FREE CHAPTER from ‘A Practical Guide to Material Contribution in Clinical Negligence’ by Rhodri Jones


The need to establish causation

Any analysis of material causation in clinical negligence causation must start with the traditional formulation of causation. In Fairchild, Lord Bingham set this out:

In the generality of personal injury actions, it is of course true that the claimant is required to discharge the burden of showing that the breach of which he complains caused the damage for which he claims and to do so by showing that but for the breach he would not have suffered the damage[1]

Establishing causation is thereby essential in all cases where an individual having sustained loss is seeking damages from a potential tortfeasor. Where a claimant brings a claim in personal injury of any kind, the burden is upon them to prove it on the balance of probabilities. Establishing legal causation may in some circumstances be complicated, but its essence is based upon common sense. As stated by Lord Reid in the case of McGhee:

it has often been said that the legal concept of causation is not based on logic or philosophy. It is based on the practical way in which the ordinary man’s mind works in the everyday affairs of life[2]

In the same case Lord Salmon referred to his own judgment in the case of Alphacell Ltd:

The nature of causation has been discussed by many eminent philosophers and also by a number of learned judges in the past. I consider, however, that what or who has caused a certain event to occur is essentially a practical question of fact which can best be answered by ordinary common sense rather than abstract metaphysical theory[3]

The traditional test for causation in personal injury is the ‘but for’ test: absent the tortious acts or omissions of a defendant, a claimant would not on the balance of probability have suffered the particular injury complained of. An often cited albeit extreme example of the ‘but for’ test in action is exemplified in the case of Barnett[4]. The claimant had consumed a quantity of arsenic and having presented himself to hospital, he was negligently discharged. He later died of arsenic poisoning. The court found that applying the traditional ‘but for’ principles of causation, absent the negligence he would on the balance of probabilities have died in any event.

Most cases in clinical negligence will succeed or fail on the application of traditional probabilistic causation principles. ‘But for’ causation most readily applies to cases where a singular act or omission has been identified by the Claimant as the cause of their injury. In this type of case, a single causative agent is in operation.

A claimant’s primary injury will often be reasonably well defined. Issues may however arise where the Claimant has gone on to develop further injuries, for example a chronic pain condition or a psychiatric injury. In those circumstances, the claimant may seek to invoke principles such as the “eggshell skull”. Similarly, defendants may seek to invoke principles of foreseeability and apportionment. The concept of material contribution does not need to be considered in the vast majority of personal injury cases.

The need for an alternative test in causation

The courts have on occasion raised the question of whether material contribution can in fact be viewed as freestanding legal principle in causation. HHJ Auerbach giving judgment in the case of Davies stated in relation to material contribution:

while Bonnington Castings was viewed in Bailey as establishing a novel principle, later authorities of the Court of Appeal, House of Lords and Privy Council view it as having resulted in an anomalous outcome, for peculiar reasons, and not as standing for any novel legal principle, distinct from the general jurisprudence on co-contribution to divisible or indivisible harms. This conclusion appears to me to accord with deep principle, and with the prevailing view at the highest level, ever since Fairchild, that it stands alone as an exception to orthodox principles, in a tightly circumscribed type of case”[5]

Assuming that it is a legal principle, material contribution in causation may be applicable when, having considered the facts of a particular case, the ‘but for’ test is insufficiently sophisticated to provide an answer. These types of cases often involve diseases and injuries where gaps exist in the scientific understanding of the relevant underlying physiological and pathological process. In this respect, material contribution may be characterised as a necessary means of dealing with causation in complex injury and disease processes.

In the industrial disease case Fairchild, the conundrum for the Court of Appeal was to decide on the balance of probabilities which fibre or fibres of asbestos from different sources caused the claimant’s injury. Lord Bingham described this issue as the “Rock of uncertainty[6]. Sienkiewicz was another case in which asbestos was the causative agent. Lord Roger described the predicament of the courts when facing inherent medical uncertainty equipped with only standard causation principles:

Faced with the problem that, in the present state of medical science, a claimant can never prove his case to the standard that the law usually requires, a legal system may react in a variety of ways. It may simply adhere to its usual stance and say that, since the claimant has not proved on the balance of probability that the defendant actually caused his disease, the claim must fail…”[7]

Lord Phillips also highlighted how in certain circumstances applying the “but for” test can lead to inherent unfairness to both claimants and defendants:

It is a basic principle of the law of tort that the claimant will only have a cause of action if he can prove, on balance of probabilities, that the defendant’s tortious conduct caused the damage in respect of which compensation is claimed. He must show that, but for the defendant’s tortious conduct he would not have suffered the damage. This broad test of balance of probabilities means that in some cases a defendant will be held liable for damage which he did not, in fact, cause. Equally there will be cases where the defendant escapes liability, notwithstanding that he has caused the damage, because the claimant is unable to discharge the burden of proving causation”[8]

Scientific understanding and uncertainty

Legal analysis of how a disease or injury manifests is inherently dependent upon an understanding of the underlying science. The courts are reliant in this respect on the evidence of experts. Their function is in assisting the court to explain how a particular causative agent interacts with an individual’s physiology to cause disease. In medical terms an agent causative of a disease may be referred to a pathogen and the process by which a pathogens acts upon an individual described as its aetiology or pathophysiology. Litigation in clinical negligence is led by principles of medical science and thereby limited by scientific uncertainty. In this way, the analysis of causation naturally evolves in step with the scientific understanding of disease processes.

AB[9] was a case involving historic exposure to radiation. Smith LJ confirmed the court’s reliance on expert analysis of the likely extent of exposure to the radiation and to provide scientific and epidemiological evidence of the likely effect in relation to the injuries complained of. The case of Davies involved an alleged delay in the treatment of meningitis. HHJ Auerbach, considered the importance to the court in making its determination of understanding the aetiology of the disease process “It therefore helps at the start, to get one’s general bearings by reflecting on what the experts, and the literature, tell me about the broad characteristics of this particular disease”[10].

In case of CNZ[11] the task for the court was to consider the effect of delays at birth upon the injury to the claimant where the relationship between the causative agent and the brain was beyond scientific knowledge. In that case, the expert evidence assisted Ritchie J in finding for the claimant by applying standard causation principles. In obiter he acknowledged that whilst gaps in the understanding of science would make causation difficult to prove, this was no bar to the court making an assessment of causation by applying the principles developed in occupational disease to clinical negligence.

Ritchie J identified five “tricky causation issues[12] that can arise in cases where there are gaps in the scientific understanding:

  1. Multiple causative factors, one negligent and the others non negligent (naturally occurring / idiopathic / genetic / environmental etc.).
  2. One known causative factor, part of which was an innocently inflicted and part of which was caused by the breach.
  • Multiple different defendants responsible for exposing the claimant to the same causative factor.
  1. Multiple known risk factors.
  2. Multiple different outcomes which can occur from one known causal factor.

The approach of the court in matters where medical science cannot prove as a fact the causative potency of a particular was explored by Lord Phillips in Sienkiewicz:

Most claims for death or personal injury result from accidents. In such cases the cause of death or personal injury will seldom be in issue. A body of knowledge which I shall call “medical science” will enable a witness, expert in that science, to describe the precise mechanism by which the accident brought about the death or injury. I shall describe this as the “biological cause” of death or injury. It is sometimes referred to by the more general description of the “cause in fact”. In some cases, however, medical science will not yet have identified the precise mechanism by which an injury gives rise to a particular physical consequence. In such cases it may be possible to deduce that there was a causative link between the two by evidence of what usually happens. Epidemiological evidence that a particular injury or disease usually follows a particular type of bodily insult may enable a court to conclude in a particular case, on balance of probability, that the former was caused by the latter. “Post hoc ergo propter hoc”. A finding of causation based on such evidence is sometimes described as “the cause in law”[13].

Lord Phillips highlights here the potential for a distinction between medical and legal causation. The applicable standard of the latter being whether an agent more likely than not caused the injury or disease.

Causative agents

In most personal injury cases there is a single recognised causative agent, most often a physical accident, from which all identifiable loss flows. In clinical negligence and occupational disease litigation there may be multiple agents and multiple tortfeasors each playing a potential causative role to the injury. In the case of multiple agents these are often distinguishable as negligent and non-negligent in origin. The status of the agent refers to whether it was born of a breach of duty. In the case of Bonnington Castings[14] both negligent and non-negligent causative agents were operating simultaneously to cause the claimant’s injury. In that case the claimant worked in a brick kiln where both “guilty dust” and “innocent dust” coexisted. Bonnington Castings is widely regarded as a touchstone for cases concerning material contribution.

In occupational disease cases, as was the case in Bonnington Castings, the damage and loss was caused by a single identifiable agent: dust. The only distinction between the dust was its origin. The argument in Bonnington Castings was the extent to which, if any, the defendant should be liable for the damage caused by the so-called “guilty dust”.

Many industrial disease cases where material contribution has been pleaded have a similar fact pattern. The role of the court in such circumstances is the determination and apporting of liability between multiple potential sources of the same causative agent. In clinical negligence cases there may be a range of different potential causative agents in operation which can make the determination of causation, and any apportionment more complicated.

The test for material contribution in clinical negligence

The courts have applied themselves to define the principles of material contribution. The clearest iteration of the circumstances in a clinical negligence case in which material contribution may be applicable is set out Waller LJ in Bailey:

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 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[15]

In accordance with Waller J’s basic statement of principle, the circumstances required to establish causation applying material contribution are:

  1. The inability of medical science to demonstrate on the balance of probabilities that “but for” the negligence the injury would not have occurred.
  2. The ability to demonstrate that the contribution of the negligence to the injury was more than minimal.

The standard upon which the more than minimal contribution requires to be proved is the balance of probabilities. The reference to Hotson[16] is to the exclusion of cases where on the balance of probabilities the damage would likely have occurred in any event (i.e., where “but for” the negligence the damage would still have occurred). The burden is upon the Claimant to prove that the causative agent caused damage.

“More than minimal”

The accepted threshold for a negligent causative agent applying material contribution is something more than a minimal contribution to the injury. It is important to bear in mind that whilst the threshold is more than minimal, to qualify as a causative agent in the first place it must be proved to have a causative role on the balance of probabilities. Given that the causative role of an agent that has materially contributed to an injury is an inherent unknown, it is perhaps unsurprising that the courts have not provided any consensus as to what might constitute de minimis.

In Bonnington Castings Lord Keith referred to the accumulation of dust for which the defendant bore responsibility as “In cumulo it must have been substantial, though it might remain small in proportion”[17]. In the same case, Lord Reid considered the extent to which a contribution would need to operate to be “material”.

What is a material contribution must be a question of degree. A contribution which comes within the exception de minimis non curat lex is not material, but I think that any contribution which does not fall within that exception must be material[18]

In the case of Saunderson (a group litigation for personal injuries following a fire) Jay J referred to the threshold whereby an injury may be actionable as “Ultimately, in my judgment, this must be a question of fact and degree. A transient, trifling, self-limiting, reversible reaction to an irritant is not “actionable injury” for the purposes of the law of tort [19]

Lord Reid in Bonnington Castings uses the word “substantially” in looking at both the effect of the “guilty” dust and the “innocent” dust. However, he concluded “In my opinion it is proved not only that the swing grinders may well have contributed but that they did in fact contribute a quota of silica dust which was not negligible to the pursuer’s lungs and therefore did help to produce the disease[20]

Lord Hoffman in Gregg, whilst referring to Wilsher[21] considered that “The defendant was only liable if the lack of oxygen caused or substantially contributed to the injury [22]. When considering what the necessary threshold was for a material contribution, Waller LJ in Bailey questioned whether the use of the word “substantial” implied a higher requisite causative potency than simple de minimis:

The use of the word “substantial” also seems to connote a higher causative potency than Lord Reid’s in excess of de minimis. The word “substantial” has also appeared in the language of others since Wardlaw’s case. Examples are Lord Hoffmann in Gregg v Scott [2005] 2 AC 176, para 77 in dealing with Wilsher’s case [1988] AC 1074, where he said that “The defendant was only liable if the lack of oxygen caused or substantially [my italics] contributed to the injury”. This is also the language used by Lord Simon of Glaisdale in McGhee’s case [1973] 1 WLR 1, 8 d.[23]

A more pragmatic position is that of Lord Phillips in SienkiewiczI doubt whether it is ever possible to define, in quantitative terms, what for the purposes of the application of any principle of law, is de minimis. This must be a question for the judge on the facts of the particular case[24].

Chapter Summary

  1. The starting point for the analysis of causation in any case should be the consideration of whether standard ‘but for’ causation principles apply.
  2. Whether applying standard causation or material causation, findings of fact will be based on the evidence and burden is upon the claimant to prove all aspects of their case on the balance of probabilities. As confirmed by Jay J in the case of Heneghan:

In a case where medical causation is in issue, strict adherence to logic and principle would demand proof on the balance of probabilities either of the whole of the damage suffered or of a material part of it. For these purposes, there is no distinction between the whole and the part, provided that the part is significant[25]

  • An appreciation of the causative potency of a particular agent is likely to require an understanding of the aetiology of a particular disease or injury. It is likely that as medical science evolves, the understanding of the causes of diseases will reveal more about the relationship between diseases and their causative agents.
  1. To be a causative agent, it much be proved on the balance of probabilities to have causative potential.
  2. The test for whether a causative agent then reaches the threshold for being more than de minimis is likely to be factually dependent.


[1]   Fairchild v Glenhaven [2002] UKHL 22 [8]

[2]   McGhee v National Coal Board 1973 S.C. (H.L.) [37]

[3]   Alphacell Ltd v Woodward [1972] AC 824 [847]

[4]   Barnett v Chelsea & Kensington Hospital Management Committee [1968] 2 WLR 422.

[5]   Davies v Frimley Health NHS Foundation Trust [2021] EWHC 169 [209].

[6]   Fairchild v Glenhaven [2002] UKHL 22 [7]

[7]   Sienkiewicz v Grief (UK) Ltd [2011] UKSC 10 [140]

[8]   Ibid [16]

[9]   AB v Ministry of Defence [2010] EWCA Civ. 1317

[10]  Davies v Frimley Health NHS Foundation Trust [2021] EWHC 169 [123]

[11]  CNZ (Suing By Her Father and Litigation Friend MNZ) v Royal Bath Hospitals NHS Foundation Trust and Another [2023] EWHC 19 (KB)

     Ibid [339]

[13]  Sienkiewicz v Grief (UK) Ltd [2011] UKSC 10 [6]

[14]  Wardlaw v Bonnington Castings [1956] AC 613

[15]  Bailey v MOD [2009] 1 WLR 1052 [46]

[16]  Hotson v East Berkshire Area Health Authority [1987] 3WLR 232

[17]  Wardlaw v Bonnington Castings [1956] AC 613 [626]

[18]  Wardlaw v Bonnington Castings [1956] AC 613 [621]

[19]  Saunderson & others v Sonae Industria (UK) Ltd [2015] EWHC 226 [179]

[20]  Wardlaw v Bonnington Castings [1956] AC 613 [623]

[21]  Wilsher v Essex Area Health Authority [1988] 2 W.L.R. 557

[22]  Gregg v Scott No. [2005] UKHL 2 [77]

     Bailey v MOD [2009] 1 WLR 1052 [41]

[24]  Sienkiewicz v Grief (UK) Ltd [2011] UKSC 10 [108]


[25]  Heneghan v Manchester Dry Docks Ltd & Others [2016] EWCA Civ 86 [51]