b. Legal causation
Legal causation limits liability even after factual causation is proved. Even if a defendant is a “but for” cause, legal causation asks whether that wrongdoing should be treated as the operative or effective cause for the purpose of attributing legal responsibility. It is an evaluative judgment often guided by judicial common sense and policy.
Intervening acts
Even if a defendant factually caused an injury, he might be “off the hook” if a new, independent event breaks the chain of causation (novus actus interveniens). An intervening act is an event that occurs after a defendant’s initial wrongdoing but before the final damage, which is significant enough to break the chain of causation. Courts ask whether the later event was so independent and potent that the defendant’s breach should no longer be treated as an operative cause.
Courts generally classify these acts into three categories: acts of nature, the conduct of a third party, and the conduct of the plaintiff himself.
(i) Acts of nature
An overwhelming and unforeseeable natural event may break the chain of causation. For example, a ship damaged by the defendant’s negligence suffers further damage in a storm during a subsequent voyage. The storm may be treated as a supervening event that makes the original collision merely part of the history of events.
(ii) Acts of third parties
A third party’s conduct may break the chain if it is deliberate or so wholly unreasonable that it eclipses the defendant’s original negligence. To decide whether another person’s actions break the chain between the defendant’s wrongdoing and the harm, courts look at:
- how unreasonable the other person’s act was;
- whether it was something that could have been expected; and
- whether the defendant had a duty to protect against that kind of act.
For example, poor medical treatment after the original injury will only break the chain if it is grossly negligent. Ordinary mistakes, emergency decisions, or normal complications in treatment usually do not break the chain.
If the defendant was under a duty to guard against the specific intervention that occurred, the chain is not broken. For example, a decorator who leaves a door unlocked is liable if a thief enters, because preventing the theft was the very purpose of the duty to secure the premise.
(iii) Conduct of the plaintiff
If the plaintiff’s act is a reasonable or instinctive response to a danger created by the defendant, the chain remains intact. For example, a plaintiff who fell because a neck brace (necessitated by the defendant’s negligence) impeded her vision was allowed to recover for the second fall.
The chain may be broken by the plaintiff’s own actions if he behaves in a wholly unreasonable way and thereby causes further injury. For example, if someone knows their leg has been weakened by an earlier accident but then jumps down a flight of stairs, the defendant will not usually be liable for the new injury.
Non-intervening acts
(i) Foreseeable intervening acts do not break the chain of causation
Not every later event will sever the chain of causation. Where a subsequent act is a normal, natural, or reasonably foreseeable response to the danger created by the defendant, the defendant will generally remain liable for the resulting harm. For that reason, later medical treatment or rescue efforts, even if carried out negligently, will not automatically break the chain of causation. If such treatment or rescue is a reasonable reaction to the injury or risk created by the defendant’s original wrongdoing, it is not considered sufficiently independent to relieve the defendant of responsibility.
(ii) Conduct of the defendant
A defendant cannot rely on his or her own subsequent negligence to argue that the chain of causation has been broken between the original wrong and the plaintiff’s loss. In one case, a firm of solicitors committed two separate acts of negligence. They could not argue that the second act broke the chain of causation from the first in order to escape liability, especially since the claim based on the second act was already statute-barred.
