Source: p 200, Thinking Like a Lawyer: An Introduction to Legal Reasoning (2010, 2 ed) by Kenneth J. Vandevelde

  This “but for” test has proved inadequate where two defendants jointly cause an injury that would have occurred even if only one of them had acted. For example, if two men negligently discharge firearms, both of which fire fatal shots into the victim, in a lay sense of the term both men “caused” the injury. Yet by applying the “but for” test, neither man can be shown to have caused it. If the first man had not fired his weapon, the victim would still have been killed by the second shot. Thus, one cannot say that but for the first man’s negligence, the injury would not have occurred. The same reasoning exonerates the second man as well. [1.] The “but for” test, in other words, would absolve both gunmen of liability for the shooting.
  Accordingly, many courts have adopted a rule that provides an alternative definition of actual causation. Under this rule, a defendant’s breach of duty is considered the actual cause of an injury if it was a substantial factor in bringing about the injury, even though the breach may not have been the “but for” cause. The outcome of applying this test to the shooting circumstances would be that either of the two gunmen would be considered the cause of the death.

Why is 1 true? Why cannot both shooters be found liable for the shooting?


They both can be found liable, but not by using the but-for test.

Suppose that person A and person B each independently negligently discharge firearms and that each on its own would be sufficient to kill person C.

Is it true that, "but for the actions of A, C would still be alive?" No. Is it true that, "but for the actions of B, C would still be alive?" No.

Using the but-for test would not be able to assign liability to either A or B.

"But for" is not an obvious phrasing for non-native English speakers. It's the same as asking, "If it were not for the actions of A, would C still be alive?".

However, courts and juries are not limited to using the but-for test for causation. See Corey v Havener, 182 Mass. 250 (1902):

It makes no difference that [...] it is impossible to determine what portion of the injury was caused by each. If each contributed to the injury, that is enough to bind both.


The but-for test is a fairly simple test -- it says that action A caused effect E only if, had action A not been performed, effect E would not have happened. So, let's say I'm a doctor, and I forget to note that a patient needs some lifesaving medication at 9:00 tomorrow; without it, they will die. If they then die because they didn't get it, their death would not have happened without my mistake. In contrast, if they have a heart attack this evening and die, my mistake would have killed them, but their death would have happened exactly the same whether or not I remembered the note. So, under the but-for test, I didn't cause the death in the second case. Sure, I was negligent, and sure, my negligence would have killed them, but in this case it wasn't what killed them. If you took me out of the situation entirely, nothing would change about the death.

Now, go to the given scenario. Two people negligently fire shots, and either shot would have killed the victim. So, if you took either of them out of the situation entirely, the victim still dies. But-for means that "without A, E wouldn't happen," and that's obviously not true for either negligent discharge.

So, the rule can be modified to consider substantial factors. Sure, the victim would die whether or not either individual shot him, but since he died from a gunshot wound the gunshots were clearly both major factors in the death.

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