8. Intervening acts (novus actus interveniens)

Intervening Acts (Novus Actus Interveniens)

In negligence, a defendant is only liable for damage directly caused by their breach of duty. If an independent event occurs after the defendant's breach that causes or worsens the plaintiff's damage, courts must decide whether this breaks the chain of causation. This intervening event is called a novus actus interveniens (a new intervening act). When it applies, it breaks the causal link and relieves the defendant of legal liability for the resulting damage.

In The Oropesa, Lord Wright explained that to break the chain of causation, the intervening act must be something "ultraneous, unwarrantable, a new cause which disturbs the sequence of events, something which can be described as unreasonable, extraneous or extrinsic."

1. Intervening Act of the Plaintiff

Unreasonable conduct: If the plaintiff acts so carelessly or unreasonably after the defendant's breach that their subsequent injury is attributable to their own conduct rather than the original negligence, the chain of causation is broken. In McKew v Holland & Hannen & Cubitts (Scotland) Ltd, the plaintiff injured his leg due to the defendants' negligence. Later, despite knowing his leg was unstable, he attempted to descend a steep flight of stairs without assistance and fell. Lord Reid held that the defendants were not liable for the additional injury, stating: "If an injured man acts unreasonably, he cannot hold the defendants liable for injury caused by his own unreasonable conduct."

Reasonable conduct and contributory negligence: Not every careless act by a plaintiff breaks the chain of causation. If the plaintiff's action is merely minor carelessness, it constitutes contributory negligence, which reduces the damages recoverable rather than wiping out liability altogether. Furthermore, if the plaintiff acts reasonably to cope with the situation created by the defendant, the chain remains intact. In Wieland v Cyril Lord Carpets Ltd, the plaintiff suffered a neck injury due to the defendants' negligence and had to wear a surgical collar. The collar interfered with her bifocal glasses, causing her to fall down stairs. The court held that she was acting reasonably in coping with the vicissitudes of life, so the defendants were liable for the further injury.

2. Intervening Act of a Third Party

Reasonable and involuntary acts: If a third party's independent act is the immediate cause of the plaintiff's damage, it will usually break the chain of causation if the defendant's breach merely provided the occasion for it. However, the defendant remains liable if the third party's act is reasonable or involuntary. In The Oropesa, after a collision caused by the defendant ship, the captain of the damaged vessel launched a lifeboat to save his ship, but the boat capsized and crew members drowned. The court held that the captain's action was a reasonable attempt to handle the emergency, so it did not break the chain of causation. Similarly, involuntary acts by third parties do not break the chain, as seen in Scott v Shepherd.

Negligent acts of third parties: Where a third party intervenes negligently, their conduct is far more likely to break the chain of causation than non-negligent acts. In Knightley v Johns, after an accident in a tunnel caused by the defendant's negligence, a police inspector made several errors, including ordering an officer to ride a motorcycle against traffic to close the tunnel entrance. The officer was struck and injured. The court held that the inspector's negligence was an intervening act that broke the chain of causation, relieving the original defendant of liability for the officer's injury.

3. Intervening Act of Nature

Acts of nature: If an independent natural event causes damage to the plaintiff after the defendant's negligent breach, the defendant is not liable for that weather-related damage. In Carslogie Steamship Co Ltd v Royal Norwegian Government, the plaintiff's ship was damaged in a collision caused by the defendant's negligence. While sailing across the Atlantic to undergo repairs, the ship suffered severe storm damage. The court held that the defendants were not liable for the storm damage, as the bad weather was an independent intervening act in the ordinary course of the voyage. The defendant's original tort had merely provided the historical backdrop that placed the ship at that location at that time.