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In the law, a proximate cause is an event sufficiently related to a legally recognizable injury to be held 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. For an act to cause a harm, both tests must be met; proximate cause is a legal limitation on cause-in-fact.
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 unreasonably, 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.
- Market share evidence. Injury or illness is occasioned by a fungible product made by all the manufacturers joined together in a lawsuit. The injury or illness is due to a design hazard, with each having been found to have sold the same type of product in a manner that made it unreasonably dangerous, there is inability to identify the specific manufacturer of the product or products that brought about the Plaintiff’s injury or illness and there are enough of manufacturers of the fungible product joined in the lawsuit, to represent a substantial share of the market. Any damages would then be divided according to the market share ratio.
Proximate Cause 
Since but-for causation is very easy to show and does not assign culpability (but for the rain, you would not have crashed your car – the rain is not morally or legally culpable but still constitutes a cause), there is a second test used to determine if an action is close enough to a harm in a "chain of events" to be a legally culpable cause of the harm. This test is called proximate cause.
There are several competing theories of proximate cause.
The most common test of proximate cause under the American legal system is foreseeability. It determines if the harm resulting from an action was reasonably able to be predicted. The test is used in most cases only in respect to the type of harm. It is foreseeable that throwing a baseball at someone could cause them a blunt-force injury. But proximate cause is still met if a thrown baseball misses the target and knocks a heavy object off a shelf behind them, which causes a blunt-force injury.
This is also known as the "extraordinary in hindsight" rule.
Direct Causation 
Direct causation is a minority test, which addresses only the metaphysical concept of causation. The main thrust of direct causation is that there are no intervening causes between an act and the resulting harm. An intervening cause has several requirements: it must 1) be independent of the original act, 2) be a voluntary human act or an abnormal natural event, and 3) occur in time between the original act and the harm.
Direct causation is the only theory that addresses only causation, and does not take into account the culpability of the original actor.
The plaintiff must demonstrate that the defendant's action increased the risk that the particular harm suffered by the plaintiff would occur. If the action were repeated, the likelihood of the harm would correspondingly increase. This is also called foreseeable risk.
Harm Within the Risk 
The Harm Within the Risk (HWR) test determines whether the victim was among the class of persons who could foreseeably be harmed, and whether the harm was foreseeable within the class of risks. It is the strictest test of causation, made famous by Benjamin Cardozo in Palsgraf v. Long Island Railroad Co. case under New York state law.
The first element of the test is met if the injured person was a member of a class of people who could be expected to be put at risk of injury by the action. For example, a pedestrian, as an expected user of sidewalks, is among the class of people put at risk by driving on a sidewalk, whereas a driver who is distracted by another driver driving on the sidewalk, and consequently crashes into a utility pole, is not.
The second prong of the test is whether the type of harm was in the class of expected harms. Giving a loaded gun to a young child gives rise to the expectation that someone could be shot by the child's firing the gun. If the child drops the gun and it breaks someone's foot, the HWR test fails, as a broken foot was not the type of harm anticipated.
The HWR test is no longer much used, outside of New York law. When it is used, it is used to consider the class of people injured, not the type of harm. The main criticism of this test is that it is preeminently concerned with culpability, rather than actual causation.
The "Risk Rule" 
Referred to by the Reporters of the Second and Third Restatements of the Law of Torts as the "scope-of-the-risk" test, the term "Risk Rule" was coined by the University of Texas School of Law's Dean Robert Keeton. The rule is that “[a]n actor’s liability is limited to those physical harms that result from the risks that made the actor’s conduct tortious.” Thus, the operative question is "what were the particular risks that made an actor's conduct negligent?" If the injury suffered is not among those risks, there can be no recovery. Two examples will illustrate this principle:
- The classic example is that of a father who gives his child a loaded gun, which she carelessly drops upon the plaintiff’s foot, causing injury. The plaintiff argues that it is negligent to give a child a loaded gun and that such negligence caused the injury, but this argument fails, for the injury did not result from the risk that made the conduct negligent. The risk that made the conduct negligent was the risk of the child accidentally firing the gun; the harm suffered could just as easily have resulted from handing the child an unloaded gun.
- Another example familiar to law students is that of the restaurant owner who stores rat poison above the grill in his luncheonette. The story is that during the lunch rush, the can explodes, severely injuring the chef who is preparing food in the kitchen. The chef sues the owner for negligence. The chef may not recover. Storing rat poison above the grill was negligent because it involved the risk that the chef might inadvertently mistake it for a spice and use it as an ingredient in a recipe. The explosion of the container and subsequent injury to the chef was not what made the chosen storage space risky.
The most obvious objection to this approach is that it requires courts to consider an arguably endless possibility of hypothetical situations. Not only can such an undertaking be an exercise in futility, but this approach lacks even a minimal amount of precision such that parties might be able to predict outcomes and results during litigation. Notwithstanding the already-complex nature of this and other questions relating to proximate or legal cause, this fluid standard could be misused by plaintiff-friendly or defense-favoring judges in attempts to vindicate their own personal philosophies regarding the appropriate reach of tort law.
The doctrine of proximate cause is notoriously confusing. The doctrine is phrased in the language of causation, but in most of the cases in which proximate cause is actively litigated, there is not much real dispute that the defendant but-for caused the plaintiff's injury. The doctrine is actually used by judges in a somewhat arbitrary fashion to limit the scope of the defendant's liability to a subset of the total class of potential plaintiffs who may have suffered some harm from the defendant's actions. For an understanding of the broader view of causation which proximate cause circumscribes, see butterfly effect.
For example, in the two famous Kinsman Transit cases from the 2nd Circuit (exercising admiralty jurisdiction over a New York incident), it was clear that mooring a boat improperly could lead to the risk of a boat drifting away and crashing into another boat, and that both boats could crash into a bridge, which collapsed and blocked the river, and in turn, the wreckage could flood the land adjacent to the river, as well as prevent any traffic from traversing the river until it had been cleared. But under proximate cause, the property owners adjacent to the river could sue (Kinsman I), but not the owners of the boats or the cargoes which could not move until the river was reopened (Kinsman II).
Therefore, in 2001, the American Law Institute proposed in a draft of the Restatement (Third), Torts: Liability for Physical Harm (Basic Principles) that proximate cause should be replaced with scope of liability.
Efficient proximate cause 
A related doctrine is the insurance law doctrine of efficient proximate cause. Under this rule, in order to determine whether a loss resulted from a cause covered under an insurance policy, a court looks for the predominant cause which sets into motion the chain of events producing the loss, which may not necessarily be the last event that immediately preceded the loss. Many insurers have attempted to contract around efficient proximate cause through the use of "anti-concurrent causation" (ACC) clauses, under which if a covered cause and a noncovered cause join together to cause a loss, the loss is not covered. A minority of jurisdictions have ruled ACC clauses to be unenforceable as against public policy, but they are generally enforceable in the majority of jurisdictions.
See also 
- Corey v. Havener, 182 Mass. 250.
- Summers v. Tice, 199 P.2d 1 (Cal. 1948).
- See Sindell v. Abbott Labs.
- See Restatement (Second) of Torts.
- Palsgraf v. Long Island Rail Road Co., 162 N.E. 99 (N.Y. 1928).
- See RESTATEMENT (THIRD) OF TORTS: LIAB. FOR PHYSICAL HARM § 29 cmt. d (Proposed Final Draft No. 1, 2005); RESTATEMENT (SECOND) OF TORTS § 281 cmt. g (1965).
- ROBERT E. KEETON, LEGAL CAUSE IN THE LAW OF TORTS 9–10 (1963).
- RESTATEMENT (THIRD) OF TORTS: LIAB. FOR PHYSICAL HARM § 29 (Proposed Final Draft No. 1, 2005).
- Benjamin C. Zipursky, Foreseeability in Breach, Duty and Proximate Cause, 44 Wake F. L. Rev. 1247, 1253 (2009). The full text of this article is available online at http://lawreview.law.wfu.edu/documents/issue.44.1247.pdf. Accord Lubitz v. Wells, 113 A. 2d 147 (Conn. 1955).
- The exact etymology of this hypothetical is difficult to trace. Adaptations are set forth and discussed in Joseph W. Glannon, The Law of Torts: Examples and Explanations (3d ed. 2005) and John C. P. Goldberg, Anthony J. Sebok, and Benjamin C. Zipursky, Tort Law: Responsibilities and Redress (2004) among others.
- "When defendants move for a determination that plaintiff’s harm is beyond the scope of liability as a matter of law, courts must initially consider all of the range of harms risked by the defendant’s conduct that the jury could find as the basis for determining that conduct tortious. Then the court can compare the plaintiff’s harm with the range of harms risked by the defendant to determine whether a reasonable jury might find the former among the latter." RESTATEMENT (THIRD) OF TORTS: LIAB. FOR PHYSICAL HARM § 29 cmt. d (Proposed Final Draft No. 1, 2005).
- PPG Indus., Inc. v. Transamerica Ins. Co., 20 Cal. 4th 310, 316 (1999).
- See In re Kinsman Transit Co., 338 F.2d 708 (2nd Cir. 1964) and Kinsman Transit Co. v. City of Buffalo, 388 F.2d 821 (2nd Cir. 1968).
- Leonard v. Nationwide Mut. Ins. Co., 499 F.3d 419 (5th Cir. 2007) (surveying cases).
Further reading 
- Michael S. Moore, The Metaphysics of Causal Intervention, 88 calif l. rev. 827 (2000).
- Leon A. Green, The Rationale of Proximate Cause (1927).