Causation — factual and legal

Causation is the conceptual bridge between breach of duty and recoverable damage. A claimant who has established that the defendant owed a duty of care and breached it must still prove that the breach caused the relevant harm. This requirement performs a double function: it ensures the defendant is held liable only for losses genuinely attributable to wrongdoing, and it cabins the law's reach in a world where consequences ripple indefinitely.

English law conventionally divides causation into two stages. Factual causation asks whether the defendant's breach was a conditio sine qua non of the damage—a necessary link in the chain of events. This enquiry is dominated by the but-for test: would the claimant have suffered the same loss if the defendant had not been in breach? Legal causation (sometimes labelled remoteness or scope of duty) asks whether, as a matter of law and policy, the defendant ought to bear responsibility for that damage. Even when factual causation is established, the law may refuse recovery if the damage is too remote, if an intervening act (a novus actus interveniens) breaks the chain, or if the loss falls outside the scope of the duty breached.

The distinction between the two stages is analytically convenient but often unstable in practice. Courts have recognised that purely factual enquiries can be manipulated by choice of counterfactual, and that legal causation frequently smuggles in considerations—such as fairness, policy, and moral intuition—that are denied at the breach stage. Recent scholarship, particularly by Stapleton and Steel, has challenged the utility of the but-for test as a universal gateway and has argued for a more explicit engagement with the normative reasons that underpin causal attribution. The Supreme Court's decisions in Sienkiewicz and Hughes illustrate the doctrinal sophistication—and the remaining tensions—in this area.

This note proceeds chronologically and thematically. After sketching the historical emergence of causal requirements in nineteenth-century trespass and case, we set out the core principles governing each stage. Statutory interventions (most notably the Compensation Act 2006) are examined, before turning to the landmark authorities that have shaped modern doctrine. Doctrinal development traces the erosion of rigid rules in favour of multi-factorial enquiries, and the academic debates section canvasses competing theoretical accounts of causal responsibility in tort.

Causal requirements emerged slowly and unevenly in the common law. In medieval pleading, the forms of action determined liability largely by reference to the manner of harm—trespass vi et armis for direct and forcible injury, case for indirect or consequential loss—rather than by any explicit causal test. So long as the defendant's act was direct, liability followed; the question of whether intervening factors might absolve the defendant was submerged in questions of intent or inevitable accident.

By the late eighteenth century, treatise writers such as Comyn and Blackstone began to describe liability in terms of causation. Yet the vocabulary remained imprecise. In Scott v Shepherd (1773) 2 W Bl 892—the famous "squib case"—the majority of the Court of King's Bench held the original thrower liable even though two others had thrown the firework onward, reasoning that the initial act set the chain in motion. The dissent would have broken the chain at each successive throw. The debate prefigured modern disputes about intervening acts, but neither side articulated a clear test.

The consolidation of negligence as a discrete tort in the nineteenth century—especially after Heaven v Pender (1883) 11 QBD 503 and Donoghue v Stevenson [1932] AC 562—forced courts to articulate limiting principles. If liability were to be based on foreseeable risk, rather than on formal boundaries of action, then some criterion was needed to distinguish harms caused by the breach from those merely coinciding with it. Victorian judges borrowed the Latin maxim causa causans (the effective cause) and opposed it to causa sine qua non (a mere condition). In Leyland Shipping Co v Norwich Union Fire Insurance Society Ltd [1918] AC 350, Lord Shaw of Dunfermline remarked that "causation is not a matter of metaphysics but of practical politics"—a candid acknowledgment that legal policy infiltrates ostensibly factual determinations.

The crystallisation of the but-for test is often attributed to the mid-twentieth century. Lord Reid's speech in McGhee v National Coal Board [1973] 1 WLR 1 and the subsequent clarification in Wilsher v Essex Area Health Authority [1988] AC 1074 mark the high-water points of doctrinal debate. By the 1990s, academic commentary by Hart and Honoré (Causation in the Law, 2nd ed, 1985) had codified the orthodox two-stage model, and courts began to cite their work as authoritative. Nevertheless, unease persisted: the but-for test, while simple in principle, proved difficult to apply in cases of multiple sufficient causes, cumulative exposure, and statistical evidence. The House of Lords' decision in Fairchild v Glenhaven Funeral Services Ltd [2003] 1 AC 32 departed from strict but-for logic in the interests of justice, acknowledging that legal causation is ultimately a normative judgment informed by policy.

The but-for test

Factual causation is most commonly established by the but-for test: the claimant must show that, on the balance of probabilities, the damage would not have occurred but for the defendant's breach. This is a counterfactual enquiry. The court asks: what would have happened if the defendant had discharged the duty of care?

In Barnett v Chelsea and Kensington Hospital Management Committee [1969] 1 QB 428, the casualty officer negligently failed to examine a patient who presented with vomiting; the patient died hours later from arsenic poisoning. Nield J held that, even if the doctor had examined him, the arsenic would have been undetectable and the patient would have died regardless. The breach did not satisfy the but-for test. The outcome may appear harsh—the hospital certainly acted wrongfully—but liability in tort is confined to causally connected harms.

The test is applied on the balance of probabilities (>50%). A claimant who proves a 49% chance that the breach caused the loss will fail entirely: Hotson v East Berkshire Health Authority [1987] AC 750. English law rejects proportionate recovery for loss of chance in personal injury claims, although it permits it in professional negligence: Allied Maples Group Ltd v Simmons & Simmons [1995] 1 WLR 1602.

Multiple sufficient causes

The but-for test fails when two or more factors, each independently sufficient, combine to cause harm. Suppose two defendants negligently light separate fires, each of which would have destroyed the claimant's property. Neither fire is a but-for cause, yet both are intuitively responsible. English law has addressed this through the concept of a material contribution to the damage. In Bonnington Castings Ltd v Wardlaw [1956] AC 613, the House of Lords held that where the defendant's breach made a material (i.e. more than negligible) contribution to an indivisible injury, causation is established even if other factors also contributed.

The test was refined in Bailey v Ministry of Defence [2008] EWCA Civ 883, where Waller LJ held that the claimant need not prove that the breach was the sole or even the principal cause, provided it made a material contribution to the cumulative state that resulted in harm. The line between material contribution to injury and material increase in risk (see Fairchild, below) remains contested.

The Fairchild exception: material increase in risk

In cases of indeterminate causation—where the claimant cannot identify which of several exposures caused the disease—the House of Lords has carved out a narrow exception. Fairchild v Glenhaven Funeral Services Ltd [2003] 1 AC 32 concerned mesothelioma induced by asbestos exposure during employment with multiple successive employers. Medical science cannot identify which exposure triggered the disease; a single fibre may suffice. Strict application of the but-for test would deny recovery. The House of Lords held (Lord Bingham, Lord Nicholls, Lord Hoffmann, Lord Hutton, Lord Rodger) that where (i) the claimant was exposed by the negligence of two or more defendants, (ii) each exposure was capable of causing the disease, and (iii) the disease is indivisible and the precise causal mechanism scientifically indeterminate, proof of a material increase in risk suffices for liability. Each defendant is liable in full (joint and several), subject to contribution proceedings.

Barker v Corus UK Ltd [2006] UKHL 20 initially interpreted Fairchild as imposing several (proportionate) liability, but Parliament reversed that holding within months by enacting the Compensation Act 2006, s 3, which restores joint and several liability in mesothelioma claims.

This section offers a narrative synthesis of the leading authorities; full structured summaries appear in the key cases section below.

Barnett v Chelsea and Kensington Hospital Management Committee [1969] 1 QB 428 remains the canonical illustration of the but-for test. The casualty officer's breach—failing to examine the deceased—did not cause the death, because examination would not have revealed arsenic poisoning in time to prevent it. The case is a salutary reminder that fault without causation yields no liability.

McGhee v National Coal Board [1973] 1 WLR 1 introduced enduring controversy. The pursuer contracted dermatitis after working in brick kilns. The defenders provided no washing facilities, requiring him to cycle home caked in dust. Medical evidence could not distinguish between dermatitis caused during work (for which there was no breach, as dust was inherent in the job) and that caused by the failure to provide showers. The House of Lords held for the pursuer. Lord Wilberforce's speech suggested that materially increasing the risk of injury could be treated as having materially contributed to it. Later courts debated whether McGhee modified the but-for test or merely applied orthodox principles on the balance of probabilities.

Wilsher v Essex Area Health Authority [1988] AC 1074 clarified—and arguably confined—McGhee. The plaintiff, a premature baby, was given excess oxygen, one of several possible causes of his blindness. The House of Lords (Lord Bridge) held that the claimant had failed to prove on the balance of probabilities that the excess oxygen caused the injury, given the presence of multiple competing causes. The Court rejected any broad principle that increase in risk equals contribution to injury. McGhee was explained as a case where the only two competing causes were tortious and non-tortious phases of the same agency (brick dust), permitting an inference that the tortious phase contributed.

Fairchild v Glenhaven Funeral Services Ltd [2003] 1 AC 32 represents the high point of judicial creativity in causation. The House of Lords recognised an exception to the but-for test where the claimant has been wrongfully exposed to a risk by multiple defendants, each of whose exposure could have caused an indivisible disease, and where the current limits of scientific knowledge make but-for proof impossible. Lord Bingham emphasised that the exception was confined by six cumulative conditions and grounded in principles of justice and policy.

Barker v Corus UK Ltd [2006] UKHL 20 extended Fairchild to cases of both negligent and non-negligent (e.g. self-) exposure and introduced several (proportionate) liability. The majority (Lord Hoffmann, Baroness Hale, Lord Walker) reasoned that, since liability was founded on increase in risk rather than causation of injury, it should be proportionate to the risk created. The result was controversial and short-lived.

Sienkiewicz v Greif (UK) Ltd [2011] UKSC 10 clarified the evidential threshold for Fairchild. The Supreme Court held that the exception applies even where the defendant's contribution to total exposure was small, provided it more than doubled the background risk (satisfying the balance of probabilities for a material increase in risk). The decision also confirmed that the Fairchild framework, as modified by the Compensation Act 2006, s 3, now provides the sole route to recovery in indeterminate mesothelioma claims.

Hughes v Lord Advocate [1963] AC 837 is the leading authority on remoteness. The precise mechanism of injury (an explosion rather than a simple burn) was unforeseeable, but the House of Lords held that burns from a paraffin lamp were a foreseeable type of harm, and the injury fell within that type. The case illustrates the generous interpretation given to "type" or "kind" of damage under the Wagon Mound test.

Collectively, these decisions demonstrate the law's struggle to reconcile principled rules with justice in hard cases. They also show how causal doctrine is shaped as much by normative commitments—about responsibility, fault, and compensation—as by logic or science.

See practice questions below.

See further reading below.