A useful reminder (via the Privy Council) of the need for an expert witness to respect the limits of his role. The Judgment of the Privy Council was delivered by Lord Kerr,
23. [The expert] was asked by the appellant’s solicitors to consider the reliability of confessions made by [the Appellant] to police and to some of his relatives; and to evaluate any other matters that he thought relevant, including the appellant’s performance in the witness box and any possible disadvantage that he might have had at trial.
This was a wide-ranging brief and [the expert] undertook a commensurately all-embracing review of all aspects of the case which included interviews and psychometric testing of the appellant, review of video recordings of the crime scene, crime scene reconstruction, police interrogation, witness statements, and the results of intellectual and neuropsychological tests, which he had asked to be undertaken. He also prepared an extensive appendix consisting of “extracts from time line, questions and answers in police interviews during [the Appellant’s] period in police custody, and observations (comments)”.
24. [The expert] can certainly not be faulted for any lack of thoroughness in his approach to the preparation of his evidence. But the Board would wish to make three general observations about that approach before commenting on some particular aspects of his reports and appendix.
It is the duty of an expert witness to provide material on which a court can form its own conclusions on relevant issues. On occasions that may involve the witness expressing an opinion about whether, for instance, an individual suffered from a particular condition or vulnerability. The expert witness should be careful to recognise, however, the need to avoid supplanting the court’s role as the ultimate decision-maker on matters that are central to the outcome of the case.
[The expert] trenchantly asserts that[the Appellant’s] confessions are unreliable and he advances a theory as to why the appellant confessed. In the Board’s view this goes beyond his role. It is for the court to decide if the confessions are reliable and to reach conclusions on any reasons for their possible falsity.
It would be open to [the expert] to give evidence of his opinion as to why, by reason of his psychological assessment of the appellant, [the Appellant] might be disposed to make an unreliable confession but, in the Board’s view, it is not open to him to assert that the confession is in fact unreliable.
Decisions of the Privy Council are of persuasive force only in England and Wales.
The citations from the case have been edited for ease of reading. See other cases on expert evidence under this tag.