By Law Teacher

3.1.1 Causation – Introduction

Welcome to the first lesson of the third topic in this module guide – Causation! For a successful claim in negligence, it must be possible to show that a defendant’s conduct in fact caused the damage that the claimant suffered. Causation is established on the balance of probabilities, using the ‘but for’ test.

At the completion of this section, you should be comfortable understanding how to apply the ‘but for’ test to establish causation. You will understand how concurrent and subsequent causes can muddy the waters, but also how to deal with these different factual scenarios. You will also learn to deal with cases where the harm involved is a loss of chance.

This section begins by discussing factual causation and the application of the ‘but for’ test, explaining the burden of proof and the all or nothing approach to damages. It then goes on to discuss specific rules of factual causation, which will involve a discussion of both multiple concurrent causes and multiple successive causes. It will finally discuss the situation where the damage has caused the loss of a chance.

Goals for this section

  • To understand how to establish causation using the ‘but for’ test.
  • To understand how to establish causation where there are multiple or successive causes.

Objectives for this section

  • To be able to define the ‘but for test’.
  • To understand the all or nothing approach to liability.
  • To know the burden of proof regarding issues of causation.
  • To understand how to deal with multiple concurrent causes using the material contribution rule.
  • To understand how to deal with multiple concurrent cases where an employer has contributed materially to the risk of an injury occurring.
  • To be able to deal with cases of multiple successive causes by knowing that the perpetrator of the former cause will be held liable for the damage.
  • To understand how courts deal with ‘lost chance’ cases by applying the 50% rule.
  • To understand how this may be departed from in favour of the claimant where it would vindicate their rights.

3.1.2 Causation Lecture

A defendant’s conduct must cause the damage that the claimant has suffered.

Factual Causation

Tort law uses a ‘but for’ test in order to establish a factual link between the conduct of the defendant and the injuries of the claimant. In other words, the question asked is ‘but for the defendant’s actions, would the harm have occurred?’ If the answer to this question is yes, then causation cannot be shown, and vice versa.

A relatively modern description of the test can be seen in Cork v Kirby MacLean Ltd [1952] 2 All ER 402, where the it was held that “if the damage would not have happened but for a particular fault, then that fault is the cause of the damage; if it would have happened just the same, fault or no fault, the fault is not the cause of the damage.”

In, Barnett v Chelsea and Kensington Management Committee [1956] AC 613, the courts found that because injury to the claimant would have occurred regardless of the defendant’s conduct, there was no factual causation.

The All or Nothing Approach and the Burden of Proof

Matters of causation are decided on the balance of probabilities (i.e. 51%). Since the burden of proof rests with the claimant, the onus is on him or her to argue that had the defendant not acted negligently, their harm would likely not have occurred. Thus, if a court finds that there is a 55% chance that a defendant caused a claimant’s harm, they will hold the defendant entirely responsible for the harm.

This approach can be considered problematic because the courts are essentially treating the defendants as if they were 100% the cause of the claimant’s injuries, whilst in fact it is entirely possible that those injuries would have occurred regardless. The all or nothing approach can be seen as particularly problematic when dealing with ‘lost chance’ cases – these are described in detail in a later section of this chapter.

Specific Rules of Factual Causation

There will often be scenarios in which there are multiple causes of the claimant’s harm. In such scenarios, you should first work out if you’re dealing with concurrent causes (causes which happen at the same time) or successive causes (causes which take place one after the other). Each of these situations have their own rules which apply, as follows.

Multiple Concurrent Causes – The General Rule

Where there exists more than one possible cause of an injury or harm, the claimant does not have to show that the defendant’s actions were the sole cause of the injury suffered. Instead, it must simply be shown that the defendant’s actions materially contributed to the harm (Bonnington Castings Ltd v Wardlaw[1956] for two concurrent causes; Wilsher v Essex Area Health Authority [1988] AC 1074 for five different possible causes). According to the case law, whilst a 50% contribution is enough to bring a successful case, a 20% contribution is not.

Multiple Concurrent Causation – Exposure to Risk

Finally, there exists a line of cases where claimants have been unable to show that their harm has occurred as a result of the defendant’s conduct, but have been able to show that their employer has contributed materially to the risk of an injury occurring. In McGhee v National Coal Board [1973] 1 WLR 1, the claim was successful where the defendants had materially increased the risk of the claimant developing dermatitis.

This principle has become important where cases involve multiple illegitimate exposures to a risk. Its application can be seen in Fairchild v Glenhaven Funeral Services [2002] UKHL 22 where the claimants were unable to demonstrate which of their multiple employers had actually caused the damage, just that one of them had. On balance of probabilities, it was improbable that each individual employer had caused it. The courts applied McGhee to deal with this problem, ruling that as long as each of the claimants could show that an employer had materially increased their risk of contracting mesothelioma (by illegitimately exposing them to asbestos), then they were entitled to claim full damages from that employer.

This did not mean that each employee could claim three times – they were only entitled to claim once for their injury. Whilst this might appear to punish the singled-out employer more than the others, that employer still had the option of suing the others for their contribution to the exposure, meaning that the cost of compensation could effectively be spread amongst the employers.

The same principle applies in cases even where the claimant has exposed themselves to asbestos voluntarily, as a matter of self-employment. In Barker v Saint Gobain Pipelines Plc [2004] EWCA Civ 545, the claimant was exposed to asbestos for nearly 9 years whilst under the employment of the defendant. For the rest of his 30-year career, the claimant was self-employed, working with asbestos on three different occasions. It was held that Fairchild still applied, and that the defendant was liable for the claimant’s mesothelioma because of the material contribution by the defendant to the claimant’s illness. A 20% reduction in the claim’s value was made due to the claimant’s own contribution to exposure.

This type of liability can lead to individual employers being singled out for the activities of their peers. In Barker v Corus UK [2006] UKHL 20, some of the potential defendants had since gone insolvent. The decision before the court regarding the defendant employer, therefore, was that, contrary to Fairchild, that each employer was only liable for a percentage of damages in proportion to their contribution to the claimant’s risk. In effect, this meant that the idea of joint and several liability applied in Fairchild was overturned, and instead it was held that the idea of ‘proportionate liability’ applied. This also meant that the share of damages attributed to insolvent defendants was not payable.

There was significant backlash from various groups representing mesothelioma victims, and s.3 of the Compensation Act 2006 has reversed the Barker v Corusposition – but only in relation to mesothelioma.

Multiple Successive Causes

There also exists a small band of cases involving multiple, successive causes of damage – that is, the claimant is harmed by two sources of damage, one after the other. The primary example here is Performance Cars Ltd v Abraham [1962] 1 QB 33, where the courts ruled that a claimant could not be compensated for the same loss twice. It was ruled that in such situations, the former, earlier defendant is liable.

Lost Chance Cases

There is also a category of cases which deal with the idea of losing a chance – for example, a claimant might have a disease with a 40% recovery prognosis if it is caught early enough. However, due to negligent misdiagnosis the disease might go unchecked, and become terminal in nature. This claimant would therefore want compensation for losing their 40% chance at recovery. Such situations are referred to as ‘lost chance cases’.

As a general rule, the courts are unlikely to compensate a claimant for the loss of a chance, where the lost chance is less than 50%. In Hotson v East Berkshire Area Health Authority [1987] AC 750 the claimant was deprived of a 25% chance of not developing a hip deformity. Whilst the Court of Appeal awarded the claimant 25% of total damages on this basis, this reasoning was rejected by the House of Lords; since there was only a 25% chance that the hospital defendant had caused the injury, this did not satisfy the balance of probabilities.

In Gregg v Scott [2005] 2 WLR 268, the rule was applied regarding chance of survival. A doctor misdiagnosed the claimant’s lump as benign, when it actual fact it was cancerous. This caused a nine-month delay in the claimant’s treatment, and reduced his chance of survival from 42% down to 25%. Again, the Hotson rule meant that this was not actionable because it was more likely than not that the claimant would have died regardless.

It can therefore be seen that the law regarding lost chances is not without criticism. The problem with the status quo can be seen when the margins are made smaller; consider a situation in which Hotson had a 49% chance of recovering had the hospital properly diagnosed him. He would still be unable to claim, despite the hospital effectively changing his prognosis from a coin flip to certain deformity.

However, there is some merit to the Hotson position for claimants. For example: consider if a defendant had a 51% of getting better, but lost this chance due to medical malpractice. Under the Hotson principle, that claimant would be entitled to the full extent of damages, because on balance, they would have got better had malpractice not occurred. If, however, damages were calculated based on the lost chance itself, then this particular claimant would only be entitled to 51% of the total compensation for their harm, since there was a 49% chance the harm would have occurred regardless of malpractice.

Furthermore, the decision in Hotson makes it a lot simpler for the courts to work out the proper amount of compensation to be paid. If the decision of the Court of Appeal in Hotson were to stand, courts would have to spend significant time and resources working out the probability of certain things happening, down to the single percentage point. In contrast, the current situation means that there is no material difference between losing a 75% percent chance of recovery and losing a 70% chance of recovery – in either situation the claimant would be entitled to full damages for their condition.

Vindication of Rights Cases

Although lost chance cases demonstrate the problem with adhering strictly to the all-or-nothing approach to causation, there does exist a slim category of cases in which the courts have been willing to ignore the general rule due to policy considerations. Whilst examples are sparse, in Chester v Ashfar [2004] 3 WLR 927, the defendant doctor failed to warn the claimant that there was a 1-2% risk of her becoming permanently disabled as a result of an operation. She underwent the operation, and despite the operation being carried out correctly, the disabling complication occurred anyway. The claimant could not show that she would have avoided the operation had she known of the risk, but was able to argue that she would have delayed the operation by some time.

In applying the conventional but-for test, the courts found that they could not help the claimant – she would have likely undergone the operation anyway, and so the doctor’s negligence could not be described as having caused the harm – the risk would still have been run, simply at a later date. Nevertheless, the courts acknowledged the fact that the doctor had done wrong – patients have a right to know what they are consenting to. They therefore found against the defendant, despite the decision appearing to run contrary to conventional causation.

3.1.3 Causation Lecture – Hands on Example


Jack works for the Buoy Company – an organisation which specialises in the production of maritime safety gear. He has been working for the organisation for 20 years. During this time, he has worked with vulcanised rubber, used in the production of high quality sea buoys.

Ten years ago, following a government study into the long-term health effects of working with vulcanised rubber, a law was passed requiring factories which work with rubber to have sufficient ventilation systems in place to protect their employees – it emerges that unventilated exposure can lead to a lung-condition which can lie dormant for many years before becoming fatal. This is novel medical research – those companies which have been working with rubber had no way of knowing about the condition up until this government announcement and subsequent legislation. Unfortunately, the Buoy Company failed to take note of this new law, and has not put a sufficient ventilation system in place to date, exposing their employees to unsafe levels of rubber by-products.

Jack notices that he has developed a significant cough, and goes to visit his GP. He is sent for an MRI scan, and it is revealed that Jack has developed bouncy lung – a condition which occurs when an individual is exposed to vulcanised rubber without proper ventilation. Jack’s GP is unable to tell him exactly when he developed the condition, only that it must have been at some point during his career with the Buoy Company, and that the only possible cause of the condition would have been his exposure to rubber within the factory. Jack takes time off work as required, and informs the Buoy Company of his condition. They immediately install a new ventilation system for the benefit of the other employees working there.

Jack is prescribed a specialised medicine designed to take care of his condition. Unfortunately, his pharmacist misreads the prescription whilst distracted by another customer, and sends Jack away with the wrong medicine. This mistake is only discovered three months later when Jack goes for a check-up with his GP: unfortunately, because Jack has not been taking his medicine, his condition has worsened in such a way as to be fatal within the next five years.

Jack is understandably upset, and seeks legal advice. As part of this advice, a medical expert is consulted, and it is established that had Jack been taking the correct medicine, he would have had a 40% chance of surviving the disease completely, but this has now diminished to a 0% chance.

Advise Jack as to the validity of his claim against both the Buoy Company and his pharmacist, concentrating on issues of causation.


Jack’s claim against the Buoy Company is based on the illegitimate exposure to unventilated vulcanised rubber Jack experienced whilst under their employment. However, the length of Jack’s career somewhat complicates things. Whilst Jack was exposed to the rubber in an unventilated environment for 20 years, it has only been in the last 10 years that his exposure has been illegitimate, since that was when the law regarding ventilation was passed. In turn, this means that when Jack acquired bouncy lung is important – if in the last 10 years, then it will likely be actionable, because his employer should have been ventilating the dangerous rubber from Jack’s workplace. If the exposure occurred in the first 10 years, however, then this will not give rise to an action – the factory was not aware of the risks, and had to statutory obligation to ventilate. Unfortunately, it is impossible to tell when the dangerous exposure actually occurred. This means Jack’s situation is arguably analogous to the one in Bonnington Castings Ltd v Wardlaw [1956] AC 613 – there are two different possible causes for his ailment, but it is impossible to tell which is which. He has been exposed to ‘legitimate rubber’ for the first 10 years of his career, and ‘illegitimate rubber’ for the last 10 years. Since it is impossible to tell which caused his condition, there is a 50% likelihood that Buoy Company was responsible for his illness, and a 50% chance it is not. As per Bonnington, Jack does not need to demonstrate that his disease was definitely caused by the illegal exposure over the last 10 years, but simply that the Buoy Company made a material contribution to his illness. Since there is a 50% likelihood that his illness was caused by an illegal exposure to vulcanised rubber, this is a sufficiently large contribution – indeed, it was 50% likely in Bonnington that the claimant’s illness was illegitimately caused by his employer.

Regarding the negligence of Jack’s pharmacist, this is a matter of lost chance, and thus, Hotson v East Berkshire Area Health Authority [1987] AC 750. As per this case, the courts will examine what would have happened had Jack’s pharmacist provided him with the correct medicine in order to work out the effects of the pharmacist’s negligence.

According to expert evidence, if properly treated Jack would have had a 40% chance of surviving, which means that the most likely outcome for Jack would still have been death, at a 60% chance.

This will mean Jack does not have a claim against his pharmacist: with negligence, Jack’s health outcome is death. Even if that negligence had not occurred, Jack’s likely health outcome is still death. This means that causation cannot be established on the balance of probabilities. To confirm this reading of the case, Gregg v Scott [2005] 2 WLR 268 is founded on similar facts – a medical failing caused the claimant to lose a (near) 40% chance at survival. Nevertheless, this was regarding as inactionable for the same reason as in Hotson.