But For Test: Legal Causation

Introduction

The “but for” test is the starting point for causation in English law. It asks a simple question: but for the defendant’s conduct, would the harm have occurred? If the answer is no, the conduct is a factual cause of the harm. If the answer is yes, the defendant did not cause the loss in fact.

Although the question is simple, applying it can be tricky when there are multiple potential causes, scientific uncertainty, or intervening events. Courts distinguish between factual causation (often addressed by the “but for” test) and legal causation (which considers limits on liability, such as intervening acts and the thin skull rule). The test features in negligence, breach of statutory duty, contract damages, and criminal offences that require a prohibited outcome.

This guide sets out the core ideas, key cases, and practical steps to apply the test with confidence.

What You’ll Learn

• What the “but for” test asks and how to apply it
• The difference between factual and legal causation
• When “but for” fails and what courts do instead
• How material contribution and material increase in risk work
• Leading cases: Barnett, Bonnington Castings, Fairchild, R v White, R v Hughes, and more
• How intervening acts and the thin skull rule affect liability
• A step-by-step method for problem questions in tort and criminal law

Core Concepts

Factual causation: the “but for” question

• Core question: but for the defendant’s breach or act, would the claimant’s loss (or the prohibited result in crime) have happened?
• Civil standard: balance of probabilities. The claimant must show that it is more likely than not that the harm would not have occurred without the defendant’s conduct.
• Criminal standard: beyond reasonable doubt. The prosecution must prove that the defendant’s conduct caused the outcome.
• Practical method:
  1. Identify the relevant conduct (breach/act).
  2. Construct the counterfactual: remove that conduct from the timeline.
  3. Ask whether the same harm would still have occurred.
  4. If yes, factual causation fails; if no, it is established.

Illustrative case: In Barnett v Chelsea & Kensington Hospital, the hospital’s failure to treat was not a factual cause of the death because, on the medical evidence, the patient would have died anyway.

When “but for” is not enough: multiple causes and uncertainty

“But for” can break down in cases with cumulative exposures or where science cannot pinpoint which exposure triggered the disease.

• Material contribution to harm (cumulative causes):

  • Where multiple exposures combine to produce a single condition, the claimant can succeed by showing the defendant’s breach made a material (more than minimal) contribution to the harm.
  • Bonnington Castings Ltd v Wardlaw: exposure to silica dust from both innocent and negligent sources; the negligent exposure materially contributed to the pneumoconiosis.

• Material increase in risk (exceptional category):

  • If scientific uncertainty makes it impossible to prove which defendant caused an indivisible disease such as mesothelioma, liability can arise where the defendant materially increased the risk of the disease.
  • McGhee v National Coal Board suggested this approach; Fairchild v Glenhaven Funeral Services confirmed it for mesothelioma.
  • Wilsher v Essex Area Health Authority cautions that the approach is confined; it is not a general rule for all uncertain causation.

• Statutory development:

  • Compensation Act 2006, s 3: in mesothelioma cases, a responsible defendant is liable in full, with contribution rights between defendants.

• Modern clinical negligence:

  • Some cases accept material contribution to harm where the negligent act contributes to a cumulative process leading to injury (e.g., Bailey v Ministry of Defence; Williams v Bermuda Hospitals Board).

Legal causation: intervening acts and limits on responsibility

Factual causation is not the end of the analysis. Courts also ask whether it is fair and sensible to treat the defendant’s act as a cause in law. Key filters include:

• Novus actus interveniens (intervening act):
  • A new, independent act can break the chain of causation if it is voluntary, informed, and not reasonably foreseeable.
  • R v Kennedy (No 2): a fully informed adult’s voluntary self-administration of drugs broke the chain from the supplier.
• Thin skull rule:
  • Take your victim as you find them. You remain liable for the actual extent of damage even if the claimant is unusually vulnerable.
  • R v Blaue: refusal of a blood transfusion for religious reasons did not break the chain.
• De minimis threshold:
  • The defendant’s conduct must contribute in more than a minimal way to the outcome.
  • R v Hughes: mere presence on the road while uninsured did not amount to a causative contribution to the death without some fault in the manner of driving.

Key Examples or Case Studies

Barnett v Chelsea & Kensington Hospital [1969] 1 QB 428

• Context: A&E sent a patient away without examination; he later died from arsenic poisoning.
• Point: Even if there was a breach, the death would have occurred anyway. “But for” not satisfied.

Bonnington Castings Ltd v Wardlaw [1956] AC 613

• Context: Claimant developed pneumoconiosis from exposure to silica dust from both lawful and unlawful sources.
• Point: A material contribution to the disease sufficed. The negligent dust exposure need not be the sole or main cause.

McGhee v National Coal Board [1973] 1 WLR 1 and Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22

• Context: Claimants developed diseases after exposures at work; scientific uncertainty made it impossible to identify which exposure triggered the condition.
• Point: Where appropriate (notably mesothelioma), a material increase in risk can establish causation. Confirmed and refined in Fairchild; later supported by Compensation Act 2006, s 3 for mesothelioma.

Wilsher v Essex Area Health Authority [1988] AC 1074

• Context: Multiple distinct potential causes (different agents) of a baby’s eye condition.
• Point: Material increase in risk is not a general solution; the claimant must still show causal connection unless within the Fairchild exception.

Bailey v Ministry of Defence [2008] EWCA Civ 883; Williams v Bermuda Hospitals Board [2016] UKPC 4

• Context: Clinical negligence contributing to a cumulative process leading to harm.
• Point: Courts have accepted material contribution to harm in clinical settings where the negligent act is one of the cumulative causes producing the injury.

R v White [1910] 2 KB 124

• Context: Defendant poisoned his mother’s drink, but she died of a heart attack unrelated to the poison.
• Point: “But for” not satisfied; guilty of attempted murder, not murder.

R v Hughes [2013] UKSC 56

• Context: Uninsured driver involved in a fatal collision. His driving was not faulty; the other driver was at fault.
• Point: Mere presence as a “but for” cause is not enough. There must be a causative contribution in the manner of driving, more than de minimis.

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

• Context: Supplier prepared a syringe of heroin; the victim self-injected and died.
• Point: The informed, voluntary act of the victim broke the chain. Supplier not the legal cause of death.

Practical Applications

• Use a clear structure in problem questions:

  1. Identify duty/breach (tort) or actus reus (crime).
  2. Apply “but for”: remove the breach/act; would the same harm have happened?
  3. If yes, explain why causation fails (e.g., Barnett).
  4. If no, causation in fact is established; move to legal causation.
  5. Consider legal causation filters: intervening acts, thin skull, de minimis.
  6. Conclude and, in tort, address remoteness and apportionment if needed.

• Dealing with multiple causes:

  • Ask whether the case involves cumulative exposures to the same agent (Bonnington) or scientific uncertainty about which exposure caused an indivisible disease (Fairchild).
  • If cumulative: argue material contribution to harm.
  • If Fairchild-type uncertainty (especially mesothelioma): argue material increase in risk; note Compensation Act 2006, s 3.
  • In clinical negligence, consider whether negligence made a material contribution to a cumulative process causing injury (Bailey; Williams).

• Intervening acts and claimant conduct:

  • Voluntary, informed acts by a competent adult may break the chain (Kennedy).
  • Ordinary negligence by third parties does not always break the chain, particularly if it is a foreseeable response to the situation created by the defendant.
  • The claimant’s own behaviour may reduce damages for contributory negligence rather than break the chain altogether.

• Evidence tips:

  • In civil claims, target the balance of probabilities with clear medical or technical evidence mapping the timeline and exposures.
  • Do not rely on “loss of chance” in personal injury (e.g., Hotson; Gregg v Scott); courts require causation of actual harm. In contract, loss of chance damages may be available in some economic cases (e.g., Allied Maples), but the approach differs.

• Criminal law focus:

  • Prove factual causation and then assess whether any intervening act breaks the chain.
  • Remember the de minimis threshold: there must be a real contribution to the result (Hughes).
  • Apply the thin skull rule to the extent of harm.

Summary Checklist

• Can you state the “but for” question clearly for the key act or breach?
• Have you identified the standard of proof (civil vs criminal)?
• If “but for” fails, is there a route via material contribution to harm (Bonnington) or, in a Fairchild-type case, material increase in risk?
• Have you considered legal causation filters: intervening acts, thin skull, and de minimis?
• For mesothelioma, have you noted Compensation Act 2006, s 3?
• In clinical negligence, have you considered cumulative processes (Bailey; Williams)?
• Have you ruled out “loss of chance” where it does not apply?
• Is your answer structured: duty/breach, factual causation, legal causation, remoteness, apportionment?

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