Negligence: causation 

Causation is where well-argued negligence answers are most often lost, because two genuinely different questions are run together. The civil liability statutes separate them expressly, and an answer should follow that structure.

• Factual causation — was the negligence a necessary condition of the harm?
• Scope of liability — is it appropriate for the defendant's liability to extend to the harm so caused?

The first is a question of historical fact. The second is normative: the statutes require the court to consider whether and why responsibility should be imposed, and to state its reasons. The plaintiff bears the onus on both.

Factual causation: the but-for test

Would the harm have occurred but for the defendant's negligence? If it would have occurred anyway, factual causation fails, and no amount of culpability in the defendant's conduct repairs it.

March v E & MH Stramare Pty Ltd (1991) 171 CLR 506¹ is the leading authority for treating the but-for test as a valuable filter rather than the exclusive determinant of causation — common sense and, ultimately, a value judgment about responsibility also inform the enquiry. The test is a filter, not a formula, and it fails in known situations:

• Multiple sufficient causes. Where two independent negligent acts would each have caused the whole harm, but-for reasoning absolves both, which cannot be right.
• Evidentiary gaps in disease cases where the mechanism of injury cannot be traced.

Adeels Palace Pty Ltd v Moubarak (2009) 239 CLR 420³ shows the but-for test applied under the New South Wales statute. Two men were shot by a gunman during a brawl at a New Year's Eve function at a licensed restaurant, and sued the operator for not providing security. In a single joint judgment the High Court held that factual causation under s 5D(1)(a) of the Civil Liability Act 2002 (NSW) is determined by the but-for test, and that the plaintiffs had not proved it: it was not shown to be more probable than not that, but for the absence of security personnel, the shootings would not have taken place. Pointing to possibilities that might have eventuated had circumstances been different did not prove factual causation, and that the injury was of the kind the duty guarded against could bear on scope of liability but did not answer the question of factual causation. The Court did not decide whether the operator was in breach.

One phrase in March should not be carried forward. In Young v Chief Executive Officer (Housing) [2023] HCA 31² Gordon and Edelman JJ said at [60] that "[a]lthough causation was once described in this Court as a concept of 'common sense', it has since, and repeatedly, been emphasised that the concept of common sense should be eschewed when applying the principles of causation". That is two Justices of five, in a case about mitigation of loss — but their footnotes collect the same criticism across five earlier decisions of the Court. What March decided about the but-for test is undisturbed; the invitation to reason by "common sense" is not something to repeat in an answer.

For these, the statutes provide an exceptional case limb: the court may find causation established where responsibility should be imposed notwithstanding that the but-for test is not satisfied. It is deliberately narrow, and it requires the court to reason openly about why liability is appropriate rather than to assert causation.

Material contribution to harm, or to the risk of harm, has been accepted in a limited class of cases, principally industrial disease with an indivisible injury and an unattributable mechanism. Do not generalise it into an ordinary substitute for but-for reasoning.

Loss of a chance

The High Court has held that the loss of a chance of a better medical outcome is not compensable damage in negligence: a plaintiff who cannot prove on the balance of probabilities that the negligence caused the physical injury does not recover a proportionate share for the lost prospect. The balance of probabilities operates as an all-or-nothing threshold.

This is distinct from cases in which the lost chance is itself a commercial opportunity with a value — a different kind of damage, most often pleaded in contract or in professional negligence over a lost transaction or claim. Students conflate the two routinely; the distinction is between a lost chance of avoiding physical injury and a lost chance that was itself the thing of value.

Intervening acts

A later act may relieve the defendant of responsibility for harm that follows it. The question is properly one of scope of liability, not of factual causation: the original negligence remains a necessary condition, and the issue is whether liability should extend that far. A deliberate wrongful act by a third party is more likely to break the chain than a foreseeable careless response; medical treatment of an injury the defendant caused usually does not, unless it is grossly substandard. The plaintiff's own careless conduct is normally handled as contributory negligence rather than as a break in the chain.

Failure to warn

Where the breach is a failure to warn of a risk, factual causation asks what this plaintiff would have done had they been warned — a subjective question, decided on all the evidence. Most jurisdictions add a statutory rule that the plaintiff's own statement about what they would have done is inadmissible except to the extent it is against their interest. The rule exists because such evidence is inevitably given with knowledge of how things turned out.

Applying this in a problem question

1. State the two limbs and keep them apart.
2. Apply but-for precisely, identifying the counterfactual: what would have happened had the precaution been taken?
3. If but-for fails, ask whether this is a recognised exceptional case, and reason about why responsibility should nonetheless be imposed.
4. For a failure to warn, ask the subjective question and note the evidentiary rule.
5. Under scope of liability, address intervening acts, the purpose of the duty breached, and whether the harm is of the kind the duty existed to guard against.

Where the authority sits

The civil liability statute of the relevant jurisdiction states the two-limb structure, and the common law supplies the but-for test, the exceptional cases, and the Australian rejection of loss of a chance in personal injury.