Core principles of criminal liability - Causation in crimina...

Learning Outcomes

This article explains causation in criminal law for result crimes, setting out how to identify and apply the requirements of factual and legal causation, differentiate between background conditions and substantial, operating causes, and use the "but for" test alongside the "more than minimal" threshold in SQE1-style problem questions. It explains how to deal with multiple contributing factors, acceleration of death or harm, and overlapping causes while maintaining a structured, step-by-step analysis. It examines when the chain of causation is broken or preserved by novus actus interveniens, with particular attention to free, deliberate and informed third-party acts, victim responses such as escape, instinctive self-defence and rescue, ordinary and negligent medical treatment, self-administration of drugs, and natural events. It further explains the thin skull rule and its application to physical frailty, psychiatric vulnerability, religious or moral beliefs, and refusal of treatment. Finally, it explains how causation principles apply to complex end-of-life scenarios, including withdrawal of life support and rare cases involving voluntary euthanasia, enabling precise reasoning and exam-focused evaluation of criminal liability.

SQE1 Syllabus

For SQE1, you are required to understand the principles of causation as part of the core elements of criminal liability, with a focus on the following syllabus points:

• the meaning and application of factual causation (the "but for" test)
• the requirements for legal causation (substantial and operating cause)
• the effect of intervening acts (novus actus interveniens), including third-party actions, victim conduct, and medical treatment
• the thin skull rule and its consequences for liability
• how to apply causation principles to realistic problem scenarios
• how foreseeability and proportionality guide “escape” cases and other victim responses
• self-administration of drugs and the effect of free, deliberate and informed decisions
• the impact of natural events (acts of God), withdrawal of life support, and rare cases involving euthanasia

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. What is the difference between factual causation and legal causation in criminal law?
2. When will an intervening act break the chain of causation?
3. How does the thin skull rule affect a defendant’s liability?
4. Can negligent medical treatment ever break the chain of causation? If so, when?

Introduction

Causation is a core requirement for criminal liability in result crimes. It ensures that a defendant is only held responsible for a prohibited consequence (such as death or injury) if their conduct actually caused that result. Causation is assessed in two stages: factual causation and legal causation. Both must be satisfied for liability to arise. This article explains the key principles, common pitfalls, and how to apply causation rules in SQE1 scenarios.

Causation is not a purely scientific exercise. It includes normative judgments about responsibility. A defendant can be a “but for” cause without being the legal cause if their conduct merely sets the stage rather than substantially contributes to the result. Conversely, a defendant remains liable even where others also contribute, provided their conduct remains a more than minimal, operating cause at the time of the result.

Factual Causation

Factual causation asks whether the result would have occurred "but for" the defendant’s conduct. If the answer is yes, factual causation is not established. If the answer is no, factual causation is present. In most cases the test is straightforward; however, two recurring patterns require attention:

• Acceleration: where the victim was already on course to die, but the defendant’s conduct accelerates the death, factual causation is satisfied because the result occurred sooner than it otherwise would have.
• Multiple factors: the “but for” test does not require the defendant to be the sole or even the main cause—only that their conduct was a necessary part of the causal set leading to the result. There may be multiple concurrent factual causes.

Key Term: factual causation
Factual causation is established if the result would not have occurred but for the defendant’s act or omission.

Worked Example 1.1

A pushes B into a river. B cannot swim and drowns. Would B have died but for A’s action?

Answer:
No. B would not have drowned but for A’s push. Factual causation is established.

Worked Example 1.2

D finds V gravely ill from natural causes. D smothers V, and V dies immediately. Would V have died when they did but for D’s conduct?

Answer:
No. Even if V would have died soon, D accelerated the death. Factual causation is present.

Legal Causation

Legal causation considers whether the defendant’s conduct was a substantial and operating cause of the result, and whether any intervening act has broken the chain of causation. It is not enough that the defendant is a “background condition”: their conduct must contribute in a way that is more than minimal and still operating at the time of the prohibited consequence.

Key themes to keep in view:

• More than minimal: often expressed as “more than a slight or trifling link”.
• Operating: the original contribution must still be an effective cause when the result occurs.
• Not necessary to be sole cause: multiple contributors can exist; liability remains if the defendant’s contribution meets the threshold.

Key Term: legal causation
Legal causation requires that the defendant’s act is a significant and continuing cause of the result, not merely a minimal or trivial contribution.

In practice, courts ask whether the defendant’s conduct is an “operating and substantial cause”. This guards against overreach (e.g., transforming mere presence or faultless background conditions into legal causes) while ensuring that defendants cannot escape responsibility because others also contributed.

Intervening Acts (Novus Actus Interveniens)

An intervening act (novus actus interveniens) is an event that occurs after the defendant’s conduct and contributes to the result. If the intervening act is sufficiently independent and unforeseeable, it may break the chain of causation and relieve the defendant of liability. Not every intervening event does so: instinctive acts in self-defence or rescue, reasonable medical treatment, and reasonable victim responses will generally not break the chain.

Key Term: novus actus interveniens
An intervening act is a new, independent event that breaks the causal link between the defendant’s conduct and the result.

Courts often apply three organising ideas:

• Free, deliberate and informed interventions by responsible actors (e.g., a third party making an independent choice) tend to break the chain.
• Unreasonable or “daft” victim responses can break the chain; reasonable and proportionate responses will not.
• Medical treatment only breaks the chain if it is so independent and potent a cause that the defendant’s contribution is rendered insignificant.

Third-Party Acts

A third party’s act will only break the chain if it is free, deliberate, informed, and outside what is reasonably foreseeable. By contrast, instinctive acts in self-defence or law enforcement typically do not break the chain because they are not truly free, deliberate choices in the legal sense and are within the realm of foreseeability in response to the defendant’s threat.

Worked Example 1.3

D stabs V. On the way to hospital, a paramedic crashes the ambulance, and V dies from injuries in the crash. Is D liable for V’s death?

Answer:
If the crash was a grossly unforeseeable event, it may break the chain. If the crash was a foreseeable risk of seeking medical help, D remains liable.

Victim’s Own Actions

If the victim’s response is reasonable and foreseeable in the circumstances, the chain is not broken. Only a "daft" or wholly unexpected reaction will break causation. In assessing reasonableness, a court considers the “range of responses” likely from a person in the victim’s situation, acting in the agony of the moment, and has regard to salient characteristics (such as age, known frailty, or intoxication) when judging what responses might be expected.

Worked Example 1.4

D threatens V with violence. V jumps out of a moving car to escape and is injured. Is D liable for V’s injuries?

Answer:
If V’s reaction was a foreseeable attempt to escape, D is liable. If V’s response was grossly disproportionate or bizarre, the chain may be broken.

Worked Example 1.5

D, a driver, threatens V. V opens the door and jumps from the car:

• (a) at approximately 10 mph on a quiet residential road;
• (b) at approximately 60 mph on a busy dual carriageway.

Answer:
(a) Injury from jumping at low speed may well be within a range of foreseeable escape responses; the chain is likely unbroken. (b) Jumping at high speed on a busy road may be so dangerous as to be “daft” and unforeseeable, potentially breaking the chain.

Medical Treatment

Negligent medical treatment will only break the chain if it is so independent and potent in causing the result that the defendant’s contribution is rendered insignificant. Ordinary negligence rarely suffices. Even where treatment is suboptimal, liability generally remains if the original injury remained an operating and substantial cause of the result.

Worked Example 1.6

D stabs V. At hospital, V is given the wrong medication and dies. The original wound was healing. Is D liable for V’s death?

Answer:
If the medical error was so gross and the wound no longer life-threatening, the chain may be broken. Ordinary negligence will not usually break causation.

Self-Administration of Drugs

Where D supplies drugs and the victim freely, deliberately and with full knowledge self-administers them, the victim’s decision generally breaks the chain. However, liability may still arise under different doctrines (e.g., gross negligence manslaughter) if D creates a dangerous situation, assumes responsibility, and then fails to act with due care.

Worked Example 1.7

D supplies heroin to V, prepares the syringe at V’s request, and V self-injects. V overdoses and dies. Does D’s supply cause death?

Answer:
V’s free, deliberate and informed self-injection usually breaks the chain for causation of death from unlawful act manslaughter. Liability may still arise on a different basis (e.g., gross negligence) if D created a danger and then failed to take reasonable steps to avert it.

The Thin Skull Rule

The defendant must take the victim as found, including any physical or psychological vulnerabilities.

Key Term: thin skull rule
The defendant is liable for the full extent of harm caused, even if the victim has an unusual weakness or condition that makes the result more severe.

The rule extends beyond physical conditions to religious or moral beliefs and recognised psychiatric vulnerabilities. A refusal of treatment on religious grounds, or a particular susceptibility to harm, does not break the chain; it aggravates the outcome for which D remains responsible, provided D’s conduct remains an operating and substantial cause.

Worked Example 1.8

D punches V, who suffers a rare blood disorder and dies. Is D liable for V’s death?

Answer:
Yes. The thin skull rule means D is responsible for the unexpected severity of the outcome.

Worked Example 1.9

D stabs V, who is a Jehovah’s Witness and refuses a blood transfusion. V dies from treatable blood loss. Is the chain broken?

Answer:
No. V’s refusal of treatment due to religious belief does not break the chain. D remains liable for the death that flowed from the injury.

Practical Guide to Common Patterns

To apply these principles with confidence, keep these recurring patterns in mind.

• Multiple causes: A defendant’s act need not be the sole cause—only more than minimal and still operating. If others contribute (including the victim or medical staff), legal causation persists unless a novus actus interveniens applies.
• Rescue and self-defence: Rescue attempts and instinctive defensive reactions are to be expected and usually do not break the chain. The law does not penalise reasonable rescue or self-protection.
• Escape cases (fright and flight): The victim’s response is assessed for reasonable foreseeability and proportionality to the threat. A response within a range of expected human reactions does not break the chain; extreme, “daft” responses can.
• Medical treatment: Only treatment so independent and potent that it renders the original contribution insignificant breaks the chain. Life support withdrawal, taken for clinical reasons, does not break the chain because the original injury remains the operating cause of death.
• Natural events (acts of God): A natural event breaks the chain only if unforeseeable and not a hazard the defendant’s act exposed the victim to. Leaving a victim where a foreseeable tide or weather hazard materialises will not exculpate the defendant.

Summary Table: Causation Principles

Applying Causation: Additional Illustrations

These focused illustrations show how the principles interplay in practice.

• Acceleration of death: Where D accelerates an inevitable death, both factual and legal causation are satisfied, unless a novus actus interveniens interposes a new, independent and potent cause.
• Foreseeable consequences of seeking help: Injuries suffered en route to or during ordinary medical care are, in general, foreseeable incidents of the situation D created; the chain remains intact.
• Euthanasia and complex end-of-life decisions: In rare factual matrices where a victim seeks lawful assisted death abroad due to injuries inflicted by D, the question is whether that decision was within the range of reasonably foreseeable responses to the harm. Even then, D’s liability turns on whether their act remained an operating and substantial cause; the chain is not automatically broken.
• Life support: Clinically indicated withdrawal of life support in accordance with medical judgment does not break the chain. The initial injury remains the operating cause of death.

Key Point Checklist

This article has covered the following key knowledge points:

• Causation is required for result crimes in criminal law.
• Factual causation uses the "but for" test.
• Legal causation requires a substantial and operating cause.
• Intervening acts (novus actus interveniens) may break the chain of causation if they are independent and unforeseeable.
• A third party’s free, deliberate and informed act can break the chain; instinctive self-defence or reasonable rescue typically does not.
• Victim escape and “fright and flight” responses do not break the chain if reasonable and foreseeable in context.
• Negligent medical treatment rarely breaks the chain; only treatment so independent and potent as to render D’s contribution insignificant will do so.
• Self-administration of drugs by a fully informed adult can break the chain for unlawful act causation; separate duties may arise where D creates a dangerous situation and fails to act.
• The thin skull rule means the defendant is liable for the full extent of harm, even if the victim is unusually vulnerable or refuses treatment for reasons of belief.

Key Terms and Concepts

• factual causation
• legal causation
• novus actus interveniens
• thin skull rule

Worked Example 1.10

D injures V. V is taken to hospital where doctors perform an operation. Due to an error, V develops complications and dies. The original injury would, without surgery, likely have led to serious harm but not death. Does the medical error break the chain?

Answer:
Only if the error is so independent and potent in causing death that D’s contribution is rendered insignificant. Ordinary negligence in treatment will not usually break the chain; D’s act remains an operating and substantial cause.

Worked Example 1.11

D leaves an unconscious V on a beach below the high-tide line. The tide later comes in and V drowns. Has the chain been broken by a natural event?

Answer:
No. The incoming tide is foreseeable. The chain remains intact and D is liable for the drowning.

Worked Example 1.12

D assaults V, who later, in despair at their injuries and prognosis, deliberately aggravates their wounds and dies. Is D still the legal cause?

Answer:
If D’s injury remains an operating and substantial cause of death, subsequent self-harm does not necessarily break the chain. The key is whether the original injury continues to materially contribute to the death.

Worked Example 1.13

D supplies strong sedatives to V and watches V ingest them, noting signs of overdose but doing nothing. V dies. Is D the legal cause?

Answer:
If V freely and deliberately ingested the drugs, this may break the chain for unlawful act causation. However, D may be liable where a duty to act arises from creating a dangerous situation and failing to take reasonable steps to prevent the fatal outcome.