A Mixed Method Investigation of the Factors that Influence Juror Interpretation of Forensic Science Testimony Presented for the Prosecution in Nine Homicide Trials in the United States

  • Alicia Brīd McCarthy Wilcox

Student thesis: Doctoral ThesisDoctor of Philosophy


The judicial system calls upon expert witnesses to testify in court when complex or specialized knowledge, beyond that of the lay person, is needed to interpret the evidence. Expert witnesses explain the meaning of the evidence and can express their opinion of its significance within the context of the circumstances of a given case. In a jury trial, it is the responsibility of the jury to listen to, assimilate, comprehend and to place the appropriate weight on the expert testimony in their final decision making.

Knowledge of how juries comprehend forensic science evidence and how this type of testimony influences decision-making is sparse. The United States National Academy of Science report, “Strengthening Forensic Science in the United States: A Path Forward” (National Research Council, 2009) called upon the greater forensic science community to address weaknesses in the forensic science disciplines. One of the weaknesses highlighted was a lack of research into how juries use and comprehend forensic evidence.

This study aimed to investigate the perceptions jurors have of expert witness testimony, delivered as oral evidence, by prosecution forensic scientists in homicide cases in the United States. Data was gathered from juries after they had completed jury duty in one of nine homicide cases through both questionnaire (n=29) and direct one to one interviews (n=22). How jurors determined the credibility of an expert witness and their views of the reliability of evidence presented to them by the witnesses they were exposed to were explored. The importance of forensic evidence in the jury decision making process and the tensions between the expertise and experience of the witness were also explored together with jurors’ view of technology and their understanding of the science presented.

The role of the use of narrative in evidence was explored in particular through the use of demonstrative aids to explain the scientific evidence. Jurors described a deeper understanding as a result of such testimony. Jurors also suggested that the expert witness’s education and years of experience were move favored over certification and laboratory accreditation and the credibility of the witness was reported to be a key factor in the juror’s acceptance that the evidence presented was reliable. The results of this study underpin the importance of the role of the judge as a gate keeper to ensure only valid and reliable scientific evidence is admitted and presented to the jury in court. This work has also exposed the degree to which jurors evaluate the reliability of forensic science evidence based on the credibility of the expert witness.
Date of Award2017
Original languageEnglish
SupervisorNiamh Nic Daeid (Supervisor)

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