Causation is the battleground of personal injury law. No one needs a lawyer to tell them they were injured because they fell on a wet floor. The question is one step back – can they prove that the wet floor was the result of the cleaner’s negligence? And another step back from that, can they prove that the cleaner’s negligence caused the accident?
The Two-Step Test Nobody Explains Properly
Many people assume causation is one question. It’s actually two, and conflating them is where a lot of claims go off the rails.
The first question is factual causation – the ‘but for’ test. Would the injury have occurred but for the negligence of the defendant? It’s a physical, close to mechanical question. In jurisdictions that have legislated the issue in a style of the Civil Liability Act (2002) (WA), that question is determined as a preliminary question before you get anywhere near damages.
The second question is scope of liability, sometimes still called legal causation or remoteness. This is a normative question, not a factual one. Even if the defendant’s conduct was a factual cause of the harm, should the law hold them responsible for it? So, the driver that runs the red light and causes a collision is factually responsible for the crash. But is he responsible if the other driver, shaken by the accident, has a heart attack three weeks later during an unrelated argument? Factual causation might be satisfied. Scope of liability is where that claim usually collapses.
Treating these as one question is the single most common mistake we see in poorly prepared claims. They require separate evidence, separate submissions, often separate expert evidence.
When “But For” Isn’t Enough: Material Contribution
Multi-cause injuries often do not pass the “but for” test. For example, consider a worker who’s exposed to a harmful substance at two different worksites; or a patient who deteriorates after both a missed diagnosis and an unrelated pre-existing illness. In these scenarios, you simply cannot say “but for the defendant’s negligence, the harm wouldn’t have occurred” with any confidence, as there are multiple potential causes at play.
This is where the material contribution test comes in. Instead of establishing that the defendant was the sole cause, a plaintiff must only show that their negligence materially contributed to the risk or extent of the harm. It’s an easier standard to meet, and courts have recognized it as necessary since applying “but for” too strictly in such cases would lead to defendants being let off the hook merely because the harm had more than one possible cause.
To make a material contribution argument, you’ll need specific evidence about exposure levels, timing, and expert evidence that provides some actual numbers on how much the defendant’s breach increased the risk. Generic statements that “it probably contributed” might be challenged, so you’d want to present something that looks more like a dose-response relationship: more exposure, more risk, and a plausible mechanism for how they’re connected.
Neutralising The Pre-Existing Condition Defense
Defense attorneys often use pre-existing medical or psychological conditions as a defense in personal injury lawsuits. It’s the easiest way to argue causation – “the plaintiff already had degenerative disc disease” or “this psychological syndrome pre-existed the accident.” It’s a defense strategy that works often enough to be common.
The plaintiff’s response is straight from Torts 101. You take your plaintiff as you find them. A pre-existing condition will not mitigate damages in the slightest – the defendant must pay for the full extent of harm caused to the plaintiff, regardless of whether they were unusually susceptible to harm. The only question on the table is whether the defendant is the legal cause of that harm – rather than being at fault for all of it, the defendant is only liable for the portion they actually caused.
This is a hugely important aspect of modern tort law. It’s not a matter of whether the negligent driver was responsible for paralyzing your 90-year-old grandmother in a car accident. The question is whether the pre-existing osteoporosis turned multiple spinal fractures into paralysis, in which case the accident only caused some of the damages. Try explaining that to your grieving client, and see how it goes over. Don’t forget to mention that your fee arrangement is contingent on successful recovery of damages.
Intervening Acts and The Chain Of Causation
Novus actus interveniens refers to an intervening act that defense argues broke the chain of causation between negligence and the final harm. For instance, a faulty secondary surgery, second accident or unreasonable response time to medical care. Defendants raise this defense in pretty much every case involving ongoing medical treatment.
In response, you have to persuade the judge that the subsequent event is not what they call a “novus actus interveniens” – a totally new cause that overtook the effects of the initial negligence. In many (most) medical negligence cases that isn’t likely because the plaintiff hasn’t taken any acts. The reasonable management of ongoing medical treatment (especially if the plaintiff’s condition is a result of the defendant’s negligence) does not typically break the chain of causation.
Getting The Expert Evidence Right
None of this works without expert medical evidence that’s actually built for the legal test being applied, not just clinical diagnosis. This is where a lot of otherwise sound claims lose momentum. A treating GP’s letter confirming an injury exists doesn’t address causation. A properly briefed specialist report does.
The brief to your expert needs to ask the legal question directly: on the balance of probabilities, did this event cause, materially contribute to, or accelerate this condition? Vague instructions produce vague reports, and vague reports get picked apart in cross-examination. Selecting the right specialist matters too – an orthopaedic surgeon can speak to structural damage, but a claim involving cumulative injury or occupational disease often needs an occupational physician who understands dose-response relationships and can speak to material contribution specifically.
Early case evaluation is where this gets decided, often before a claim is even filed. The statutory frameworks governing causation, limitation periods, and expert evidence rules are technical enough that getting the sequencing wrong – instructing an expert too early, before records are complete, or asking the wrong question – can be difficult to fix later. This is exactly the kind of situation where specialised legal advice from experienced personal injury lawyers like Trewin Norman makes the difference between a claim that survives scrutiny and one that doesn’t, particularly when evidentiary deadlines and proof standards are involved.
Psychological Injury Without A Scan To Point To
Physical injuries are often easier to prove in court, as imaging can visually demonstrate the source of the problem. However, psychological injuries cannot be detected through images, which makes it harder to provide evidence in cases such as PTSD, secondary depression, or anxiety disorders following a traumatic event.
When there’s no image, proving causation heavily relies on expert clinical assessment, including timelines and the extraction of detailed histories, both mental and social – work, family, relationships, you name it. Your lawyer and treating team are also important for providing medical records and past counselling notes that show pre-existing vulnerabilities.
A psychiatrist will also write a report that explains the details of the diagnosis and directly addresses causation. They don’t just say, Jennifer has X – PTSD/depression/an anxiety disorder – and leave it at that. They consider all the alternative possible causes and reasons why it might not be the incident in question: bad events/stresses in other areas of life that predate the incident, symptom delay, age, pre-existing family/medical history, and so on. Any good defence lawyer will easily pick apart a generic report that doesn’t address alternative causes, so the psychiatrist’s job is to rule those out methodically.
Why Timing Of Evidence Matters More Than People Think
Defense arguments suggesting that a plaintiff’s pain complaints are exaggerated, psychosomatic, or part of a pre-existing degenerative condition are also legion. Any of these arguments might resonate with a jury, but without evidence to support them, they are nothing more than guesses, and the party making them does have the burden of proof. In the absence of supporting medical evidence or expert testimony, these defenses are typically just desperate assertions, and a quality lawyer will know how to turn them to their client’s advantage.
Contemporaneous evidence is the real fix here. Immediate post-accident medical records, ambulance notes, witness statements taken close to the event, and even digital telemetry from vehicles or wearable devices can establish that symptoms were present, or at least consistent with mechanism of injury, from the start. Gaps in this record don’t automatically kill a claim, but they hand the defense a ready-made argument, and closing that gap after the fact is far harder than documenting it properly at the time.
Failure To Warn and The “What Would This Patient Have Done” Question
Claims of medical negligence concerning a lack of warning about the risks of a procedure or treatment involve a somewhat different causation issue: not what happened physically, but what the plaintiff would have decided had they been properly informed. Courts tend to use a mostly subjective test here, considering what this specific plaintiff – in light of their circumstances, values, and risk tolerances – would have chosen, rather than what a hypothetical reasonable patient would have opted for. This evidence may include previous statements about risk aversion, family situation, or the patient’s declination of a similar procedure.
Apportioning Damages When Causes Overlap
Once causality is established, the tortfeasor and non-tortious cause issues arise. This often involves questions of proportionate liability for tortious and non-tortious diseases or injuries. How those settlements can be divided depends on whether or not the contribution between the tortious factor and the non-tortious factor is additive. If it is, as in the case of pneumoconiosis and COPD, then each party pays in relation to the full extent of the injury caused by their product or action. If not, then the parties pay in relation to the loss that their separate negligence would have caused. The same principle applies to tortious and innocent aggravation of non-tortious diseases.
This will happen in cumulative injury or occupational disease cases. We still need a logical basis for apportionment even when the injury wouldn’t have met a non-tortious cause standard on its own. That will usually be by the duration of exposure, the intensity of the dose, or by the relative timing of each contributing factor.
Causation disputes, not disputes over whether a duty was breached, are the reason a significant share of medically complex negligence claims fail to secure compensation. That single fact should shape how every complex personal injury case gets built from day one – not as an afterthought once liability is settled, but as the central battleground the whole case is constructed around.
