify that eyewitness confidence is not strongly related to identification accuracy and that eyewitness confidence levels can be inflated by information provided by cowitnesses or investigators that validates the eyewitness’s selection from the lineup. Expert witnesses do not offer opinions about the accuracy of eyewitness identification in a given case.

The decision to admit expert psychological testimony about eyewitness identification into court is normally left to the discretion of the trial judge, and the likelihood of admission varies considerably from state to state and in federal courts. Typical reasons for excluding expert psychological testimony about eyewitness identification include the following. Many judges have ruled that the content of the expert testimony is merely a matter of common sense and, therefore, of little benefit to jurors. Judges have also ruled that the research is not commonly accepted within the relevant discipline. Other judges have ruled that the testimony is not helpful to jurors, because it addresses only research and does not inform the jury about the accuracy of the identification in the specific case at trial. Still other judges have ruled that the testimony will prejudice the jury by making them unnecessarily skeptical about the eyewitness identification.

Psychological research on expert testimony has taken various forms. Some research has directly examined the validity of the reasons for excluding the expert testimony. For example, as noted above, judges have ruled that the content of the testimony does not go beyond the jurors’ common sense. This implies that the average juror is well-informed about the factors that influence eyewitness identification accuracy. Scholars have developed several ways of testing the degree to which jurors are so informed. Some have conducted surveys of juror knowledge by administering test questions about factors affecting eyewitness identification. Others have described eyewitness identification experiments to students and asked them to predict the outcomes of the experiments. Their predicted outcomes are then compared with the actual outcomes of the experiments. Still other scholars have developed simulated trials (transcripts or videotaped enactments of trials) in which some evidence is held constant while other evidence (e.g., the suggestiveness of lineup procedures) is systematically manipulated. This methodology enables the researcher to examine what factors influence juror decisions and whether those factors are consistent with what is known from the research on eyewitness identification. This body of research on common sense supports the conclusion that what psychologists have to offer to the jurors well exceeds their common sense. For example, contrary to conclusions from the research literature, eligible jurors lack a full understanding of the impact of cross-race recognition, high stress, and weapon focus on identification accuracy and believe that eyewitness confidence is a stronger predictor of identification accuracy than the research suggests.

Research has also examined the extent to which the content of expert testimony is generally accepted among psychologists with expertise in eyewitness identification. This research paper takes the form of surveys of these experts and shows that many (but not all) of the factors about which experts testify are generally accepted in the relevant scientific community.

Does expert testimony about eyewitness identification prejudice the jury? What effect does expert testimony have on juries? Some researchers have addressed these questions by conducting trial simulation experiments. In these experiments, students or jury-eligible community members serve as “mock-jurors” and either read trial transcripts or view videotaped enactments of trials. Some trials would contain expert testimony while others would not. The research findings are mixed, with many studies showing that expert testimony makes jurors more skeptical about the accuracy of eyewitness identification, meaning they are less likely to convict a defendant based on eyewitness identification after hearing expert testimony. Other studies have found that expert testimony improves juror decision making by teaching jurors to rely on factors that are known to influence the likelihood of false identification (e.g., high stress, weapon focus, suggestive lineup procedures) and to not rely on factors that do not predict identification accuracy (e.g., witness confidence).

In sum, expert psychological testimony about eyewitness identification is a relatively new safeguard developed to prevent mistaken identifications from resulting in erroneous convictions. Its effectiveness is limited by virtue of the fact that it is often not allowed in criminal cases. Some of the reasons that judges typically give for not allowing this form of expert testimony are not supported by empirical research. Research on the effectiveness of expert testimony suggests that it can be helpful but not universally so.


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