Core principles of criminal liability - Partial defences

Learning Outcomes

This article covers partial defences to murder in criminal law, including:

• The statutory requirements for loss of control under ss 54–55 Coroners and Justice Act 2009 and how to apply them
• The statutory requirements for diminished responsibility under s 2 Homicide Act 1957 (as amended) and how to apply them
• The distinction between partial and complete defences and when each can be raised in practice
• Qualifying triggers for loss of control, exclusions, and the objective “might have reacted similarly” test
• Assessing abnormality of mental functioning from a recognised medical condition and determining substantial impairment
• The evidential gatekeeping role of the judge under s 54(5) and the respective burdens and standards of proof
• Sentencing consequences of a successful partial defence, including reduction to voluntary manslaughter and potential hospital orders
• Common pitfalls to avoid, such as sexual infidelity standing alone, considered revenge, and voluntary intoxication alone for diminished responsibility

SQE2 Syllabus

For SQE2, you are required to understand the practical operation and consequences of partial defences to criminal liability, especially in the context of homicide, with a focus on the following syllabus points:

• the definition and key legal elements of loss of control
• the definition and key legal elements of diminished responsibility
• the effect of each partial defence on criminal liability for murder
• the differences and overlaps between partial and complete defences in criminal law
• the burden and standard of proof for partial defences

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. Which two partial defences can reduce a charge of murder to voluntary manslaughter?
2. What are the three core requirements for the defence of loss of control?
3. Who bears the burden of proof in establishing diminished responsibility, and to what standard?
4. True or false? The partial defence of loss of control requires the loss to be sudden.

Introduction

When advising clients or preparing for the SQE2 exam, it is essential to understand partial defences—specifically, loss of control and diminished responsibility. These partial defences do not provide a full acquittal but can reduce liability for murder to voluntary manslaughter, affecting both sentencing and criminal records. This article explains the core requirements of each partial defence and the legal and practical consequences of raising them.

Key Term: loss of control
A statutory partial defence to murder, available if the defendant lost self-control due to a qualifying trigger and an objective person might have acted similarly.

Loss of Control

The partial defence of loss of control allows a defendant charged with murder to have their liability reduced to voluntary manslaughter if strict statutory criteria are met.

Loss of control was introduced to replace the old defence of provocation and is governed by s 54 and s 55 of the Coroners and Justice Act 2009. Loss of control can only be pleaded in response to a charge of murder.

To establish this partial defence, the following must be shown:

1. The defendant must have lost self-control. This means a loss of the ability to act with considered judgment or normal powers of reasoning. There is no requirement that the loss must be sudden, but greater delay makes its success less likely.
2. The loss of self-control must result from a qualifying trigger. There are two possible triggers:
   • Fear of serious violence from the victim against the defendant or another identified person.
   • Things said or done of an extremely grave character causing the defendant to have a justifiable sense of being seriously wronged.
3. A person of the defendant's sex and age, with a normal degree of tolerance and self-restraint and in the defendant's circumstances, might have reacted as the defendant did. This is an objective test.

There are important exclusions:

• If the loss of control was induced by a considered desire for revenge, the defence will fail.
• Sexual infidelity cannot by itself be a qualifying trigger (but may be part of the overall context).
• The defendant cannot rely on a trigger they incited for the purpose of providing an excuse for violence.

Key Term: qualifying trigger
A reason for the loss of self-control that meets strict statutory definitions: fear of serious violence, or things done/said of extremely grave character causing justifiable sense of being seriously wronged.

The statutory structure imposes both evidential and legal burdens. The defendant carries an evidential burden to adduce sufficient evidence so the issue can be left to the jury. Under s 54(5), the judge acts as a gatekeeper: if a properly directed jury could not reasonably conclude the defence might apply, the judge must withdraw it. If left to the jury, the prosecution must disprove loss of control beyond reasonable doubt.

The first element is subjective. The jury assesses whether this particular defendant lost self-control, not whether a reasonable person would have done so. Evidence of planning, arming oneself, or calm deliberation can undermine a claim that self-control was actually lost. Equally, in cases involving cumulative abuse, the loss may occur after a build-up of events; the statute recognises that the loss need not be “sudden”. Nonetheless, the more reflective and delayed the response, the more likely it is to be characterised as revenge or calculated retaliation.

The second element focuses on qualifying triggers:

• Fear trigger: the defendant must fear serious violence from the victim, directed at the defendant or another identified person. The fear must be genuine; it can be based on prior threats or patterns of violence. While the defendant’s perception is relevant, fabricated or self-induced triggers are excluded where the defendant acted to provide an excuse for violence or deliberately incited the situation.
• Anger trigger (things said or done): the stimulus must be of an “extremely grave” character and must cause a “justifiable” sense of being “seriously wronged”. Ordinary insults, minor provocations, or lawful conduct that is merely upsetting will not suffice. The assessment has a mixed objective flavour: the words or conduct must be sufficiently grave, and the sense of wrong must be justifiable when viewed objectively, albeit assessed in the defendant’s circumstances as to gravity.

Sexual infidelity cannot, by itself, constitute a qualifying trigger. However, it may form part of the background circumstances that give context to other qualifying triggers. The mere presence of infidelity does not automatically exclude the defence where other triggers exist, but a case framed solely as a response to infidelity will fail.

The third element requires the jury to consider whether a person of the defendant’s sex and age, with normal tolerance and self-restraint, and in the defendant’s circumstances, might have reacted similarly. This is an objective test intended to exclude reliance on unusually low tolerance or poor self-control. “Circumstances” includes relevant contextual factors affecting gravity (e.g. a history of abuse) but does not adjust the standard of self-restraint beyond age and sex. Intoxication should not be imported into the objective standard: a drunken defendant cannot lower the benchmark of normal tolerance and self-restraint by reference to intoxication.

Self-induced triggers, calculated revenge, and retaliatory violence are excluded. Where the defendant purposely provokes the victim to generate an excuse for violence, the defence is barred. Similarly, s 54(4) excludes cases of a considered desire for revenge: that is where the defendant’s action is the product of reflection and decision, as opposed to spontaneous loss of control.

Worked Example 1.1

Case study: Sarah endured years of domestic abuse from her partner. After a particularly humiliating and threatening incident, Sarah snapped and fatally stabbed her partner the next day. Does the defence of loss of control apply?

Answer:
The delayed response does not itself prevent the loss of control defence. If Sarah's actions resulted from a loss of self-control attributable to fear of serious violence or things said/done of extremely grave character, and a person in her circumstances might have acted similarly, the defence may be raised. The jury would consider if she lost self-control, identify a qualifying trigger, and apply the objective test.

The evidential threshold is critical. A mere assertion of “I lost control” is not enough. The defence should marshal evidence of threats, prior violence, humiliation, or grave conduct, as well as evidence explaining the defendant’s state of mind. The more the evidence points to planning (e.g. acquiring a weapon, lying in wait, choosing a moment of maximum vulnerability), the more likely a judge will refuse to leave the defence to the jury. By contrast, where the evidence substantiates a genuine loss of normal powers of reasoning in response to fear or grave wrong, the defence should be left.

The boundary with complete defences must be kept clear. Self-defence (including defence of another and prevention of crime) is a complete defence where the use of reasonable force was necessary as the defendant believed it to be. Loss of control is not a justification: it acknowledges human frailty and mitigates culpability. Advise clients on both where appropriate. If self-defence fails, loss of control may still mitigate to manslaughter if the statutory criteria are met.

In practice:

• Identify the trigger(s). Separately analyse fear and anger. Check for exclusionary factors (revenge; self-induced).
• Assess “extremely grave” and “seriously wronged” objectively, while factoring the defendant’s circumstances as to gravity.
• Apply the objective “might have reacted similarly” test, maintaining normal tolerance/self-restraint, unaffected by intoxication.
• Consider whether cumulative abuse and coercive control form the context in which fear or grave wrong arose.

Worked Example 1.2

Scenario: Omar learns his partner has been unfaithful. The next day, during an argument, she taunts him about the affair and also says she will falsely accuse him of sexually assaulting their child to have him imprisoned. Omar lashes out and kills her. Can sexual infidelity ground loss of control?

Answer:
Sexual infidelity alone cannot be a qualifying trigger. However, if the taunt about false allegations constitutes “things said” of an extremely grave character that cause a justifiable sense of serious wrong, the defence may be considered. The infidelity can be part of the context but is not the basis for the trigger. The court will still examine the objective limb and any evidence of revenge.

Worked Example 1.3

Scenario: Lena, intoxicated after drinking all night, gets into a heated exchange with a neighbour who has repeatedly threatened to assault her teenage son. She stabs the neighbour. How does intoxication affect loss of control?

Answer:
Intoxication does not lower the standard of normal tolerance and self-restraint in the objective test. The jury considers whether a person of Lena’s age and sex with normal tolerance, and in her circumstances (including prior threats to her son), might have reacted similarly. Intoxication is disregarded as a characteristic for the objective limb. The fear trigger may be relevant if the neighbour’s threats were of serious violence.

Exam Warning

For SQE2, remember that loss of control requires a qualifying trigger and an objective element; motive alone (e.g. anger, embarrassment) is not sufficient. The defence is only available to murder charges—never to manslaughter or other offences. The defendant must adduce sufficient evidence; the judge may withdraw the defence if the evidential threshold is not met, after which the prosecution need not disprove it.

Diminished Responsibility

Diminished responsibility is a statutory partial defence to murder under s 2 of the Homicide Act 1957 (as amended). If successful, it reduces liability from murder to voluntary manslaughter.

Key Term: diminished responsibility
A partial defence to murder available if the defendant was suffering an abnormality of mental functioning that substantially impaired their responsibility for the killing, arising from a recognised medical condition.

To establish diminished responsibility, the defendant must prove:

1. They were suffering from an abnormality of mental functioning.
2. The abnormality arose from a recognised medical condition.
3. The abnormality substantially impaired the defendant's ability to:
   • understand the nature of their conduct,
   • form a rational judgment, or
   • exercise self-control.
4. The abnormality was a significant cause of the defendant's conduct in killing the victim.

Key Term: recognised medical condition
Any medical or psychiatric disorder accepted by the courts or medical profession, including mental illness, some personality disorders, intellectual disability, and some physical conditions like epilepsy.

The burden is on the defence to establish diminished responsibility on the balance of probabilities. If successful, the defendant is convicted of voluntary manslaughter rather than murder, allowing the judge discretion in sentencing.

Each statutory element warrants careful analysis.

Abnormality of mental functioning describes a state of mind so different from that of ordinary human beings that the reasonable person would describe it as abnormal. It is broader than “insanity” and does not require loss of contact with reality. It covers disturbances in the defendant’s mental processes that affect understanding, judgment, or self-control. Evidence from psychiatrists or clinical psychologists is almost always necessary.

A recognised medical condition includes psychiatric diagnoses such as severe depressive disorder, bipolar disorder, schizophrenia, schizoaffective disorder, PTSD, autism spectrum disorder, and certain personality disorders (e.g. emotionally unstable/borderline). It also encompasses neurological or physical conditions with mental effects, such as epilepsy and dementia. Adjustment disorders may qualify in appropriate cases. The diagnosis must be genuine and operative at the time of the killing; transient emotions or stress alone are insufficient.

“Substantial impairment” requires more than a minimal or trivial effect, but not total impairment. It must be a weighty or significant limitation on one or more of the statutory capacities. The jury decides this, guided by expert evidence. Practical application involves asking: did the condition significantly impair the defendant’s ability to understand their conduct, form rational decisions, or exercise self-control at the material time? The impairment need only affect one of the three capacities.

Causation is essential. The abnormality must provide an explanation for the killing. It need not be the sole cause, but it must be a significant contributory factor. If the killing was driven by anger or retaliation unrelated to the condition, the defence will fail. The jury must be satisfied that the abnormality substantially impaired responsibility for the homicide.

Intoxication and diminished responsibility require careful handling:

• Voluntary intoxication from alcohol or drugs alone cannot found diminished responsibility. Public policy limits reliance on voluntary intoxication in this context.
• Where the defendant suffers from a recognised medical condition and intoxication triggers or exacerbates its effects, the defence may still succeed if the abnormality, not the intoxication alone, substantially impaired responsibility.
• Alcohol dependency syndrome (ADS) is a recognised medical condition. The jury must differentiate between alcohol consumed due to the dependency and purely voluntary consumption. They focus on the effect of ADS and disregard the effect of volitional drinking unrelated to the syndrome. Expert evidence is important to assist the jury in separating these strands.

Worked Example 1.4

Scenario: James is charged with murder after killing his business partner. At trial, expert evidence shows James suffers from severe depression and at the time of the act was unable to form rational judgments. Can James plead diminished responsibility?

Answer:
If the court accepts that James was suffering from an abnormality of mental functioning (severe depression) that arose from a recognised medical condition and that his ability to form rational judgment was substantially impaired, and this was a significant cause of the killing, James may successfully plead diminished responsibility.

Worked Example 1.5

Scenario: Priya, diagnosed with alcohol dependency syndrome, killed a stranger during an intoxicated episode. Evidence shows her consumption was driven by dependency, but she also chose to drink excessively that night at a party. How does ADS affect diminished responsibility?

Answer:
ADS is a recognised medical condition. The jury should assess whether the abnormality substantially impaired Priya’s ability to exercise self-control or form rational judgment, focusing on alcohol consumed due to the dependency and excluding the effect of purely voluntary drinking. If the dependency-related abnormality was a significant cause of the killing and substantially impaired her relevant capacity, diminished responsibility may succeed.

Worked Example 1.6

Scenario: Alex kills during a flash of anger unrelated to his diagnosed paranoid personality disorder. He argues diminished responsibility because he has a recognised condition.

Answer:
A diagnosis alone is insufficient. The abnormality must substantially impair one of the statutory capacities and provide an explanation for the killing. If Alex’s act was unrelated to the condition and not causally linked to a substantial impairment, the defence fails.

Exam Warning – Diminished Responsibility

The partial defence of diminished responsibility only applies to murder. The defendant must call medical evidence to support the existence and effects of the recognised medical condition at the time of the crime. Voluntary intoxication alone cannot suffice; the abnormality must cause substantial impairment and explain the killing. The defence carries the legal burden and must be proved on the balance of probabilities.

How Partial Defences Affect Criminal Liability

Partial defences do not provide a complete acquittal. If proven, they reduce murder to voluntary manslaughter. This can significantly affect sentencing, since the mandatory life sentence for murder does not apply to manslaughter. The judge has discretion in sentencing, and a wide range of sentences may be available.

Key Term: voluntary manslaughter
An offence where all elements of murder are present but the defendant successfully raises a partial defence such as loss of control or diminished responsibility.

Sentencing for voluntary manslaughter is discretionary. Outcomes range from community sentences in exceptional cases, through determinate custodial terms, to life sentences where appropriate. Where diminished responsibility is established and the defendant requires treatment, the court may prefer a hospital order with or without restrictions under the Mental Health Act, reflecting the therapeutic and protective aims of sentencing. In loss of control cases, sentencing often turns on proportionality, the gravity of the trigger, and residual culpability.

Running both partial defences is permissible where the evidence supports each. The judge may leave both to the jury. The burdens differ: diminished responsibility must be proved by the defence; loss of control must be raised evidentially and then disproved by the prosecution. Advise clients on the distinct evidential foundations: expert medical reports for diminished responsibility; factual testimony and contextual materials for loss of control.

Partial defences sit alongside complete defences. Self-defence, defence of another and prevention of crime (under the statutory and common law framework) can, if made out, lead to full acquittal. Duress is generally not available for murder. If complete defences fail, partial defences may still reduce liability to manslaughter where criteria are met.

Worked Example 1.7

Scenario: Amal is convicted of murder after raising the defence of diminished responsibility. The jury is satisfied on the balance of probabilities. What is the effect on sentencing?

Answer:
Amal is convicted of voluntary manslaughter, and the sentencing judge now has full discretion. There is no requirement for a mandatory life sentence. Amal may receive a determinate sentence, a hospital order, or another outcome appropriate to the circumstances.

Worked Example 1.8

Scenario: Ben faces a murder charge after a fatal altercation. He raises self-defence and, in the alternative, loss of control. The jury rejects self-defence (force not reasonable), but accepts that Ben lost self-control in response to a qualifying trigger and that an ordinary person might have reacted similarly. What is the legal outcome?

Answer:
Ben is guilty of voluntary manslaughter, not murder. Self-defence would have been a complete defence, but its failure does not preclude loss of control as a partial defence if its statutory criteria are satisfied.

Revision Tip

For the SQE2 exam, always set out whether the evidence could satisfy the elements of loss of control or diminished responsibility. Apply each statutory element separately, and check who has the burden of proof and to what standard.

Summary

Key Point Checklist

This article has covered the following key knowledge points:

• Loss of control and diminished responsibility are partial defences to murder only.
• Loss of control requires proof of a loss of self-control, a qualifying trigger, and that an objective person might have acted similarly.
• Diminished responsibility requires an abnormality of mental functioning from a recognised medical condition that substantially impaired responsibility for the killing.
• Both partial defences, if successfully raised, reduce liability from murder to voluntary manslaughter, avoiding a mandatory life sentence.
• The defence has the burden of proving diminished responsibility on the balance of probabilities; for loss of control, the defence must raise evidence and the prosecution must disprove it.
• Voluntary manslaughter allows the judge discretion in sentencing.
• Sexual infidelity alone cannot constitute a qualifying trigger; revenge and self-induced triggers bar loss of control.
• Voluntary intoxication alone cannot ground diminished responsibility; alcohol dependency syndrome and other recognised conditions may.
• The judge may withdraw loss of control from the jury if the evidential threshold in s 54(5) is not met.

Key Terms and Concepts

• loss of control
• qualifying trigger
• diminished responsibility
• recognised medical condition
• voluntary manslaughter