Causation (law) - Establishing Factual Causation

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.

One weakness in the but-for test arises in situations where each of several acts alone are sufficient to cause the harm. For example, if both A and B fire what would alone be fatal shots at C at approximately the same time, and C dies, it becomes impossible to say that but-for A's shot, or but-for B's shot alone, C would have died. Taking the but-for test literally in such a case would seem to make neither A nor B responsible for C's death.

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 AC 691 (HL), 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.