[2016] UKSC 61 – Summary

Factual and Procedural Background

The Appellant was convicted by a jury of the murder of his partner. He admitted killing her; the sole live issue at trial was whether he had established the partial defence of diminished responsibility so that he should be convicted of manslaughter rather than murder.

The killing occurred at the couple’s home after a day marked by arguments at a family barbecue and later at home. The Appellant attacked the deceased in front of her two young sons. Following an earlier argument and physical assault, the Appellant armed himself with a knife which was removed by the older child. He then produced a second knife, knelt on the deceased’s arms, shouted that he was going to kill her, and inflicted about 22 knife wounds in addition to blunt-force injuries to the abdomen and liver, the latter being fatal.

The Appellant had a long history of mental disorder, including depression, paranoid fears and reports of hearing voices, and had been treated with anti‑depressant and anti‑psychotic medication. Two consultant forensic psychiatrists agreed that he suffered from an abnormality of mental functioning arising from a recognised medical condition, but differed as to diagnosis and which of the statutory capacities were impaired. Both nonetheless considered that his abilities were substantially impaired within the meaning of section 2 of the Homicide Act 1957 (as amended by the Coroners and Justice Act 2009). There was no contradictory psychiatric evidence.

At trial the judge correctly identified the four structured questions arising under the amended section 2 of the 1957 Act and provided the jury with a written “route to verdict”. On the key question of whether the Appellant’s abilities were “substantially impaired”, the judge told the jury that “substantially” was an everyday English word used in the statute, that he would not paraphrase or define it, and that they should apply their own common sense unless they encountered real difficulty, in which case they could ask for further assistance. The jury did not seek any further direction and returned a verdict of murder.

In the Court of Appeal (Criminal Division) the Appellant argued, among other grounds, that the judge ought to have directed the jury on the meaning of “substantially impaired”, and that in the absence of such direction the jury may have applied too stringent a test. It was submitted that any impairment greater than merely trivial satisfied the statutory requirement. The Court of Appeal dismissed the appeal but certified two questions of law of general public importance concerning (1) whether a judge must direct the jury on the meaning of “substantial” in section 2(1)(b), and (2) if so, whether it should be defined as “more than merely trivial” or as requiring something more (for example “significant and appreciable”).

The present opinion is the judgment of Judge Hughes, with whom the other Justices agreed, on the Appellant’s further appeal to the Supreme Court. The Court addressed the proper interpretation of “substantially impaired” under the amended Homicide Act 1957 and the circumstances in which trial judges should, or should not, attempt to define the term for juries. The appeal was dismissed.

Legal Issues Presented

1. In a murder trial where a defendant advances diminished responsibility under section 2 of the Homicide Act 1957 (as amended), is the trial judge required to direct the jury as to the definition of the word “substantial” in the phrase “substantially impaired” in section 2(1)(b)?
2. If a judge does give a direction on the meaning of “substantial”, must (or may) “substantial” be defined as “something more than merely trivial”, or should it instead be explained in a way that connotes a higher threshold, such as “something, whilst short of total impairment, that is nevertheless significant and appreciable” or otherwise of serious or weighty degree?
3. More generally, in the context of diminished responsibility, which of the two ordinary senses of “substantial” applies: (a) “more than minor or trivial” or (b) “important, weighty, of real importance”; and what are the implications of that choice for directions to juries and for psychiatric evidence?
4. In light of prior authority such as R v Brennan and R v Galbraith, in what circumstances, if any, should a trial judge withdraw the issue of murder from the jury where uncontradicted expert psychiatric evidence supports a plea of diminished responsibility?

Arguments of the Parties

Appellant's Arguments

• The trial judge erred in refusing to give any further guidance on the meaning of “substantially impaired” beyond repeating the statutory wording. It was contended that, where the phrase is central to the single live issue, the judge is required to direct the jury on its meaning.
• Because no definition was given, the jury might have applied too stringent a threshold for “substantial”, thereby wrongly rejecting the diminished responsibility defence.
• The correct interpretation of “substantially impaired” was said to be that any impairment which was more than merely trivial satisfied the statutory test. On this view, once it was accepted that there was some non‑trivial impairment caused by the recognised medical condition, the defence of diminished responsibility was established.

Respondent's Arguments

• Relying on prior authority, particularly R v Spriggs, R v Lloyd, and R v Ramchurn, the Respondent submitted that the judge was not required to define “substantial” and had directed the jury adequately by using the statutory language.
• “Substantially impaired” in the context of diminished responsibility had consistently been interpreted as requiring an impairment of real importance or serious degree, not merely something that exceeded triviality. The submission that “substantial” meant no more than “more than trivial” had already been rejected in earlier case law.
• The Respondent contended that equating “substantial” with “more than merely trivial” would undermine the policy of the partial defence, which is to reserve reduction from murder to manslaughter for cases where a weighty diminution of responsibility is shown.
• The Respondent further maintained that the circumstances of the present case, including the nature of the evidence and the psychiatrists’ agreed view that there was “substantial” impairment if the relevant condition existed, meant that any semantic debate about the word “substantially” was, in practical terms, marginal to the real factual issue: whether the Appellant was, on the balance of probabilities, suffering from such a condition at the time of the killing.

Table of Precedents Cited

Precedent	Rule or Principle Cited For	Application by the Court
Galbraith v HM Advocate 2002 JC 1	Historical and doctrinal survey of diminished responsibility in Scotland; establishes that the doctrine originated as a partial defence to mitigate the mandatory death penalty; confirms that diminished responsibility requires that “something far wrong” with the accused substantially impairing ability to determine or control conduct, without requiring a borderline of insanity.	Used to trace the origins of diminished responsibility, to show the parallel development in Scotland and England, and to support the interpretation of “substantially impaired” as denoting a serious, weighty impairment (“something far wrong”), not merely anything more than trivial. Also referenced in relation to the later Scottish statutory codification.
R v Matheson [1958] 1 WLR 474	Early application of the new partial defence under the Homicide Act 1957; no detailed discussion of “substantially impaired” because the defendant was accepted as certifiably mentally disordered.	Cited as part of the early English case law under the 1957 Act, contrasting with later decisions that examined the meaning of “substantially impaired”.
R v Spriggs [1958] 1 QB 270	Held that trial judges should generally direct juries in the statutory terms of section 2 without attempting to substitute synonyms or re‑define “substantially impaired”; discussed and rejected Scottish “borderline of insanity” formulations as obligatory tests.	Relied on as authority that the proper course is usually to leave the word “substantially” to the jury as an ordinary English term, reinforcing the Supreme Court’s conclusion that formal re‑definition is ordinarily unnecessary and potentially unhelpful.
R v Byrne [1960] 2 QB 396	Confirmed that inability to control violent or sexual impulses can constitute an abnormality of mind for diminished responsibility; emphasised that whether mental responsibility is “substantially impaired” is a question of degree for the jury, which may legitimately differ from medical opinion. Also discussed, in passing, “partial insanity” or the “border-line of insanity”.	Used to show: (a) the centrality of the jury’s role in assessing substantial impairment, (b) that references to “borderline insanity” were illustrative rather than a fixed test, and (c) that the word “substantially” indicates a serious degree of impairment, not mere triviality, while supporting the practice of not over‑elaborating the statutory wording.
HM Advocate v Savage 1923 JC 49	Example of earlier Scottish formulations referring to the borderline of insanity in diminished responsibility cases.	Cited via Spriggs and Galbraith as historical background to show why “borderline of insanity” language later came to be regarded as unhelpful and was not adopted as a necessary test.
R v Seers (1984) 79 Cr App R 261	Disapproved the use of “borderline of insanity” as a universal test for diminished responsibility, especially in depression cases; emphasised that Parliament’s use of “substantial” indicated a “serious degree of impairment of mental responsibility”.	Used to underline that “substantial” in this context denotes a serious or weighty impairment and that references to insanity are not a necessary or generally appropriate gloss on the statute.
R v Simcox The Times, 25 Feb 1964; [1964] Crim LR 402	Approved a direction asking the jury, in broad common‑sense terms, whether there was “a substantial impairment” of mental responsibility, contrasting substantial impairment with cases where there may be “some impairment” that “does not really make any great difference”.	Cited to show that directions emphasising the word “substantial” and asking juries to assess impairment in a common‑sense way are acceptable, and that the statutory wording suffices without elaborate re‑definition, while still excluding minor or moderate impairments.
R v Lloyd [1967] 1 QB 175	Rejected the submission that “substantial” meant merely “not trivial”; confirmed that “substantial” was inserted to carry real meaning and that it was not enough that there was some impairment above triviality. Approved directions stating that “substantial” does not mean total but also does not mean trivial or minimal, and that it lies “somewhere in between”.	Relied on as a key authority rejecting the argument, revived in the present appeal, that any impairment greater than trivial suffices. The Supreme Court held that Lloyd establishes that “substantially” in the diminished responsibility context requires an impairment of consequence or weight, not just more than trivial; it does not create two alternative meanings.
R v Egan [1992] 4 All ER 470	Dealt principally with diminished responsibility where both abnormality of mind and voluntary intoxication are present; referred to Gittens and repeated that intoxication is not an abnormality of mind. Also contained a passing suggestion that directions such as those in Lloyd could be used to explain “substantial”.	Discussed to show that any implication in Egan that “substantial” might mean merely “more than some trivial degree” was a misreading of Lloyd. The Supreme Court clarified that Egan did not establish two distinct legal thresholds for “substantial impairment”.
R v Dietschmann [2003] UKHL 10; [2003] 1 AC 1209	House of Lords authority on diminished responsibility where the defendant is intoxicated as well as mentally abnormal; confirmed that voluntary intoxication cannot itself found diminished responsibility and that the effect of alcohol is to be disregarded when assessing abnormality.	Cited to note that Egan’s principal reasoning on the treatment of drink in diminished responsibility was later largely endorsed in Dietschmann, while some other aspects of Egan were disapproved; serves as background on how intoxication fits within the diminished responsibility framework.
R v Gittens [1984] QB 698	Held that voluntary intoxication cannot itself constitute an abnormality of mind for diminished responsibility and that the jury should ignore the effect of drink and then consider whether the remaining matters amount to such abnormality as substantially impairs mental responsibility, referring to the meaning of “substantial” in Lloyd.	Used to show that later courts have treated Lloyd as the governing authority on “substantially impaired”, reinforcing the view that “substantial” has a single, weighty meaning rather than embracing alternative low thresholds.
R v Brown (Robert) [2011] EWCA Crim 2796; [2012] 2 Cr App R (S) 156	Sentence appeal in which the trial judge had directed the jury that “substantially impaired” meant impairment “more than minimal”; the defendant was convicted of manslaughter on that basis, but at sentence the judge found his responsibility had been “substantial” and imposed a very long sentence.	Relied on as an illustration of the practical difficulties and inconsistencies that can arise when “substantial” is equated with “more than minimal”, underscoring the need to clarify that the statutory threshold is higher than mere non‑triviality.
R v Absolam (1988) 88 Cr App R 332	Considered the exclusion of evidence under the Police and Criminal Evidence Act 1984 where police breaches of the Codes of Practice were “significant and substantial”.	Cited as an example of the use of “substantial” in the sense of “important or weighty” in another legal context, supporting the conclusion that, in diminished responsibility, “substantial” likewise carries the higher, not the “more than trivial”, meaning.
R v Keenan [1990] 2 QB 54	Another case employing the expression “significant and substantial” breaches of PACE Codes when considering the exclusion of evidence.	Used, together with Absolam, to illustrate that “substantial” can and does bear the meaning of something serious or weighty in legal contexts, reinforcing the choice of this sense in the diminished responsibility provision.
R v Dowds [2012] EWCA (Crim) 281; [2012] 1 WLR 2576	Confirmed in English law that voluntary intoxication alone cannot found a diminished responsibility plea under section 2 of the Homicide Act 1957.	Cited in parallel with Scottish law to show that both jurisdictions exclude voluntary intoxication as a sole basis for diminished responsibility, but that psychopathic personality disorder is now potentially within the scope of the defence following statutory reform in Scotland.
Brutus v Cozens [1973] AC 854	House of Lords authority that ordinary English words in statutes (e.g., “insulting”) should generally be left to the jury without judicial gloss or attempted definition.	Invoked to support the general principle that ordinary English terms such as “substantially” and “reasonable” should not usually be judicially re‑defined for juries, aligning with the Court’s conclusion that judges should usually refrain from defining “substantial” in diminished responsibility directions.
R v Ramchurn [2010] EWCA Crim 194; [2010] 2 Cr App R 18	Rejected the argument that Lloyd authorised two different legal meanings of “substantially impaired”; held that “substantially” is an ordinary English word designed to exclude trivial impairments while not requiring total impairment. The Court of Appeal described “substantial” as meaning “of real importance” or “having substance”.	Relied on heavily to confirm that there is only one standard of “substantial impairment” and that it excludes trivial or insignificant impairments without requiring near‑total loss of responsibility. Used to reinforce the rejection of the Appellant’s “more than trivial” interpretation and to support leaving the word to the jury in most cases.
Galbraith v HM Advocate (as applied in Scottish statute)	Adopted “substantially impaired” as the test for diminished responsibility in Scotland, explained as requiring “something far wrong” with the accused affecting control or determination of conduct.	Cited again to show that, when the Scottish Parliament adopted the term “substantially impaired” in the Criminal Procedure (Scotland) Act 1995, it did so against the background of Galbraith’s understanding that the impairment must be substantial in the sense of serious or far‑reaching.
R v Brennan [2014] EWCA Crim 2387; [2015] 1 WLR 2060	Concerned a defendant with undisputed serious psychiatric disorders; the Court of Appeal held that, on uncontradicted expert evidence accepted by the Crown, there was no proper basis for a murder verdict and that murder should not have been left to the jury. The decision applied the general principles in R v Galbraith (English case) to a diminished responsibility context.	Discussed to illustrate when, exceptionally, it may be appropriate to withdraw murder from the jury where the psychiatric evidence overwhelmingly and uncontestedly supports diminished responsibility and the Crown can suggest no rational basis to reject it. The Supreme Court endorsed aspects of the approach but cautioned that such withdrawal should be rare and carefully justified.
R v Galbraith [1981] 1 WLR 1039; 73 Cr App R 124	Leading authority on the circumstances in which a trial judge can withdraw a case from the jury on the basis that no reasonable jury, properly directed, could convict.	Applied by analogy in discussing Brennan and the present case: the Court explained that, in diminished responsibility cases, murder can only be withdrawn from the jury where no reasonable jury could fail to find that the defendant has proved diminished responsibility, stressing that this threshold will be met only rarely.

Court's Reasoning and Analysis

1. Statutory framework and effect of the 2009 amendments

The Court began by setting out section 2 of the Homicide Act 1957 as amended by the Coroners and Justice Act 2009. The amended provision defines diminished responsibility by reference to an “abnormality of mental functioning” arising from a recognised medical condition which (a) substantially impairs the defendant’s ability to understand the nature of conduct, form a rational judgment, or exercise self‑control, and (b) provides an explanation for the killing.

The Court noted that, although the statutory structure has changed, the expression “substantially impaired” was carried over from the original section 2(1), where it related to “mental responsibility” in a more global sense. Under the new formulation, juries are directed to answer four specific questions: (1) whether the accused suffered from an abnormality of mental functioning, (2) whether it arose from a recognised medical condition, (3) whether it substantially impaired one or more of the listed abilities, and (4) whether it caused or significantly contributed to the killing. The judge’s function is to focus the jury on the issues in dispute, not to provide general lectures on the law.

2. Historical case law on “substantially impaired”

Surveying early English decisions, the Court observed that:

• Spriggs advocated leaving “substantially impaired” in the statutory language without synonyms.
• Byrne stressed that whether impairment is “substantial” is a question of degree for the jury, and that juries may differ from medical experts on this issue. Although Byrne referred to “partial insanity” or the “border-line of insanity”, later cases such as Seers rightly treated those phrases as merely illustrative and potentially misleading, especially in cases of depression.
• Seers underlined that Parliament’s use of “substantial” indicates a “serious degree of impairment”, not a minimal one.
• In Simcox, a direction asking the jury whether, in broad common‑sense terms, the impairment was “substantial” (as opposed to something that “did not really make any great difference”) was approved.

The key development came in Lloyd, where the Court of Criminal Appeal faced the explicit submission that “substantial” meant merely “not trivial” or “really present”. The court firmly rejected this. It approved a direction explaining that “substantial” does not mean total, but also does not mean trivial or minimal; it is “something in between”, to be assessed by the jury on the evidence. The Supreme Court emphasised that Lloyd is an authority that “substantial” does not mean simply “more than trivial”.

The Court acknowledged that an “Ashworth spectrum” formulation – placing trivial impairment at one end and total impairment at the other, with “substantial” somewhere in between – had been adopted in some later directions and in Crown Court Benchbooks. However, this had at times encouraged the mistaken view that any impairment above triviality sufficed. The Court noted that some later readings of Egan and benchbook formulations had wrongly implied two possible meanings of “substantial”, when in fact Lloyd rejected a “more than trivial” standard.

3. Scottish law and statutory codification

The Court next referred to the Scottish decision in Galbraith v HM Advocate, which reviewed the law of diminished responsibility and concluded that it was not confined to “mental illness” but extended to recognised mental abnormalities, such as the disorders in that case. Galbraith required “some recognised mental abnormality” and summarised the rule as demanding that there be “something far wrong” with the accused, affecting the way he acted, so that his ability to determine or control his conduct was substantially impaired.

Following Galbraith, Scotland enacted section 51B of the Criminal Procedure (Scotland) Act 1995, using the same expression “substantially impaired” to define the level of impairment required. The Supreme Court inferred that, in Scotland, the phrase was used with the Galbraith meaning of a serious, significant impairment.

The Court contrasted the Scottish and English positions on psychopathic personality disorder and voluntary intoxication, noting that voluntary intoxication is excluded as a sole basis for diminished responsibility in both jurisdictions, but that Scottish statutory reform had now removed an earlier exclusion of psychopathic personality disorder.

4. Linguistic analysis of “substantial”

The Court accepted that “substantial” has two ordinary dictionary senses:

• First, “present rather than illusory or fanciful; more than minor or trivial” (used, for example, in the Equality Act 2010, where “substantial” is defined as “more than minor or trivial” for disability discrimination purposes).
• Second, “important, weighty, of real importance”, as in a “substantial meal” or “substantial salary”, and in legal phrases such as “significant and substantial” breaches of police Codes of Practice leading to exclusion of evidence.

Which sense applies depends on context. The Court observed that, while the first sense has been deliberately adopted in equality legislation, the case law on diminished responsibility has, whenever the issue has been confronted, used “substantially” in the second sense: an impairment of real importance, seriousness or weight.

The Court accepted that the word “substantially” is not entirely redundant even where it is defined statutorily as “more than minor or trivial”, but held that the central thrust of Lloyd and subsequent authorities in the diminished responsibility context was that a higher, weightier level of impairment is required.

5. Parliamentary intent and Law Commission recommendations

Because Parliament chose to retain the wording “substantially impaired” in the 2009 reform, applying it to the defendant’s specific capacities rather than to “mental responsibility” in general, the Court concluded that Parliament intended to preserve the established judicial meaning. In the absence of any contrary indication, the same sense of “substantially” – a serious, significant impairment – should be presumed.

The Law Commission had considered various alternative formulations in its reports Partial Defences to Murder (Law Com 290) and Murder, Manslaughter and Infanticide (Law Com 304), many of which retained the term “substantial”. The Commission ultimately recommended preserving the workable statutory formula, while adding an explicit causation requirement. The Commission noted that the concept had been pragmatically applied in practice, that Byrne remained the leading authority, and that diminished responsibility had caused relatively little appellate difficulty. This history supported the conclusion that no change of meaning was intended.

6. Policy rationale: why a serious degree of impairment is required

The Court emphasised that diminished responsibility produces a “radical alteration” in the outcome: it reduces murder, a homicide committed with murderous intent and without provocation or self‑defence, to manslaughter. Given the gravity of the offence and the public importance of a murder conviction, it is appropriate that only a substantial, weighty impairment of one or more of the statutory abilities should justify this reduction. An impairment that is only just beyond triviality does not provide a sufficient reason to downgrade murder to manslaughter.

7. Directions to juries: when to define, and how

The Court reiterated the general principle, supported by cases such as Brutus v Cozens, that ordinary English words which involve questions of degree (e.g. “reasonable”, “sure”, “insulting”) should normally be left to the jury without judicial gloss. The same applies to “substantially” in most diminished responsibility cases.

However, because “substantial” has two distinct ordinary senses, there is a danger of inconsistent usage and semantic disputes. This is particularly important in diminished responsibility cases where psychiatric evidence is central and experts routinely express opinions on the “substantial” nature of impairment. Consistent understanding of the term is needed to avoid misunderstandings and unnecessary cross‑examination.

The Court concluded that:

• The statutorily relevant sense of “substantially impaired” in diminished responsibility is the higher, weighty sense, not “anything more than trivial”.
• Ordinarily, in a murder trial where diminished responsibility is in issue, the judge need not go beyond the statutory words. The jury should be told that “substantially” is an ordinary English word, that it involves a question of degree, and that whether the impairment is substantial is for them to decide.
• If, however, the jury has been exposed (by counsel, experts, benchbooks or otherwise) to the notion that “any” impairment beyond the merely trivial suffices, or to a “spectrum” illustration that might suggest that, then the judge should make it clear that this is not the law: although the impairment must exceed mere triviality, not every such impairment is “substantial”.
• Similarly, if an expert or counsel employs the phrase “more than merely trivial”, the judge should correct any potential misunderstanding by explaining that the statutory test requires the jury to be satisfied that the impairment reaches the higher “substantial” level.
• The “Ashworth spectrum” illustration (placing trivial at one end, total impairment at the other) may sometimes assist juries by indicating that total impairment is unnecessary and that “substantial” is a matter of degree. But if used, it must be explicitly coupled with a statement that not all non‑trivial impairments are substantial. Where the illustration has not featured in the case, it is preferable that the judge does not introduce it.
• Illustrative expressions such as “significant and appreciable” (if “appreciable” is understood as “considerable”), “serious degree of impairment” (Seers), “not total but substantial” (Ramchurn), or “something far wrong” (Galbraith) may be used, so long as the jury is not told that any single synonym replaces the statutory word.

The Court stressed that, once the correct usage is understood by judges, counsel and experts, trials should not become side‑tracked by debates over the semantics of “substantially”. The focus should remain on the practical effect of the medical condition on the defendant’s abilities.

8. Role of expert evidence and guidance from Brennan

Turning to R v Brennan, the Court described it as a case where the psychiatric evidence, including a report obtained by the Crown, was uncontradicted and strongly favoured diminished responsibility: the defendant suffered serious and long‑standing mental disorders, and the expert evidence, essentially unchallenged in key respects, was that his abilities to form rational judgments and control himself were substantially impaired, independently of any intoxication.

The Court agreed that the Crown’s function in such cases is to assess the expert evidence and its interaction with factual disputes. If it is clear that the defendant was suffering from a recognised medical condition which substantially impaired one of the relevant abilities and significantly contributed to the killing, the Crown may properly accept a plea to manslaughter without a trial. If the Crown nevertheless wishes the jury to consider murder, it must articulate a rational basis upon which the jury could reject or differ from the expert opinion.

The Court acknowledged that there are legitimate bases on which a jury might disagree with experts – for example, where extensive pre‑planning suggests preserved self‑control inconsistent with the alleged level of impairment, or where drink or drugs may be the true proximate cause of the killing. However, it is not proper simply to invite the jury to convict of murder without explaining why the expert evidence should or might be rejected. Nor is the brutality or sadism of the killing, by itself, an adequate ground for rejecting diminished responsibility, as such features may themselves be manifestations of mental disorder.

9. Application of R v Galbraith (English case) and judicial caution

The Court accepted that the ordinary principles in R v Galbraith (English authority on no‑case submissions) can, in principle, apply in diminished responsibility trials: there may be cases where, on the evidence, no reasonable jury could fail to conclude that diminished responsibility is proved. However, the Court urged particular caution for several reasons:

• A contested murder trial is a highly sensitive matter; where murder with intent has occurred, it is important that verdicts ordinarily be returned by the jury.
• The burden of proof on diminished responsibility lies on the defendant (on the balance of probabilities). Withdrawing murder from the jury would only be justified where no reasonable jury could conclude that the defendant had failed to discharge that burden.
• Diminished responsibility involves multiple components: recognised medical condition, abnormality of mental functioning, substantial impairment of specific abilities, and causation. Causation and the assessment of substantial impairment are essentially jury questions.

Accordingly, although Brennan was an example where the court considered that only one verdict was realistically open, such cases are rare. Where expert evidence supports the plea and is uncontradicted, the judge should ensure that the Crown’s basis for inviting the jury to reject that evidence is properly articulated and is one upon which the jury could legitimately act. The judge should also caution the jury against becoming “amateur psychiatrists” and indicate that, in the absence of a good reason, undisputed expert evidence will usually be accepted, while making clear that the ultimate decision is the jury’s.

10. The present case

On the facts of this case, the Appellant and deceased had cohabited for around three years. On the day of the killing there were arguments about alleged past violence, financial disputes, and the Appellant’s wish to leave the barbecue early. After they returned home, further arguments and physical incidents occurred, culminating in the fatal attack. The presence and interventions of the deceased’s children, the removal of the first knife, and the Appellant’s decision to resume the attack with a second knife were all before the jury.

The psychiatric evidence showed a long history of mental health issues. One consultant diagnosed a mixed personality disorder together with an acute psychotic episode driven by persecutory beliefs at the time of the killing. The other rejected personality disorder but diagnosed a paranoid psychotic illness, most likely schizophrenia. Both concluded that the Appellant’s relevant abilities were substantially impaired, though they disagreed about which specific abilities were affected.

The Crown’s case was that the killing was driven by anger over domestic grievances rather than by psychotic thinking. Evidence relevant to self‑control included the Appellant’s prior assaults on the deceased, said to have been controlled, and the sequence of events on the night, including reaction to the children’s presence and to the removal of the first knife. The reality, as the Supreme Court saw it, was that the dispute between the parties over the meaning of “substantial” scarcely arose on these facts. If the jury accepted that the Appellant was experiencing a psychotic episode with persecutory delusions at the time of the killing, his impairment would plainly be substantial under any proper interpretation. The real factual issue was whether he was in that state at the relevant time, a question the judge left squarely to the jury.

The trial judge reminded the jury that the doctors agreed there was a medical condition substantially impairing the relevant abilities, but correctly treated the assessment of whether the statutory test was satisfied as a matter for the jury. In light of the Court’s conclusions on the correct meaning of “substantially” and the circumstances in which definition is or is not required, there was no misdirection.

Holding and Implications

Holding: The Supreme Court dismissed the appeal. DISMISSED.

The Court answered the certified questions as follows:

• First question: Ordinarily, a trial judge in a murder case where diminished responsibility is in issue is not required to define “substantially” for the jury beyond using the statutory language. “Substantially” is an ordinary English word involving a question of degree, and the jury should generally be left to apply it using their collective common sense.
• Second question: If, exceptionally, the judge does direct the jury on the meaning of “substantial” – in particular where the idea that “any impairment more than trivial” suffices has been introduced – it must be made clear that this is not the law. While impairment must exceed the merely trivial before it can be considered, “substantially impaired” in this context means an impairment of real importance, seriousness, or weight (for example, “significant and appreciable” in a strong sense), and not simply anything above triviality. No single synonym should be prescribed or substituted for the statutory term, but illustrative expressions may be used to convey this higher threshold.

Immediate implications for the parties: The Appellant’s conviction for murder stands. The jury’s verdict is upheld because the trial judge’s refusal to define “substantially impaired” and his reliance on the statutory wording were consistent with the law as clarified by the Supreme Court. Any alleged risk that the jury applied too stringent a test did not amount to a misdirection warranting reversal.

Wider implications: The decision provides authoritative clarification that, in the context of diminished responsibility under section 2 of the Homicide Act 1957 (as amended), “substantially impaired” does not mean simply “more than trivial” but denotes a serious, weighty impairment of the defendant’s ability to understand the nature of conduct, form a rational judgment, or exercise self‑control.

The judgment offers structured guidance for trial judges:

• In most cases, they should not attempt to define “substantial”; they should instead emphasise that it is an ordinary word, that it involves a question of degree, and that its application is for the jury.
• Where there is any risk that the jury has been directed, or may assume, that any impairment above triviality suffices, judges must correct that impression and affirm that a higher level of impairment is required.
• Judges and practitioners should exercise caution in deploying the “spectrum” illustration and in using expressions such as “more than minimal” or “more than trivial” when discussing the defence.
• Forensic psychiatrists and other experts are expected henceforth to understand and employ “substantially impaired” in the higher, weighty sense identified by the Court, thereby promoting consistency between expert evidence and the legal standard.

The Court also endorsed, with cautions, the application of R v Galbraith principles to diminished responsibility cases in light of R v Brennan. While uncontradicted psychiatric evidence may sometimes justify withdrawing murder from the jury, the Court stressed that such cases will be rare and that the burden remains on the defendant to prove diminished responsibility. The judgment therefore refines existing law and practice rather than creating a new doctrinal framework, offering clarity on both the meaning of “substantially impaired” and on good practice in directing juries and handling expert evidence.