202304552 B1 - [2025] EWCA Crim 1051

The test for the admissibility of expert opinion evidence is set out in Criminal Practice Direction (CPD) 7.1.1:

“Expert opinion evidence is admissible in criminal proceedings if, in summary: it is relevant to a matter in issue in the proceedings; it is needed to provide the court with information likely to be outside the courts own knowledge and experience; the witness is competent to give that opinion; and the expert opinion is sufficiently reliable to be admitted.”

This test reflects the criteria for admissibility set by the common law, as summarised in R v Dlugosz [2013] EWCA Crim 2 where this court said:

“It is essential to recall the principle which is applicable, namely, in determining the issue of admissibility, the court must be satisfied that there is a sufficiently reliable scientific base for the evidence to be admitted. If there is, then the court leaves the opposing views to be tested before the jury”.

CPD 7.1.2 sets out factors which the court may take into account in determining the reliability of expert opinion and especially of expert scientific opinion. They include:

“(a)

the extent and quality of the data on which the expert opinion is based;

(b)

the validity of the methodology employed by the expert;

(c)

the extent to which any material upon which the experts opinion is based has been reviewed by others with relevant expertise and the views of those others on that material;

…

(h)

if there is a range of expert opinion on the matter in question where in the range the expert's own opinion lies and whether the experts preference has been properly explained; and

(i)

whether the experts methods followed established practice in the field and, if they did not, whether the reason for the divergence has been properly explained.”

It was not disputed at trial that the evidence of Dr Marsden was relevant to a matter in issue (7.1.1(a)) and was needed to provide the court with information likely to be outside of the court’s own knowledge and experience (7.1.1.(b)). The issue as to which of the two adults in the caravan, or both of them, caused Alfie's injuries was plainly the central issue in the trial, and a lay juror would be unable to assess whether a particular mark was a bite mark and, if it was, who might have been responsible for inflicting it.

The argument on admissibility centred therefore on the criteria in 7.1.1(c), and 7.1.1.(d) viz. whether expert opinion on bite mark evidence was sufficiently reliable to be admitted and whether Dr Marsden was competent to give that opinion.

We turn first to the question of sufficient reliability.

The concerns about forensic odontology evidence expressed by Professor Pretty were based in the main on what had occurred in the past in the United States where forensic odontologists purported to make a positive identification (that a person had bitten another person) from odontology evidence alone, and they did so on the basis that each person's dentition was unique.

However, Dr Marsden specifically eschewed such an approach. It is now accepted and was clearly accepted by Dr Marsden in his expert reports for the trial, that the current state of the science does not permit such claims properly to be made. Expert evidence which purports to say that a particular suspect was responsible for biting a victim, relying only on bite mark comparison, is not reliable therefore and is not a claim compatible with the current state of the relevant science.

The scientific/academic papers provided by the prosecution (but not Professor Pretty) make it clear that the approach of forensic odontologists has changed. A 2018 editorial in the American Journal of Forensic Medical Pathology sets out current practice.

“Bite marks are no longer to be said to be unique. Experienced forensic odontologists accept that many dentitions can produce similar patterns. Some bite patterns do not contain sufficient information to allow them to be associated with any specific individual. Odontologists now do not continue to analyse bite marks in many cases they are asked to consider because of the shortcomings”.

Professor Pretty did not refer to this or other publications which addressed the concerns in the articles and studies he did rely on. This is unfortunate. It is the duty of any expert to draw the court’s attention to research which might contradict that used by the expert in forming his views. This enables the court to set the concluded views of one expert against the range of contemporary academic research and opinions, which in turn, is part of the assessment that must take place as to the reliability of those conclusions.

There are undoubtedly limitations on the scope of expert forensic odontology evidence on bite mark identification and comparison. An expert may face difficulties in identifying a mark as a probable or possible bite mark because of a range of factors, including skin elasticity, movement of the victim and/or the perpetrator or the quality of the imaging of the mark. The science of forensic odontology is sufficiently robust however to enable a suitably qualified expert to consider whether a sufficiently clear mark on skin was, or was not, caused by biting. There is no evidence that this is not a proper exercise to conduct and report on.

Similarly we have concluded that in a case of a probable bite mark, where there is an unusual feature of the dentition which can be identified from the mark, an appropriately qualified expert in possession of a cast of a suspect's teeth can determine whether that suspect shares that unusual feature. The science does not currently permit such an expert to move from that point to making a positive identification of the suspect as having inflicted the bite. It simply permits the properly qualified expert to include the suspect in a list of potential biters or exclude him from that list. It is for the jury to then consider that finding in the context of the other evidence in the case to see how far, if at all, that evidence assists in establishing the prosecution case.

We find nothing inconsistent with these conclusions in the Glamorgan paper to which we were taken by Ms Martin in her oral submissions and upon which she placed significant emphasis. The twenty three participants in the study ranged from absolute beginners with no experience of forensic odontology through to students and experienced odontologists. Much of the apparent inconsistency and variation in the study was due to the inclusion of participants with such varying levels of expertise. Both variation and inconsistency were much reduced when only the answers given by experienced odontologists were considered.

Turning then to Dr Marsden’s evidence, in our opinion, it was notably careful and cautious and wholly in line with modern practice.

Dr Marsden indicated that in many cases he would decline to provide a report where in his opinion there was no evidence that the mark he was asked to examine was a bite mark or where there was insufficient detail in the mark to allow any meaningful comparison between that mark and the dentition of a suspect to be made.

In this case Dr Marsden was able to find sufficient features in two of the skin marks to conclude that they were probable bite marks but was only able to say that the third mark was a possible bite mark, using the scale of certainty suggested by the British Association of Forensic Odontologists. He took a similarly careful approach when comparing the marks on the skin to the dental impressions he had taken from the applicant and Jack Benham, where he declined to draw any conclusions in respect of two of the marks. In respect of the third mark, he did not purport to make any positive identification of the applicant. That is, he did not fall into the trap either of asserting that bite marks were unique or that a positive identification could be made on an individual from such evidence on its own –assertions which had caused the concerns in some of the material upon which Professor Pretty had relied.

Dr Marsden was fully aware of the relevant limitations in bite mark evidence identified by Professor Pretty and took account of those matters when expressing his opinions. In particular, Dr Marsden used his experience to determine that there was a particular feature of the impression left in the mark on the body, namely the misalignment of a canine tooth, to make comparison possible. He demonstrated his awareness of the limitations of conducting this exercise using photographs alone (a matter referred to by Professor Pretty) but was clear that the photographs in this case were of good quality and allowed for the comparison properly to be made. Dr Marsden was also aware that there can be distortion to a bite mark impression on the skin as a result of movement caused by the natural elasticity of the skin. He therefore looked for evidence of such distortion but found none. When he considered whether the teeth of either or both of the defendants could have caused the mark, he limited his evidence to saying that the unusual feature he found in the mark on the skin was not found in the dentition of Jack Benham but was found in that of the applicant.

The further question that fell to be determined was whether Dr Marsden was a suitably qualified expert. There can be no question but that he has relevant qualifications and experience. Further, in the course of the voir dire he demonstrated his careful approach to his work. He did not seek to make exaggerated claims for his science. He showed himself to be aware of the potential difficulties of carrying out an analysis working only from photographs and because of the elastic nature of skin. Alive to those difficulties, he satisfied himself that they did not prevent him from making the analysis he attempted. Further, in the course of Dr Marsden’s evidence he explained why his reports in this case had not been peer reviewed. He explained that there were few people in this country who were suitably qualified and prepared to undertake this work, not because of the reliability of the science, but because it was time consuming and most forensic odontologists preferred to concentrate on other areas of their work such as the identification of corpses by comparing dental records with the teeth of the deceased. The absence of peer review is a factor for the court to consider when assessing the admissibility of an expert report but it is not the only relevant factor.

Although the defence relied on Professor Pretty’s evidence at the voir dire to make a general attack on the reliability of bite mark evidence, they called no evidence at trial to counter Dr Marsden’s evidence or to impugn his methodology. The applicant’s concern that Dr Marsden’s evidence involved a degree of subjective analysis is a false point. There are some branches of forensic expert evidence which rely wholly or mainly on calculation for their validity –for example, the determination of the level of alcohol in someone’s blood. There are others which rely in large part of the interpretation of results in reliance on the expert’s particular expertise. That is the case, for example, when fingerprints found at a crime scene are being compared to a set of prints taken from a suspect, or a thread of material found at a crime scene is being compared to a garment recovered elsewhere to see whether the thread could have come from that garment. In such cases, the expertise in making the subjective comparison is the very reason why an expert witness is required.

The trial judge was able to assess Dr Marsden when he gave evidence at the voir dire. He was able to assess the robustness of the science and the expertise of Dr Marsden by reference to the evidence of Professor Pretty. At the conclusion of the 2-day voir dire the trial judge concluded that Dr Marsden's evidence was sufficiently robust to be placed before a jury for their consideration in a lengthy and fully reasoned judgment. We do not regard it as arguable that he was wrong to reach the conclusions he reached.

Finally, it should be noted that this was otherwise a very strong case against the applicant. She had been in a small caravan with her partner throughout most of the night during which Alfie sustained the injuries which led to his death. There was some evidence that she was away from the caravan for a short period during the night to acquire drugs but was not away for a significant period. In any event, it was her case in her police interviews that Alfie was still well and feeding well after she returned to the caravan for the last time. (She later changed this to say that she did not really look closely at Alfie after she returned). There was no suggestion that anyone else had been in the caravan at a time when they could have caused or contributed to the injuries. The applicant accepted that she had bitten her son and might have left a mark (albeit that she disputed the strength of her bite). She had sent texts to her partner before the night Alfie died showing some annoyance with her son. She remained in the caravan until 11.30 a.m. by which time her son was dead and had obviously been so for some time. It would be fanciful to suggest that the case against her was not a strong one, even without the evidence of Dr Marsden. There was an obvious inference to be drawn from the totality of the evidence: the applicant was either responsible for some or all of the violence or acted to assist or encourage the person who inflicted it. There was no danger of the applicant being convicted on the basis of the expert opinion evidence alone.