Novus Actus Interveniens: Breaking the Chain

Introduction

Novus actus interveniens (Latin for “a new intervening act”) is a key rule in causation used in both tort and criminal law. It asks whether a later event has broken the chain between the defendant’s conduct and the final harm. Proving factual cause is not enough; the original act must also remain the legal cause of the damage or death.

Courts look closely at what happened after the initial wrongdoing. A voluntary and informed choice by the victim can sometimes sever liability. Reasonable responses to danger usually do not. Natural events may break the chain if they are sufficiently unexpected and independent. In criminal law, the Court of Appeal in R v Wallace [2018] framed the question as whether the injuries inflicted by the defendant were an “operating and significant cause” of death. This aligns with a broader approach that focuses on fairness, foreseeability, and whether the later event truly overtook the original wrongdoing.

This guide sets out the tests, explains how the courts approach common scenarios, and summarises the leading cases.

What You’ll Learn

• The difference between factual causation and legal causation, and how the chain can be broken
• When a victim’s voluntary, informed act will relieve the original wrongdoer of liability
• Why reasonable acts in self-preservation, self-defence, or duty usually keep the chain intact
• How unreasonable conduct can break the chain, and when it is merely contributory negligence
• The role of natural events and whether they are independent and unforeseeable
• How foreseeability and remoteness shape the analysis
• How the eggshell skull rule affects claims where the victim has pre-existing conditions or beliefs
• Key authorities including R v Kennedy [2007], R v Wallace [2018], R v Pagett (1983), McKew v Holland & Hannen [1969], and R v Blaue [1975]

Core Concepts

Causation and the chain

• Factual causation asks: but for the defendant’s conduct, would the harm have occurred?
• Legal causation asks whether the defendant’s conduct remains an operative cause in law, or whether a later act has broken the chain (novus actus interveniens).
• In criminal law, R v Wallace [2018] reframed the question as whether the injuries inflicted by the defendant were an “operating and significant cause” of death. A later event can be the main cause without relieving the defendant if the original injuries continue to play a significant role.

Voluntary and informed acts by the victim

• A fully informed, voluntary act by an adult of sound mind may break the chain. In R v Kennedy [2007], the victim self-injected heroin prepared by the defendant. The House of Lords held that the victim’s autonomous choice severed the causal link.
• Not every apparent “choice” is treated as free and unfettered. In R v Wallace [2018], the victim chose euthanasia following catastrophic injuries. The court held this was not a discrete, independent act of the kind that triggers novus actus interveniens because it was bound up with the defendant’s attack.

Reasonable responses to danger and legal duty

• Where a person responds reasonably to danger created by the defendant, the chain usually remains intact. In R v Pagett (1983), police returning fire during a shootout did not break the chain; the defendant’s conduct remained the legal cause of the hostage’s death.
• Acts done in the execution of a legal duty, or to prevent harm, are generally considered reasonable. Haynes v Harwood [1935] held that a rescuer injured while stopping runaway horses could recover; the response was the natural and probable result of the defendant’s negligence.

Unreasonable conduct vs contributory negligence

• A claimant’s unreasonable act may break the chain, extinguishing liability for subsequent harm. In McKew v Holland & Hannen [1969], the claimant descended a steep staircase without a handrail despite a known leg weakness; his conduct was so unreasonable it was treated as a new intervening act.
• If the claimant’s conduct is unwise but not extreme, the chain usually holds and damages may be reduced for contributory negligence. In Spencer v Wincanton Holdings [2009], the claimant’s actions after an amputation did not meet the high threshold required to break the chain; a deduction for contributory negligence was appropriate.

Natural events and “Act of God” arguments

• An external natural event can break the chain if it is independent and sufficiently unforeseeable. In Carslogie Steamship Co Ltd v Royal Norwegian Government [1952], storm damage during a voyage was a novus actus interveniens unrelated to the earlier collision damage.
• But not all weather or external conditions meet that standard. In Southern Water Authority v Pegrum [1989], heavy rain and a blocked drain did not displace liability; the defendants’ conduct remained a cause of the pollution.

Foreseeability and remoteness

• An intervening act is less likely to break the chain if it was foreseeable. The more extraordinary the intervening conduct, the more likely it is to be treated as a new cause.
• In Knightley v Johns [1982], the negligent order given by a senior police officer (to ride the wrong way down a tunnel) was not a foreseeable result of the defendant’s initial negligent driving and did break the chain.

The eggshell skull rule

• Defendants take their victims as they find them. That includes physical vulnerabilities, beliefs, and pre-existing conditions.
• In R v Blaue [1975], a stabbing victim refused a blood transfusion on religious grounds and died. The refusal did not break the chain; the attacker remained liable.
• Baker v Willoughby [1970] illustrates that pre-existing or subsequent conditions do not automatically interrupt liability for the damage already caused.

Key Examples or Case Studies

• R v Kennedy [2007]

  • Context: Defendant supplied heroin; victim self-injected and died.
  • Point: A fully informed, voluntary act by an adult of sound mind can break the chain.
  • Use: Consider autonomy and whether the act was truly free of coercion or compulsion.

• R v Wallace [2018]

  • Context: Victim requested euthanasia after a severe attack.
  • Point: The original injuries were an operating and significant cause of death; the euthanasia decision did not sever liability.
  • Use: Not every later decision is considered independent; look for whether the original harm continues to operate.

• R v Pagett (1983)

  • Context: Police returned fire during a shootout; a hostage was killed.
  • Point: Reasonable acts in self-preservation or duty do not break the chain.
  • Use: Responses to danger created by the defendant usually keep liability in place.

• Haynes v Harwood [1935]

  • Context: Rescuer injured stopping runaway horses.
  • Point: Rescue attempts are the natural and probable result of the defendant’s negligence and do not sever liability.
  • Use: Treat rescue as foreseeable where the defendant creates a hazardous situation.

• McKew v Holland & Hannen [1969]

  • Context: Claimant took an unjustifiable risk by descending a steep stairway with a known weak leg.
  • Point: Highly unreasonable conduct breaks the chain.
  • Use: Apply a demanding threshold before finding a novus actus interveniens based on the claimant’s actions.

• Spencer v Wincanton Holdings [2009]

  • Context: Claimant’s post-accident mishap after an amputation.
  • Point: Conduct was unwise but not extreme; damages reduced for contributory negligence rather than ending liability.
  • Use: Distinguish between truly unreasonable acts and ordinary misjudgment.

• Carslogie Steamship Co Ltd v Royal Norwegian Government [1952]

  • Context: Subsequent storm damage during a voyage after a negligent collision.
  • Point: A powerful, independent natural event can be a new intervening cause.
  • Use: Assess independence and foreseeability of external events.

• Southern Water Authority v Pegrum [1989]

  • Context: Pollution with heavy rain and a blocked drain as contributing factors.
  • Point: Conditions did not overwhelm the defendants’ role; chain intact.
  • Use: Weather alone rarely severs liability unless it is extraordinary and independent.

• Knightley v Johns [1982]

  • Context: Senior officer’s negligent order led to a collision.
  • Point: Extraordinary third-party negligence can break the chain.
  • Use: Consider whether third-party acts were within the range of what the defendant should reasonably anticipate.

• R v Blaue [1975]

  • Context: Victim refused a blood transfusion on religious grounds.
  • Point: Eggshell skull rule applies; the refusal did not break the chain.
  • Use: Pre-existing beliefs and vulnerabilities are taken as found.

Practical Applications

• Step 1: Separate factual from legal causation

  • Confirm “but for” causation first.
  • Then ask whether a later event was a new intervening act that displaced the defendant’s conduct as a legal cause.

• Step 2: Identify any intervening act and classify it

  • Victim’s act: Was it voluntary, informed, and genuinely free (R v Kennedy)?
  • Third-party act: Was it a reasonable response to danger or duty (R v Pagett; Haynes v Harwood) or an extraordinary act (Knightley v Johns)?
  • Natural event: Was it independent and sufficiently unforeseeable (Carslogie)?

• Step 3: Apply reasonableness and foreseeability

  • Reasonable conduct in self-preservation, self-defence, or duty typically does not break the chain.
  • Unreasonable conduct by a claimant may break it (McKew), but the threshold is high; otherwise, consider contributory negligence (Spencer).

• Step 4: Consider the eggshell skull rule

  • Physical, psychological, or religious traits of the victim do not usually break the chain when triggered by the defendant’s act (R v Blaue).

• Step 5: For criminal cases, apply the “operating and significant cause” test

  • Ask whether the injuries inflicted remained an operating and significant cause of death (R v Wallace), even if another factor later became the main cause.

• Step 6: Evidence checklist

  • Autonomy: Was the victim informed and free from pressure?
  • Foreseeability: Was the later act within the scope of what could reasonably be anticipated?
  • Independence: Did the later event arise from the defendant’s act or was it truly independent?
  • Reasonableness: Were responses proportionate to the danger created?
  • Pre-existing conditions: Record medical history, beliefs, and vulnerabilities.

• Step 7: Remedies and liability allocation

  • If the chain is broken: no liability for subsequent harm.
  • If the chain holds but the claimant contributed: apportion damages for contributory negligence.
  • If multiple causes operate: assess whether the defendant’s act remains significant (criminal) or a material cause (civil).

Summary Checklist

• Confirm “but for” causation, then test for legal causation.
• Identify any intervening acts and classify them (victim, third party, natural event).
• Victim’s fully informed, voluntary act can break the chain (R v Kennedy).
• Reasonable acts in self-preservation or duty keep the chain intact (R v Pagett; Haynes v Harwood).
• Unreasonable claimant conduct can break the chain (McKew); otherwise consider contributory negligence (Spencer).
• Extraordinary third-party negligence can break the chain (Knightley v Johns).
• Natural events break the chain only when independent and unexpected (Carslogie); routine conditions usually do not (Pegrum).
• Apply the eggshell skull rule: take the victim as found (R v Blaue; Baker v Willoughby).
• In criminal law, ask if the defendant’s act was an operating and significant cause (R v Wallace).

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