Duties and Obligations of expert witnesses

An expert witness can be instructed at any stage of a case, such as in the investigation stage (when a case is being built), the court stage (where the case is being heard), or the post-court stage (where sentencing is being delivered) (Davies & Beech, 2017). Depending on the stage, different services would pay to retain an expert witness. For example, if an expert is hired at the investigation stage, then this is often paid for by the police, whereas during the court stage, this is often paid for by the CPS.

An expert witness has many duties and obligations they must meet (Davies & Beech, 2017):

• They must provide independent, unbiased assistance regardless of which side retains them. For example, if an expert is hired by the defence to argue that the defendant is not fit to plea (perhaps due to some cognitive impairments), the expert should not automatically attempt to support this hypothesis and should instead provide an unbiased account of the literature on fitness to plea, providing a likelihood for the current defendant based on this.
• They should only comment in areas that they are experts in. For example, an expert on memory should not also be providing expert evidence related to whether a defendant is fit to plea, unless the argument for this is related in some way to their memory. In this regard, an expert is obligated to clearly define their expertise when providing testimony and provide an indication of the limits of their expertise.
• They should be transparent and make all materials and evidence used available to both the prosecution and the defence, regardless of which side retained them. Similarly, any materials or evidence that were not used must also be disclosed, regardless of their credibility. As such, experts must keep a clear and organised record of the documents they have generated and their full involvement in the case.
• They need to be familiar with court processes. While expert witnesses are not expected to be legal professionals, they must have some understanding of how to give evidence in court, some understanding of relevant legislation in their area (e.g. Police and Criminal Evidence Act 1984 and Mental Health Act 2007- UK Government, 1984, 2007) and the guidelines of good court etiquette by the Ministry of Justice and other services (e.g. Civil Justice Council, 2014; Criminal Prosecution Service, 2022).
• When giving evidence in court, they should be providing a balanced argument, disclosing all of the evidence and materials they have used and considering new information that might come to light during the case itself. As such, experts should attempt to address their biases and not be too narrow in their thinking.
• Before giving evidence, experts must make an oath to provide only truthful information.

While there are a number of duties that an expert must carry out and a number of obligations they must meet to ensure their evidence is admissible, there are many checks that need to be conducted by legal professionals as well to ensure that the testimony is admissible and required, and that the expert is credible (Davies & Beech, 2017).

We'll further consider the admissibility of expert testimony in the following section.