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Individuals who give witness testimony can include victims of crimes, bystanders or all emergency services. Stakeholders, including psychologists, police officers and legal professionals, agree that witness testimony can provide useful evidence for investigations. Witness testimonies are often seen as a strong form of evidence by police, judges and jurors. However, as with any evidence, the reliability of witness testimony decreases if it is not carefully obtained, managed and handled during investigations and prosecutions. For example, research by the Innocence Project, an organisation that seeks to exonerate wrongfully convicted individuals, indicates that witness testimony that incorrectly implicated a defendant was a leading factor in wrongful convictions. In a 2018 review of 365 wrongful conviction cases in the USA where an individual was later exonerated by DNA evidence, 69% involved inaccurate witness identifications of the suspect. In the UK, there are no available data on how frequently witness testimony is used in trials or how many wrongful convictions are caused by inaccurate testimony.

Key Points

  • When an individual experiences an event, it is encoded as a memory that may be retrieved later. For many reasons, an individual’s memory will not be a full and accurate version of an event. For example, the viewing circumstances (such as how far away the event occurs from the witness) can influence the quantity and quality of detail encoded by a witness or the memory can be altered after it has been encoded (for example, individuals tend to forget details from events as time passes).
  • During a criminal investigation, there is a risk of witness testimony being contaminated. However, evidence suggests there are several ways to reduce/mitigate contamination, including asking open questions instead of leading questions, asking witnesses how confident they are in their testimony, and letting witnesses know they can opt out of answering a question or choosing an individual in an identity parade.
  • During crime scene interviews, the risk of witness testimony being contaminated comes from potentially leading questions asked by emergency service staff and interactions between different witnesses. Evidence indicates these risks can be reduced by using the Structured Interview Protocol and the Self-Administered Interview booklet.
  • During investigative interviews, the environment in which the interview takes place and the rapport between the witness and the interviewer influence how much detail is elicited. The most commonly used format of investigative interview is the Cognitive Interview, which has been shown to increase the amount of information gathered compared to standard interviews.
  • When creating facial composites (images of a perpetrator’s face constructed by a witness with the help of a trained professional), methods that focus on one facial feature (such as eyes or nose) at a time have been shown to be less accurate than methods that try to construct the face as a whole.
  • Identity parades are when a witness tries to identify a perpetrator after being shown a suspect and at least eight other similar-looking individuals who are not suspects (fillers). Video identification parades (where the witness is shown videos of the suspect and fillers) can be less expensive to arrange than live parades (where the witness is shown the suspect and fillers in person) and have the benefits of witnesses not needing to confront a suspect and being able to take place anywhere.
  • Vulnerable groups (including children, older people and individuals with a disability/disorder) may find standard procedures more intimidating, which can affect the quantity and quality of their testimony. Therefore, there are several adaptations that can reduce intimidation, including conducting interviews in more comfortable environments, allowing witnesses to draw events instead of describing them, and providing communication specialists (Registered Intermediaries) who can help vulnerable witnesses give evidence.
  • Careful management of testimony in criminal investigations can be undermined if it is not presented appropriately in court. Research suggests that jurors’ beliefs about memory (such to what extent stress can affect encoding) are often inconsistent with research evidence on memory. The ways in which these beliefs are currently countered include the reading out of statements about memory myths or using expert witnesses. However, stakeholders have criticised these procedures and evidence suggests that the reading of a statement may not improve juror evaluations.



POSTnotes are based on literature reviews and interviews with a range of stakeholders and are externally peer reviewed. POST would like to thank interviewees and peer reviewers for kindly giving up their time during the preparation of this briefing, including:

  • College of Policing*
  • Home Office*
  • Identification Unit, South Wales Police Headquarters
  • Ministry of Justice*
  • National Policing Chief Council*
  • Northern Irish Research Service*
  • SPICEe (Scottish Parliament Information Centre)*
  • Professor Bernice Andrews, Royal Holloway*
  • Wayne Collins, National VIPER Bureau, Office of the Police & Crime Commissioner for West Yorkshire*
  • Dr Melissa Colloff, University of Birmingham*
  • Professor Martin Conway, University City London
  • Stephen Farmer, Open University and Innocence Project
  • Dr Ryan Fitzgerald, University of Portsmouth*
  • Dr Heather Flowe, University of Birmingham*
  • Professor Charlie Frowd, University of Central Lancashire*
  • Dr Rebecca K. Helm, University of Exeter*
  • Professor Lucy Henry, City University of London
  • Louise Hewitt, Innocence Project*
  • Professor Lorraine Hope, University of Portsmouth*
  • Dr Peter Hills, Bournemouth University
  • Dr Katie Maras, University of Bath
  • Professor Amina Memon, Royal Holloway*
  • Professor Laura Mickes, Royal Holloway*
  • Professor Becky Milne, University of Portsmouth*
  • Dr Dara Mojtahedi, University of Huddersfield
  • Dr Katherine Puddifoot, Durham University
  • Dr Kev Smith, National Vulnerable Witness Advisor*
  • Dr Rachel Wilcock, University of Winchester

*denotes people and organisations who acted as external reviewers of the briefing.

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