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?