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Causation is the "causal relationship between conduct and result." That is to say that causation provides a means of connecting conduct, complete with actus reus, with the resulting harm or result element. It should be noted that causation is only applicable where a result has been achieved and therefore is immaterial with regards to inchoate offenses.
Background concepts
Most legal systems are to a greater or lesser extent concerned with the notions of fairness and justice. If a state is going to penalise a person or require that person to pay compensation to another for losses incurred, this imposition of liability will be derived from the idea that those who injure others should take responsibility for their actions. Although some parts of any legal system will have qualities of strict liability, in which the mens rea is immaterial to the result and subsequent liability of the actor, most look to establish liability by showing that the defendant was the cause (applying various legal tests in order to establish this) of the particular injury or loss.
Even the youngest children quickly learn that, with varying degrees of probability, consequences flow from physical acts and omissions. The more predictable the outcome, the greater the likelihood that the actor caused the injury or loss intentionally. There are many ways in which the law might capture this simple rule of practical experience: that there is a natural flow to events, that a reasonable man in the same situation would have foreseen this consequence as likely to occur, that the loss flowed naturally from the breach of contractual or tortious duty, etc. However it is phrased, the essence of the degree of fault attributed will lie in the fact that reasonable people try to avoid injuring others so, if harm was foreseeable, there should be liability to the extent that the extent of the harm actually resulting was foreseeable
The relationship between causation and liability
Causation of an event by itself is not sufficient to create legal liability.
Sometimes causation is one part of a multi-stage test for legal liability. For example for the defendant to be held liable for the tort of negligence, the defendant must have (1) owed the plaintiff a duty of care; (2) breached that duty; (3) by so doing caused damage to the plaintiff; and (4) that damage must not have been too remote. Causation is but one component of the tort.
On other occasions causation is the only requirement for legal liability (other than the fact that the outcome is proscribed). For example in the law of product liability, the fact that the defendant's product caused the plaintiff harm is the only thing that matters. The defendant need not also have been negligent.
On still other occasions, causation is irrelevant to legal liability altogether. For example, under a contract of indemnity insurance, the insurer agrees to indemnify the victim for harm not caused by the insurer, but by other parties.
Because of the difficulty in establishing causation, it is one area of the law where the case law overlaps significantly with general doctrines of analytic philosophy to do with causation. The two subjects have long been somewhat intermingled.
Establishing causation
Where establishing causation is required to establish legal liability, it is usually said that it involves a two-stage inquiry.
The first stage involves establishing ‘factual’ causation. Did the defendant act in the plaintiff’s loss?
The second stage involves establishing ‘legal’ causation. This is often a question of public policy: is this the sort of situation in which, despite the outcome of the factual enquiry, we might nevertheless release the defendant from liability, or impose liability?
Establishing factual causation
The usual method of establishing factual causation is the but-for test. The but for test inquires ‘But for the defendant’s act, would the harm have occurred?’ A shoots and wounds B. We ask ‘But for A's act, would B have been wounded?’ The answer is ‘No.’ So we conclude that A caused the harm to B. The but for test is a test of necessity. It asks was it ‘necessary’ for the defendant’s act to have occurred for the harm to have occurred.
The but-for test often gives us the right answer to causal problems, but sometimes not. Two difficulties are immediately obvious. The first is that under the but-for test, almost anything is a cause. But for a tortfeasor's grandmother's birth, the relevant tortious conduct would not have occurred. But for the victim of a crime missing the bus, he or she would not have been at the site of the crime and hence the crime would not have occurred. Yet in these two cases, the grandmother's birth or the victim's missing the bus are not intuitively causes of the resulting harm. This often does not matter in the case where cause is only one element of liability, as the remote actor will most likely not have committed the other elements of the test. However, this situation can arise in strict liability situations.
The other problem is that of overdetermination. Imagine two hunters, A and B, who each negligently fire a shot that takes out C's eye. Each shot on its own would have been sufficient to cause the damage. But for A's shot, would C's eye have been taken out? Yes. The same answer follows in relation to B's shot. But on the but-for test, this leads us to the counterintuitive position that neither shot caused the injury.
The courts have generally accepted the but for test notwithstanding these weaknesses, qualifying it by saying that causation is to be understood “as the man in the street” would: Yorkshire Dale Steamship Co v Minister of War Transport [1942] AC 691 (House of Lords), or by supplementing it with “common sense”: (March v Stramare (1991) 171 CLR 506 (High Court of Australia).
This dilemma was handled in the United States in State v. Tally, 15 So. 722, 738 (Ala. 1894), where the court ruled that: “The assistance given ... need not contribute to criminal result in the sense that but for it the result would not have ensued. It is quite sufficient if it facilitated a result that would have transpired without it.” Using this logic, A and B are liable in that no matter who was responsible for the fatal shot, the other "facilitated" the criminal act even though his shot was not necessary to deliver the fatal blow.
However, legal scholars have attempted to make further inroads into what explains these difficult cases. Some scholars have proposed a test of sufficiency instead of a test of necessity. H. L. A. Hart and Tony Honoré, and later Richard Wright, have said that something is a cause if it is a ‘necessary element of a set of conditions jointly sufficient for the result’. This is known as the NESS test. In the case of the two hunters, the set of conditions required to bring about the result of the victim's injury would include a gunshot to the eye, the victim being in the right place at the right time, gravity, etc. In such a set, either of the hunters' shots would be a member, and hence a cause. This arguably gives us a more theoretically satisfying reason to conclude that something was a cause of something else than by appealing to notions of intuition or common sense.
Hart and Honore, in their famous work Causation in the Law, also tackle the problem of 'too many causes'. For them, there are degrees of causal contribution. A member of the NESS set is a "causally relevant condition". This is elevated into a "cause" where it is a deliberate human intervention, or an abnormal act in the context. So, returning to our hunter example, hunter A's grandmother's birth is a causally relevant condition, but not a "cause". On the other hand, hunter A's gunshot, being a deliberate human intervention in the ordinary state of affairs, is elevated to the status of "cause". An intermediate position can be occupied by those who "occasion" harm, such as accomplices. Imagine an accomplice to a murder who drives the principal to the scene of the crime. Clearly the principal's act in committing the murder is a "cause" (on the but for or NESS test). So is the accomplice's act in driving the principal to the scene of the crime. However the causal contribution is not of the same level (and, incidentally, this provides some basis for treating principals and accomplices differently under criminal law). Leon Green and Jane Stapleton are two scholars who take the opposite view. They consider that once something is a "but for" (Green) or NESS (Stapleton) condition, that ends the factual inquiry altogether, and anything further is a question of policy.
Establishing legal causation
Notwithstanding the fact that causation may be established in the above situations, the law often intervenes and says that it will nevertheless not hold the defendant liable because in the circumstances the defendant is not to be understood, in a legal sense, as having caused the loss. In the United States, this is known as the doctrine of proximate cause. The most important doctrine is that of novus actus interveniens, which means a ‘new intervening act’ which may ‘cut the chain of causation’.
Imagine the following. A critically injures B. As B is wheeled to an ambulance, she is struck by lightning. She would not have been if she had not been injured in the first place. Clearly then, A caused B's whole injury on the ‘but for’ or NESS test. However, at law, the intervention of a supervening event renders the defendant not liable for the injury caused by the lightning.
The effect of the principle may be stated simply:
if the new event, whether through human agency or natural causes, does not break the chain, the original actor is liable for all the consequences flowing naturally from the initial circumstances. But if the new act breaks the chain, the liability of the initial actor stops at that point, and the new actor, if human, will be liable for all that flows from his or her contribution.This is usually expressed as a question of 'foreseeability'. An actor is liable for the foreseeable, but not the unforeseeable, consequences of his or her act. For example it is foreseeable that if I shoot someone on a beach and they are immobilized, they may drown in a rising tide rather than from the gunshot wound or bloodloss. However it is not (generally speaking) foreseeable that they will be struck by lightning and killed by that event.
This type of causal foreseeability is to be distinguished from foreseeability of extent or kind of injury, which is a question of remoteness of damage, not causation. For example, if I conduct welding work on a dock that lights an oil slick that destroys a ship a long way down the river, it would be hard to construe my negligence as anything other than causal of the ship's damage. There is no novus actus interveniens. However, I may not be held liable if that damage is not of a type foreseeable as arising from my negligence: The Wagon Mound (No 1) [1961] AC 388 (Privy Council). That is a question of public policy, and not one of causation.
Other relevant considerations
Because causation in the law is a complex amalgam of fact and policy, other doctrines are also important, such as foreseeability and risk. Particularly in the United States, where the doctrine of 'proximate cause' effectively amalgamates the two stage factual then legal causation inquiry favoured in the English system, one must always be alert to these considerations in assessing the postulated relationship between two events.
Foreseeability test
Some aspects of the physical world are so inevitable that it is always reasonable to impute knowledge of their incidence. So if A abandons B on a beach, A must be taken to foresee that the tide comes in and goes out. But the mere fact that B subsequently drowns is not enough. A court would have to consider where the body was left and what level of injury A believed that B had suffered. If B was left in a position that any reasonable person would consider safe but a storm surge caused extensive flooding throughout the area, this might be a novus actus. That B was further injured by an event within a foreseen class does not of itself require a court to hold that every incident falling within that class is a natural link in the chain. Only those causes that are reasonably foreseeable fit naturally into the chain. So if A had heard a weather forecast predicting a storm, the drowning will be a natural outcome. But if this was an event like a flash flood, an entirely unpredictable event, it will be a novus actus.