Self-Defence and the Reliability of Expert Evidence

The applicant was convicted on two counts of causing GBH with intent and sentenced to concurrent terms of 42 months’ imprisonment. He had stabbed two young men during a confrontation between a group of pupils from his school, who were attending a party, and a group from another school who came to the house where the party was held but were not let in. The defence was that he had acted in self-defence. He had taken a knife from the kitchen in a state of panic, having been seriously injured in a previous incident involving boys from the other school. He described being punched, kicked and scratched – which was disputed – and waving the knife around to deter his attackers, without any intention of causing serious injury.

After conviction, the applicant was examined by a psychologist, Professor Farrell, who reported that he was suffering from PTSD as a result of the earlier incident. The report was taken into account as a mitigating factor in sentencing but the applicant applied for leave to introduce it as fresh evidence for the purpose of an appeal against conviction.

Professor Farrell's report stated: ‘An existing condition of PTSD would, in strong likelihood, result in miscalculation of threat level, appraisal of current situation, a more heightened perception of threat, which can result in more extreme reactions and engagement in safety behaviour’.

Held, refusing the application: evidence of PTSD could be relevant in a case of self-defence, both to the question of what threat the defendant honestly believed he faced, and to the level of force he believed was necessary, which was relevant though not decisive factor in assessing the reasonableness of the force used (R v Thompson [2013] EWCA Crim 1849 followed).

The evidence must, however, be ‘sufficiently reliable to be admitted’ (Criminal Practice Direction (CPD) 2023 [7.1.1]). In this context it was usually necessary for the expert's report to be tethered to the evidence about the events, and in particular, any account given by the defendant; otherwise, it was likely only to confuse a jury. It was unlikely to be helpful for a psychiatrist or a psychologist to give general evidence diagnosing a condition in a defendant and saying what its effects might have been without also saying how, in the opinion of the expert, that related to the accounts given by the defendant, and by any relevant witnesses, as to what took place. The report must also set out the expert's reasons for the opinions given (R v Jacobs [2024] 4 WLR 8 considered).

Professor Farrell's opinion was not tethered to the evidence in this way. It did not say how any particular circumstance could have been viewed differently by the applicant because of PTSD and hypervigilance. It did not even deal with Mazzer's taking a knife from the kitchen drawer. The defence at trial had not been that the defendant miscalculated the threat he faced, but that he had been punched and kicked and used reasonable force to defend himself. It was for the jury to decide whether that might have been correct. The absence of psychological evidence at the trial did make the convictions arguably unsafe.

Commentary

This case raises an important question about the meaning of ‘sufficiently reliable’ in CPD 7.1.1. Does it mean that the scientific basis of the evidence is valid, i.e., that it can accurately identify the phenomena it claims to identify? (Until the latest amendments to the CPD, it referred to the evidence having ‘a sufficiently reliable scientific basis’.) Or does it mean that the evidence is something that the tribunal of fact can safely rely on for a particular purpose in the context of the case at hand? There appears to have been no challenge to the reliability of Professor Farrell's evidence in the first sense: that he had correctly applied the accepted diagnostic criteria for PTSD and explained the effects that PTSD typically has on the traumatised person's sense of threat. When the Court finds that the evidence is not sufficiently reliable it is using the term in the second sense: the evidence was not of such a nature that the jury could safely rely on it, in conjunction with the other evidence, to draw an inference as to the likelihood of the defendant's mental state being such as to afford a defence of self-defence.

The Court's preferred interpretation of ‘sufficiently reliable’ has significant advantages. The judgment correctly roots the test in the common law as a particular aspect of the more general principle that to be admissible the evidence must be ‘helpful’ to the jury – it must help them to draw a sound inference, and its value in that respect must not be outweighed by its unhelpful or prejudicial effects such as a tendency to confuse the jury. Framing the question in this way makes it one that is well within the competence of the judge as an expert in managing trials, rather than one that is primarily concerned with complex scientific issues of the validity of clinical diagnoses.

One can applaud the judgment for clarifying this important conceptual point while still having doubts about its application to the facts of the case. The difficulties largely arose from a difference in strategy between the legal team who represented the defendant at trial, and did not make use of psychological evidence, and those who represented him in the sentencing hearing and the application for leave to appeal. The Court of Appeal tends to be unsympathetic to defendants who come to regret the tactical decisions of their trial lawyers, and the judgment delivered by Holgate J in this case is no exception. It emphasises that Mazzer's ‘defence was not based upon a miscalculation of a threat posed to him, or to anyone else for that matter, by people in the vicinity’ (at [30]). The onus, however, was not on the defence to raise this particular issue. It was for the prosecution to make the jury sure that the defendant either had not honestly miscalculated, or that any miscalculation was ‘deluded’ and thus inherently unreasonable (R v Press and Thompson [2013] EWCA Crim 1879), or that even in the light of any miscalculation he had used an unreasonable level of force.

From the brief account of the evidence given in the judgment the possibility of such a miscalculation seems obvious, even if the defence for tactical reasons chose not to mention it, and even if the defendant's own evidence (honestly, ‘deludedly’, or otherwise) painted the assault on him as more serious than other evidence indicated. Moreover, irrespective of whether Professor Farrell's evidence was ‘tethered’ to a specific piece of evidence, there is enough in what is quoted from his report to make clear that throughout the incident (which would inevitably have reminded the defendant of the original trauma, i.e., an assault by pupils at the same school) the defendant was more likely than a person without PTSD would have been to misjudge any threat that he faced. He said in evidence that he took the knife from the drawer because he was ‘scared’, and the evidence of PTSD would appear to support this account even if the link was not made explicit in Professor Farrell’ s report. These issues could have been clarified by oral evidence, cross-examination, and perhaps by further expert evidence called by the prosecution, all of which might have been more helpful to the jury than the wording of the report. It is questionable whether the verdict of a trial that did not even consider the question of PTSD and the complex issues it raises can be considered truly safe.