A separate act or omission that breaks the direct connection between the defendant's actions and an injury or loss to another person, and may relieve the defendant of liability for the injury or loss.
Civil and criminal defendants alike may invoke the intervening cause doctrine to escape liability for their actions.
A defendant is held liable for an injury or loss to another person if the defendant's negligent or reckless conduct was the proximate cause of the resulting injury or loss. This means that the defendant's conduct must have played a substantial part in bringing about or directly causing the injury or loss. However, the defendant may escape liability by showing that a subsequent act or event, or intervening cause, was the real cause of the injury.
Not all intervening causes relieve a defendant of liability. An intervening cause relieves a defendant of liability only if it would not have been foreseeable to a reasonable person, and only if damage resulting from the defendant's own actions would not have been foreseeable to a reasonable person.
For example, assume that a farmer agrees to store a large, heavy sculpture for an artist. The sculpture is designed for outdoor display, so the farmer leaves it in her backyard. A tornado throws the sculpture several thousand feet, ruining it.
If the artist sues the farmer for damage to the sculpture, the farmer may argue that the tornado intervened between her negligent storage and the damage, relieving her from any liability. The farmer may claim that she could not have anticipated any detrimental effects of outdoor storage on the sculpture, because the sculpture was made for outdoor display.
At trial the issue of the farmer's liability is a question of fact to be determined by the judge or jury. The judge or jury asks whether a reasonable person would have anticipated a tornado. Generally, extraordinary weather conditions are deemed an unforeseeable intervening cause. However, if the farmer lives in Kansas, where tornadoes may be expected, and stored the sculpture outside without tethers during tornado season, the judge or jury may find that she should have anticipated the tornado and its damaging effects, and thus is liable for the damage.
Next, the fact-finder considers whether the farmer could have foreseen damage resulting from outdoor storage. Since the artist made the sculpture for outdoor display, damage to the sculpture from outdoor storage may be considered unforeseeable. Under these facts the tornado may be deemed an unforeseeable intervening cause of the damage to the sculpture, and the farmer may avoid liability.
Two types of intervening causes are considered: dependent and independent. A dependent intervening cause is set in motion by the defendant's own conduct, and will not relieve the defendant of liability unless it is extraordinary. For example, suppose the defendant poked an associate in the chest during a friendly discussion around a watercooler, and the associate subsequently jumped out a window. This unusual reaction may be deemed an extraordinary intervening cause that relieves the defendant of liability.
An independent intervening cause arises through no fault of the defendant. It relieves a defendant of liability unless it was foreseeable by the defendant.
The most common intervening causes cited by defendants are natural forces and negligent human conduct. Natural forces include extraordinary weather, earthquakes, volcanic eruptions, and the conduct of animals. Negligent human conduct is conduct that exposes a person to abnormal risks. Criminal human conduct by a third party will not be considered an intervening cause relieving the defendant of liability if the defendant's negligence has contributed to the victim's loss. For example, assume that Martin borrows Tasha's vehicle, drives it to a neighborhood notorious for its high crime rate, and leaves it unlocked with the keys in the ignition. If the vehicle is stolen, Martin may be held liable to Tasha for her loss because a reasonable person would have anticipated the theft.
Cohen v. Petty, 62 App. D.C. 187, 65 F.2d 820 (D.C. Cir. 1933), illustrates how the doctrine of intervening cause works. In Cohen, Jeanette Cohen sued Joseph Petty for permanent injuries she suffered as a passenger in a vehicle when Petty drove it into an embankment.
At trial Petty argued that he had become sick without warning and had fainted while driving. The sudden sickness and fainting spell were, Petty claimed, an intervening cause that relieved him of liability. Petty testified that he had never fainted before and that he was feeling fine up to the point of the sudden illness. Petty's wife, Theresa Petty, who was sitting in the front passenger's seat, testified that just before the accident, Petty said, "Oh, Tree, I feel sick." Cohen herself testified that shortly before the accident, she heard Petty exclaim to his wife that he felt sick.
The trial court agreed with Petty and entered judgment in his favor. On appeal the Court of Appeals of the District of Columbia affirmed. According to the appeals court, the sudden illness was an intervening cause. Petty had had no reason to anticipate the illness, and because he had not been negligent in any way prior to the accident, the illness relieved him of all liability for Cohen's injuries.
Some jurisdictions use two terms to define the intervening cause doctrine: intervening cause and superseding cause. In these jurisdictions intervening cause describes any cause that comes between a defendant's conduct and the resulting injury, and an intervening cause that relieves a defendant of liability is called a superseding cause. Other jurisdictions do not use the term superseding cause. These jurisdictions simply ask whether the intervening cause is sufficient to relieve a defendant of liability. All jurisdictions differentiate between an intervening cause that relieves a defendant of liability and one that does not: the only difference is in the terminology.
Loehr, Cynthia. 2000."Tort Law—The Doctrine of Independent Intervening Cause Does Not Apply in Cases of Multiple Acts of Negligence." New Mexico Law Review 30 (spring): 325–49.
Schlosser, William L. 1998."Intervening-Cause Defense: Is It Still Viable Under Comparative Fault?" Res Gestae 42 (July): 16.