Tort law (2022): Principles of negligence: Proximate cause

In law and insurance, a proximate cause is an event sufficiently related to an injury that the courts deem the event to be the cause of that injury. There are two types of causation in the law: cause-in-fact, and proximate (or legal) cause. Cause-in-fact is determined by the "but for" test: But for the action, the result would not have happened. (For example, but for running the red light, the collision would not have occurred.) The action is a necessary condition, but may not be a sufficient condition, for the resulting injury. A few circumstances exist where the but for test is ineffective. Since but-for causation is very easy to show (but for stopping to tie your shoe, you would not have missed the train and would not have been mugged), a second test is used to determine if an action is close enough to a harm in a "chain of events" to be legally valid. This test is called proximate cause. Proximate cause is a key principle of Insurance and is concerned with how the loss or damage actually occurred. There are several competing theories of proximate cause (see Other factors). For an act to be deemed to cause a harm, both tests must be met; proximate cause is a legal limitation on cause-in-fact.

The formal Latin term for "but for" (cause-in-fact) causation, is sine qua non causation.

But-for test.

A few circumstances exist where the "but for" test is complicated, or the test is ineffective. The primary examples are:

Concurrent causes. Where two separate acts of negligence combine to cause an injury to a third party, each actor is liable. For example, a construction worker negligently leaves the cover off a manhole, and a careless driver negligently clips a pedestrian, forcing the pedestrian to fall into the open manhole. Both the construction worker and the careless driver are equally liable for the injury to the pedestrian. This example obeys the ‘but for test’. The injury could have been avoided by the elimination of either act of negligence, thus each is a but for cause of the injury.

Sufficient combined causes. Where an injury results from two separate acts of negligence, either of which would have been sufficient to cause the injury, both actors are liable. For example, two campers in different parts of the woods negligently leave their campfires unattended. A forest fire results, but the same amount of property damage would have resulted from either fire. Both campers are equally liable for all damage. A famous case establishing this principle in the United States is Corey v Havener.

In the United States, the rule of Summers v Tice holds that where two parties have acted negligently, but only one causes an injury to a third party, the burden shifts to the negligent parties to prove that they were not the cause of the injury. In that case, two hunters negligently fired their shotguns in the direction of their guide, and a pellet lodged in his eye. Because it was impossible to tell which hunter fired the shot that caused the injury, the court held both hunters liable.