Witnesses in court can only give evidence of facts they personally perceived and not evidence of their opinion. It is for the court to draw inferences from the testimony of witnesses. The opinion of an expert witness is an exclusion to this general rule because courts need the assistance of experts to consider issues beyond their knowledge.

Lawton L.J in R v Turner established a ‘common knowledge’ rule governing expert evidence as follows:

This seems to limit psychiatric evidence to recognized mental disorder. However, expert advice is allowed which is ‘outside the experience and knowledge of a Judge or jury’. The abnormal/normal dichotomy is not a rule of law but guidance. Courts have allowed evidence on a variety of conditions which would not normally be thought of as established mental disorder, for example ‘Battered Women’s syndrome’.

Particular problems arise for the Court in respect of borderline conditions falling short of recognized mental disorder. Here admissibility will usually be determined by the court’s judgement as to whether the expert evidence addresses matters outside the experience or knowledge of a Judge or jury. Generally courts seek to limit evidence to established abnormal conditions.^(3,5) Courts have problems with evidence that utilizes leading edge or novel theory or diagnosis. The Court will require evidence that the novel theory or diagnosis is sufficiently organized or recognized to be accepted as a reliable body of knowledge by the profession. The expert will need to demonstrate that acceptance through reference to scientific literature.^{(3, 5)}

For medical experts, the Courts are able to establish expertise by reference to qualification and training. Nevertheless, the expert must be able to demonstrate that they have the requisite expertise in a particular case. For example, a psychiatrist trained in general adult psychiatry may not be an expert in a case concerning a person with moderate to severe learning disability.

Lawyers and experts enjoy immunity from suit (civil litigation) for their professional work in court.⁽³⁾ For medical expert witnesses this includes their report and any oral evidence presented in court. That immunity does not extend to immunity from report to the doctor’s regulatory body. It does not extend to subsequent actions such as duty of confidentiality in respect of disclosure of reports to third parties. The judgement in the case of GMC v Meadows lays out the legal and public policy arguments for immunity from suit but with regulation by their professional regulatory body.

Courts need to assure themselves that an expert witness’s evidence is reliable. This creates an obvious problem as the very reason an expert is giving evidence is that they have expertise which the judge or jury does not possess.

Courts have utilized three broad approaches to this problem. The first is to examine the scientific validity of evidence. The second is to devise standards for expert evidence and the third is to regulate experts through formal accreditation systems.

The landmark case is that of Daubert v Merrell Dow Pharmaceuticals in the United States courts. That ruling established stringent criteria to judge reliability to include that the technique, body of knowledge, or theory can be tested, has been subjected to peer review and publication, has a known rate of error, is subject to maintenance of standards and controls and is generally accepted by the scientific community. This judgement is problematic for a number of reasons,⁽⁶⁾ not least because the court does not have that expert’s specific knowledge but nevertheless has to make a scientific judgement on the reliability of that expert’s evidence. Judgements are unlikely to be value free determinations and there is a risk that the admissibility of evidence could be distorted by policy considerations or interfere with the use of leading edge science in the court. These considerations have made United Kingdom courts reluctant to introduce a ‘Daubert’ type test but there is pressure to do so^(4,7) with public concern about miscarriages of justice linked to expert evidence.

Courts have defined the standards expected of an expert witness; the landmark case being the judgement of Cresswell J in The Ikarian Reefer. Court judgements have been used to draw up formal rules governing civil family and criminal courts; for UK Criminal courts, the relevant rules are contained in Part 33, Criminal Procedure Rules.⁽⁸⁾ The common features are listed in Box 11.15.1 below. The understandable anxiety of the court to ensure experts adhere to these stringent standards may have the unfortunate effect of deterring psychiatrists from providing occasional expert reports for criminal courts.

In the United Kingdom, there are a number of organizations to accredit expert witnesses utilizing some combination of direct scrutiny of reports and references from legal teams. None so far have addressed the specific needs of the expert psychiatrist in court. They provide the court with some measure of an expert witness’s expertise in legal matters over and above what comes from their basic professional qualification. Critics have pointed out⁽¹⁰⁾ that once registered accreditation is unlikely to pick up poor practice, as experts with years of experience, but not necessarily competence, are unlikely to be refused accreditation. Accreditation is unlikely to prevent expert straying outside their area of expertise. These schemes have not as yet been able to deal effectively with problems of accrediting inexperienced but competent aspiring expert witnesses. To be effective they may require codes of discipline with the attendant danger of attracting vexatious complaints. The Royal College of Psychiatrists in the United Kingdom utilizes a competency based training framework together with standards for continuing professional development (CPD) following training to promote a high standard in medico-legal work. Evidence of completion of such training and CPD is likely to be the most effective way of demonstrating credibility as an expert witness.