Causation in Criminal Law: Tests, Intervening Acts, and Thin...

Introduction

Causation links a defendant’s conduct to a prohibited result. It sits within the actus reus and must be proved alongside mens rea for many offences. In short, the prosecution must show both that the result would not have happened without the defendant’s conduct (factual causation) and that the conduct was a legally significant, operating cause of that result (legal causation).

Two recurring themes are whether the chain of causation has been broken by a new intervening act (novus actus interveniens) and how the “thin skull” rule applies where the victim has a particular vulnerability. These issues arise across offences, including unlawful act manslaughter.

This guide sets out the tests, leading cases, and practical steps to apply in problem questions and practice.

What You'll Learn

• How to apply the “but for” test for factual causation (with R v White)
• What “more than minimal” and “operative” mean for legal causation (with R v Hughes)
• When a new intervening act breaks the chain (with R v Kennedy and R v Wallace)
• How the “thin skull” rule works (with R v Blaue)
• How causation operates in unlawful act manslaughter (with R v Church, R v Dawson, and R v Goodfellow)
• Practical steps to analyse causation methodically

Core Concepts

Factual Causation: The “But For” Test

Factual causation asks a simple question: but for the defendant’s conduct, would the result have happened when and as it did?

• If the answer is “no”, factual causation is made out.
• If the answer is “yes” (the result would have happened anyway, in the same way at the same time), factual causation fails.

Key point: Factual causation is a gateway. You cannot reach legal causation unless factual causation is established. However, factual causation alone is not enough to fix liability.

Common pitfalls and clarifications:

• Multiple causes: More than one person can factually cause a result.
• Acceleration: If the conduct brings the result forward (e.g., earlier death), factual causation can still be satisfied.

Illustration: In R v White [1910], the defendant put poison in his mother’s drink, but she died from a heart attack unrelated to the poison. The “but for” test failed for murder; he was not the factual cause of death.

Legal Causation: More Than Minimal and Operative

Legal causation filters out trivial contributions and checks the chain remains intact.

• More than minimal: The defendant’s conduct must be a significant contribution, not merely negligible. Courts often frame this as “more than de minimis”.
• Operative: The conduct must still be operating at the time of the result, without the chain being broken by a new intervening event.

R v Hughes [2013] confirms that liability requires more than a purely minimal cause of death. The defendant need not be the only or the main cause, but the contribution must be legally significant.

Important reminders:

• There can be multiple operating causes.
• Blameworthiness and causation are distinct; a breach or unlawful status is not enough without a causal contribution.

Intervening Acts (Novus Actus Interveniens)

A new intervening act can break the chain of causation if it becomes the immediate and independent cause of the result. Courts consider categories:

• Victim’s own conduct: If the victim’s act is free, informed, and voluntary, it can break the chain (R v Kennedy [2007]). Where the victim’s behaviour is a direct response to the defendant’s conduct and not truly voluntary (e.g., compelled by the defendant’s attack), the chain remains intact (see R v Wallace [2018]).
• Third-party acts: Acts by others (including medics) may break the chain if they are so independent and potent that they become the real cause. Ordinary medical treatment, even if negligent, usually does not break the chain unless it is exceptionally bad and wholly dominates the outcome.
• Natural events: A wholly unforeseeable and overwhelming natural event may break the chain.

What to ask:

1. Was the intervening act free, informed, and voluntary?
2. Was it reasonably foreseeable as a response to the situation created by the defendant?
3. Did it eclipse the defendant’s contribution, making the latter no longer an operating cause?

The “Thin Skull” Rule

Also known as “take your victim as you find them”, this principle fixes the defendant with the victim’s actual condition, beliefs, or vulnerabilities.

• If the defendant causes harm, they are responsible for the full extent of that harm, even if it is aggravated by a pre-existing condition.
• The rule applies to physical and, as in R v Blaue [1975], religious beliefs affecting medical treatment.

This prevents defendants from arguing that a “normal” person would have suffered less harm.

Unlawful Act Manslaughter and Causation

For unlawful act manslaughter, the prosecution must prove:

1. An unlawful act (a crime, not a civil wrong).
2. The act was dangerous: a sober and reasonable person would recognise a risk of some physical harm (R v Church [1966]).
3. The act caused the death.

Clarifications from case law:

• “Dangerous” means a risk of physical harm, not just fear or emotional distress (R v Dawson [1985]).
• The unlawful act need not be directed at the victim (R v Goodfellow [1986]).
• Where a fully informed and voluntary act by the victim intervenes (e.g., self-administration of drugs supplied by the defendant), the chain may be broken (R v Kennedy [2007]).

Key Examples or Case Studies

R v White [1910]

• Context: Poison placed in mother’s drink; death due to heart attack.
• Held: Factual causation for murder failed; the death would have occurred anyway.
• Application: Start with “but for”; if it fails, the inquiry ends for result crimes.

R v Hughes [2013] UKSC 56

• Context: Uninsured driver involved in a collision; victim’s negligent driving also contributed to death.
• Held: Liability requires more than a minimal causal contribution; mere presence is not enough.
• Application: Prove a significant, operative contribution to the result.

R v Kennedy (No 2) [2007] UKHL 38

• Context: Supplier prepared heroin; adult victim self-injected and died.
• Held: Victim’s free, informed, voluntary act broke the chain; supplier did not legally cause death.
• Application: For adult, informed victims, voluntary self-administration breaks causation.

R v Wallace [2018] EWCA Crim 690

• Context: Acid attack; victim later chose euthanasia abroad due to severe injuries and suffering.
• Held: The decision on causation was for the jury; the victim’s choice was not treated as an independent, voluntary act in the Kennedy sense given the circumstances created by the defendant.
• Application: Where the victim’s “choice” is driven by the defendant’s attack, the chain often remains intact.

R v Blaue [1975] 1 WLR 1411

• Context: Stabbing victim refused a life-saving blood transfusion due to religious beliefs and died.
• Held: Defendant liable for death; must take the victim as found.
• Application: The thin skull rule covers beliefs as well as physical conditions.

R v Church [1966] 1 QB 59

• Context: Unlawful act manslaughter; dangerousness test formulated.
• Held: The act is dangerous if a sober and reasonable person would recognise a risk of some physical harm.
• Application: “Some harm” suffices; it need not be serious harm.

R v Dawson [1985] 81 Cr App R 150

• Context: Attempted robbery; victim with a heart condition died from stress-induced heart attack.
• Held: Dangerousness is judged by the risk of physical harm apparent to a reasonable person with the knowledge the defendant actually had.
• Application: If defendants are unaware of a hidden frailty, a mere risk of fright is not enough.

R v Goodfellow [1986] 83 Cr App R 23

• Context: Arson of a council house to be rehoused; fire killed family.
• Held: Unlawful act need not target a specific person; it must be unlawful, dangerous, and causative of death.
• Application: Direction of the act is irrelevant; focus on unlawfulness, danger, and causation.

Practical Applications

• Use a two-stage test:

  1. Factual causation: Apply the “but for” test.
  2. Legal causation: Ask if the contribution was more than minimal and still operating when the result occurred.

• Check for possible breaks in the chain:

  • Victim’s voluntary act: Free, informed, and voluntary? If yes, likely breaks the chain (Kennedy). If the conduct is a pressured or natural response to the defendant’s act (e.g., driven by injuries or threat), the chain usually holds (Wallace).
  • Third-party acts: Ordinary medical treatment rarely breaks the chain. It must be so independent and potent that it becomes the real cause.
  • Natural events: Only overwhelming, unforeseeable events are likely to break the chain.

• Apply the thin skull rule:

  • Do not discount aggravation due to the victim’s pre-existing conditions or beliefs. If you caused the harm, you answer for the actual consequences (Blaue).

• Unlawful act manslaughter checklist:

  • Identify the base crime (not just a civil wrong).
  • Test for danger: would a sober and reasonable person see a risk of some physical harm?
  • Prove causation: the act must significantly contribute to death and the chain must remain intact.

• Exam and practice tips:

  • Set out causation separately from actus reus and mens rea for clarity.
  • Work chronologically: defendant’s act → any intervening events → the result.
  • Tackle categories explicitly (victim’s conduct, third-party acts, medical treatment).
  • Use case names to support each stage (White for “but for”; Hughes for legal causation; Kennedy/Wallace for intervening acts; Blaue for thin skull; Church/Dawson/Goodfellow for manslaughter).

Summary Checklist

• Apply the “but for” test (R v White).
• Confirm a more than minimal, operating cause (R v Hughes).
• Assess intervening acts:
  • Victim’s free, informed, voluntary act? Likely breaks the chain (R v Kennedy).
  • Response compelled by the defendant’s act? Chain usually intact (R v Wallace).
  • Medical treatment only breaks the chain if extraordinarily independent and potent.
• Use the thin skull rule: take the victim as found (R v Blaue).
• For unlawful act manslaughter:
  • Prove an unlawful act, dangerous to a reasonable person (R v Church).
  • Risk must be of physical harm (R v Dawson).
  • Act need not target the victim (R v Goodfellow).
  • Check causation carefully, including potential breaks.

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