merged causes

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Merged causes describes a situation where there are two causes of a single result. The term highlights an issue with the but-for causation test used in criminal law and tort law to determine actual causation. In a case where two defendants separately breach duties owed to a plaintiff, there may be an issue proving but-for causation in that each defendant can use the defense that neither of their breaches were the but-for causation of the plaintiff's harm, since the plaintiff would have been harmed even if one defendant did not breach their duty to the plaintiff. For example, if two defendants each shoot a person at the same time, it may be unclear which defendant is the but-for cause of the death. 

In dealing with cases of this nature, where but-for and proximate cause limiting factors are insufficient, courts often use the "substantial factor test” - when there is a merged causes situation, the court asks if each individual breach was itself a substantial factor, meaning that it could have caused the harm individually, even though it did not. If a defendant's breach is deemed a substantial factor, the defendant is held liable. Using this test, multiple defendants can be held jointly and severally liable for breaching a duty towards a plaintiff.

[Last updated in July of 2023 by the Wex Definitions Team]