Causation in Clinical Negligence Cases: Can there be liability where the same injury would probably have happened anyway?


    1. The approach to causation in clinical negligence cases has changed significantly over the past 20 years. In this article I consider whether as practitioners we are about to see another significant step forward or whether in practical terms little has changed.

Life before Bailey

    1. Before Bailey v. MoD, most clinical negligence practitioners thought that satisfying traditional ‘but for’ causation was a requirement for a successful claim. In a case where there had been a negligent five minute delay in delivering a baby but there would still have been an injury in an event a claim would be assumed to fail because the claimant could not prove how much damage had been caused by the breach of duty.
    1. Bailey was followed by a sea-change in practice with successful claims being brought in circumstances where it could be proved that the breach of duty had materially i.e. more than negligibly, contributed to the outcome. Many practitioners will recall some surprise at the decision in Bailey because we could immediately see that it would open the door to many more clinical negligence claims, particularly obstetric claims.  Would the door be shut again we wondered?  Academics meanwhile thought the only surprise was that material contribution had not been applied more quickly by practitioners to clinical negligence cases, seeing no basis for distinguishing clinical negligence cases from other personal injury claims.

The post-Bailey concensus

    1. A ‘post-Bailey concensus’ has emerged amongst clinical negligence practitioners that material contribution is limited to cases where it is not possible to say what the outcome would have been absent the ‘breach of duty’. We look for cases where medical science cannot give an answer on the balance of probabilities.  Until now, we have not been bringing cases where the outcome would probably have been the same in any event, no matter how egregious the breach of duty.
    1. That concensus derives from the judgment of Waller LJ in Bailey in the Court of Appeal[2]:

“In my view one cannot draw a distinction between medical negligence cases and others. I would summarise the position in relation to cumulative cause cases as follows. If the evidence demonstrates on a balance of probabilities that the injury would have occurred as a result of the non-tortious cause or causes in any event, the claimant will have failed to establish that the tortious cause contributed. Hotson’s case exemplifies such a situation. If the evidence demonstrates that but for the contribution of the tortious cause the injury would probably not have occurred, the claimant will (obviously) have discharged the burden. In a case where medical science cannot establish the probability that but for an act of negligence the injury would not have happened but can establish that the contribution of the negligent cause was more than negligible, the but for test is modified, and the claimant will succeed.” (my underlining)

What if the outcome would have been the same in any event?

    1. What though if a claimant were entitled to damages in a case of sub-standard treatment where the outcome would probably have been the same in any event? A combination of recent case law and academic writing from Professor Jane Stapleton and others, now raises the question of whether we are on the cusp of another major change in the practical approach to causation in clinical negligence cases.

The hiker in the woods

    1. Suppose that a hiker in the woods is carelessly shot by two hunters, simultaneously. The medical evidence is that the bullet from each hunter would have killed the hiker without the need for the other bullet.  Should each hunter escape liability for the negligent discharge of her gun because the hiker would have died anyway?  This is the question asked by the Supreme Court in FCA v Arch [2021] AC 649.  The Supreme Court held that the hunter should not escape liability:

“Applying the ‘but for’ test would produce the result that neither hunter’s shot caused the hiker’s death – a result which is manifestly not consistent with common-sense principles.”[3]

FCA v Arch

    1. FCA v Arch was an insurance case arising out of the Covid pandemic. Briefly, insurers argued that they were not obliged to meet claims for business interruption insurance where businesses had been closed by government restrictions during the pandemic. The wording of the policies required a business to have been shut because of cases of Covid-19 occurring within a specified radius of the insured premises (usually 25 miles). The insurers argued that Covid was so widespread that any business would have been closed by the government in any event because of cases of Covid outside the relevant radius.  Cases of Covid within 25 miles were therefore, in insurance law, not the proximate cause of the closures and there could be no claim under the policy.  The Supreme Court disagreed.  In doing so the Court wrestled with the fundamental principles of causation in tort. The result is important for clinical negligence practitioners and judges.

The importance of Holmes v Poeton

    1. Encouragement for the idea that a fresh look at causation in clinical negligence cases is needed comes too from the recent Court of Appeal case of Holmes v. Poeton[4].  This was a personal injury case where at first instance the judge had found that exposure to chemicals in the claimant’s workplace had materially contributed to his Parkinson’s disease. The Court of Appeal overtured this decision but in doing so reviewed the concept of material contribution causation. In a deft review of the authorities, where he pulls no punches, Stuart-Smith LJ makes plain that material contribution causation applies both to cases of divisible and indivisible injuries.  In doing so he puts to bed an issue which has divided clinical negligence practitioners for more than a decade.
    1. But Stuart-Smith LJ goes further. At paragraph 63 he suggests that material contribution applies even to cases where the injury would have happened anyway:

“The reasoning in Bailey has been subjected to sustained criticism, not least by Professor Stapleton (Unnecessary Causes, LQR 2013, 39-65 at 50-54). It may well be asked how a principle first enunciated in a case involving a quintessential divisible disease has come to be established as a test that bypasses the need to satisfy the “but for” test in cases of individual injury that do not come within the Fairchild exception. It might, as Professor Stapleton suggests, derive from a response to an evidentiary gap in divisible injury cases where there is no satisfactory acceptable evidentiary basis from which the disability due to the separate insults to the body could be apportioned to the individual sources. But, if so, its transference to the field of indivisible injury or disease has not been explained apart from the recognition that the principle is generous to claimants. I would, however, hold that we are bound in the light of Bailey to find that the Bonnington “material contribution” principle applies to cases of indivisible injury and that, where the principle applies, the claimant does not have to show that the injury would not have happened but for the tortious exposure for which the defendant is responsible.” (my underlining)

    1. The Stapleton approach, having been endorsed by Stuart-Smith LJ, is approved by Underhill LJ in a supporting judgment:

“I would only add one footnote on Issue 1.  Stuart-Smith LJ refers at paragraph 63 of his judgment to Professor Stapleton’s criticisms of the reasoning (though not the outcome) in Bailey.  Her preferred analysis of cases of this kind appears now to have been approved by the Supreme Court: see paras 182-185 of the judgment of Lord Leggatt and Lord Hamblen in FCA v Arch and her recent article “Unnecessary and Insufficient Factual Causes,” Journal of Tort Law 2023.”[5]

    1. The 2023 article endorsed by Underhill LJ in Holmes is important reading for practitioners. There Professor Stapleton concludes:

The 21st century has seen both the UK Supreme Court and the US Supreme Court acknowledge that necessity, and its but-for test, as the exclusive test of factual causation leads to absurd, anomalous and nonsensical results. It is time that legal academics, law students, practicing lawyers and judges accept that the common law seems to recognize a concept of factual causation that is wider than necessity and that embraces unnecessary and insufficient factual causes. Far from being as rare as is often assumed, unnecessary and insufficient factual causes are common. They may be present in the simplest of situations such as a boy falling from a height and being negligently treated by a hospital, a wheelbarrow being left in a public way, or a decision-maker taking into account a number of reasons.” (my underlining)

The Stapleton approach

    1. Perhaps the easiest way for practitioners to understand Professor Stapleton’s approach is to consider the examples that she gives to demonstrate situations where common sense demands liability be found even if the outcome would have been the same anyway.
    1. We have already looked at the hiker in the woods. The next example approved by the Supreme Court in FCA v Arch involves a bus being pushed over a cliff:

“A hypothetical case adapted from an example given by Professor Stapleton, which was discussed in oral argument on these appeals, postulates 20 individuals who al combine to push a bus over a cliff.  Assume it is shown that only, say 13 or 14 people would have been needed to bring about that result.  It could not then be said that the participation of any given individual was either necessary or sufficient to cause the destruction of the bus.  Yet it seems appropriate to describe each person’s involvement as a cause of the loss. Treating the ‘but for’ test as a minimum threshold which must always be crossed if X is to be regarded as a cause of Y would again lead to the absurd conclusion that no one’s actions caused the bus to be destroyed.”[6]

    1. Professor Stapleton invokes the idea of ‘oversubscribed’ causation. In respect of the bus which is pushed over the cliff she explains:

“Here the relevant element of the mechanism by which the indivisible outcome of the vehicle’s destruction came about (the amount of force) was not only oversubscribed,  but was so oversubscribed that no one person leaning on the vehicle was necessary or sufficient to satisfy that element and thereby produce the outcome.”[7]

    1. Back to FCA v Arch:

“Other examples of a similar nature given by Professor Stapleton include a case where the directors of a company unanimously vote to put on the market a dangerous product which causes injuries, although the decision only required the approval of a majority.  Again it cannot be said that the any individual director’s vote was either necessary or sufficient to cause the product to be marketed and yet it is reasonable to regard each vote as causative rather than to say that none of the votes caused the decision to be made.”

    1. The final example given is where multiple polluters discharge hazardous waste into a river.
    1. The Supreme Court concludes that it does not matter that the damage would have occurred in any event:

“In all these cases each individual contribution is reasonably capable of being regarded as a cause of the harm that occurs, even though it was neither necessary nor sufficient to cause the harm by itself.”

A new ‘extended but for’ test

    1. In her 2023 paper Professor Stapleton proposes a new extended ‘but for’ test for causation which she suggests has the advantage that it is a “user-friendly test, easily understood by law students and amenable to forensic formulation as a jury instruction.” Even practitioners may be able to understand it. She proposes:

“For the smooth practice of law all that a test of factual cause needs to do, to be valuable, is to describe the types of occasion on which the law has identified the connection between a factor and an indivisible outcome as a causal connection. As we have seen, there are two ways a connection between a factor and an indivisible outcome may be causal in the law: (i) by the factor being necessary for the outcome; or (ii) by the factor contributing to the mechanism by which the outcome came about, even if that contribution was neither necessary nor sufficient for that outcome. These 2 types of causal connection can be captured by a simple extension of the traditional but-for test:

A factor is a factual cause of an indivisible outcome if, but-for that factor,

(i) the outcome would have been absent or

(ii) a contribution to the mechanism by which it came about, would have been absent.”

(my emphasis)

Applying the new test – rethinking Hotson

    1. A powerful illustration of the significance of Professor Stapleton’s proposed new approach is the well-known case of Hotson.  She suggests:

“The clarity of the Supreme Court’s recognition of unnecessary and insufficient factual causes also provides a more coherent frame in which to view earlier cases which had been analyzed by the relevant court through the misleading lens of the but-for test of factual causation.  One of many available examples, Hotson’s case, concerned an everyday situation in which a young boy suffered an indivisible personal injury, in this case hip joint necrosis the mechanism of which involves the blood supply to the joint becoming inadequate.”

    1. The claimant fell from a tree damaging blood vessels which supplied his left hip joint. He was negligently sent home from hospital following a failure of diagnosis and went on to develop avascular necrosis.  His claim failed because there was a 75% chance that he would have developed avascular necrosis in any event.  Professor Stapleton proposes that on the basis of her test and the approach to causation of the Supreme Court in FCA v Arch, the claim in Hotson would have succeeded had it been framed as follows:

“Since both the fall and the subsequent medical negligence reduced the blood supply to the hip joint they both made contributions to the mechanism by which the hip joint necrosis eventually came about, and both should be recognized as a factual cause of that injury regardless of whether the plaintiff can prove that, but for the medical negligence, the necrosis would not have occurred.”[8]

The problem of quantification

    1. In her 2023 paper Professor Stapleton repeats her previously stated view that a “core principle of compensatory damages is that they should not make the victim better off than what the individual’ victim’s prospects would have been but for tortious conduct.”
    1. So in her re-worked example of Hotson the claim succeeds (breach of duty plus causation) but the damages are zero because outcome would probably have been the same in any event.
    1. She argues that the position is different where the boy is negligently pushed out of the tree: “Then the benchmark is found by asking: what would the victim’s prospects have been but for the contributions of both the person pushing and the hospital.”
    1. Writing before Holmes v Poeton, Professor Sandy Steel reminds us of the same principle i.e. a claimant cannot be put in a better position than she would otherwise have been in:

“… the law recognises that, if a breach is a cause of damage, but does not cause a person to be worse off than if the breach had not occurred, because other, non-wrongful events produced or would have produced the damage anyway, the breach does not pro tanto trigger compensatory liability: Jobling v Associated Dairies [1982] AC 794.”[9]

    1. He gives the example of two poisoners:

“A and B, independently, each in breach of a duty of care, poison C’s tea. A puts in two drops of poison and B puts in one. C drinks the tea and suffers a brain injury. Two drops, let us suppose, are sufficient to cause the brain injury which occurred. Although B’s breach was neither necessary nor sufficient for C’s brain injury, it can reasonably be described as making a causal contribution to the occurrence of the injury.”

    1. Professor Steel argues that both poisoners are fully liable for the brain damage. The situation would be different however if it were known that the brain injury would have occurred anyway. By reference to Bailey:

“If it were known that the brain injury would have occurred anyway in Bailey, there should be no substantial damages: although the breach contributed to the brain injury, it would have happened anyway due to non-wrongful causes (see Stapleton and Steel (2016).”


    1. So where does this leave us in practice?
    1. Establishing causation in cases where the outcome would have been the same in any event will usually be a pyrrhic victory if no damages are recovered.
    1. Proof of damage has always been the essence of negligence claims. There may now be cases which can be brought simply for vindication and to establish that a wrong has occurred.  Examples might be cases of negligent treatment where a child has died but would have died anyway.  From a practice perspective such a claim could in theory now succeed, even if no damages are awarded whereas previously such a claim would inevitably fail.  How courts will deal with such claims remains to be seen.
    1. The Stapleton/Steel approach also opens the door to claims for negligent medical treatment where the initial injury was caused tortiously e.g. in a road accident or a pub fight. The latter will be more important because it is unlikely that an assailant who hits me on the head with a hammer will be good for the damages but the hospital who negligently treat me afterwards will be.
    1. For the vast bulk of clinical negligence claims the practical reality is likely to remain the same: the key question is whether the outcome would have been the same in any event. If yes, then Stapleton and Steel and their endorsement by the Supreme Court and Court of Appeal have opened the door to a successful claim but not to damages.  Where the outcome would not have been the same in any event, or where it is not possible to say, the Stapleton/ Steel approach is a slightly different path to the same destination and the claim will succeed, just as it would have done previously.
    1. There is though an important difference in how we should frame claims going forward. Whilst practitioners might still ask expert witnesses in clinical negligence cases whether the breach of duty has ‘materially contributed’ to the outcome, we should also be prepared to ask the slightly different question: has the breach of duty contributed to the mechanism by which the injury or outcome occurred?





[1] John de Bono KC, Serjeants’ Inn Chambers, London.

[2] Bailey v MoD at [46]

[3] FCA v Arch at [182]

[4] [2023] EWCA Civ 1377

[5] Holmes v Poeton at [125]

[6] FCA v Arch at [184]

[7] “Unnecessary and Insufficient Factual Causes,” Journal of Tort Law 2023.


[8] Unnecessary and Insufficient Factual Causes, Journal of Tort Law 2023. 12

[9] Material contribution to damage, again, LQR 2022, 138 (Oct) 540-545